2 Life in the taiga.
1.1 The key role of the tree layer
The tree story plays a vital role in the taiga ecosystem. Its biomass is far greater than that of all the other plants and animals put together. The canopy of conifers creates a very special environment where there is shade, the air is moist, temperature variations are buffered, and the wind speed is lower. This environment is very different from the adjacent open spaces that lack a tree canopy.
The dominant role of the trees
Few plants can grow under the dense canopy of conifers. Many species of light-demanding plants cannot grow in the gloom of the boreal forests, and shade-tolerant species have a competitive advantage. The shadow cast by trees on the lower layers is not the only effect they have on the understory. Their roots also absorb the water and nutrients required by smaller plants. Conifer needles and stems fall on the smaller plants and smother them, while also releasing toxic substances. The leaf-litter layer often presents an insuperable obstacle to the successful germination of many herbaceous plants, and even the trees' own seeds. The tree population is much less diverse than in temperate deciduous forests and tropical forests. In virgin boreal forests and those that have not been recently felled or burned, it is almost impossible to distinguish substories within the tree layer, which usually consists of a single tree species, or at most two. There are very few boreal forests where three or four species coexist. If there are more than four, it can be assumed that the ecosystem is at an intermediate stage of succession.
Another feature of the tree population of virgin taiga forests is the uneven distribution of age classes. The regeneration of the forest is slow and limited, because growth is very slow in the dense shade of the boreal forest and most of the seedlings die from lack of light and competition with the adult plants. Only if an adult tree dies and then falls to the ground does a clearing form, creating a brief period of favorable conditions for the growth of young trees. The fortunate seedlings that are growing in this clearing speed up their growth and can reach adult size. The canopy layer thus contains trees of different ages, as the period of rapid growth in these trees may begin when they are 10 years old or when they are 50 years old.
The population density of a forest like this, with trees of different ages but all members of the same species, is usually very high. Yet this is only true among the boreal forests in the dark taiga, where each specific area of virgin forest contains the largest possible number of trees. The light conifer forests either occur on sites where fires in the understory recur frequently or else occupy the less favorable sites. If fires are recurrent, the forests do not have time to become very dense. In less favorable habitats, the soil conditions, or the lack of water or nutrients, prevent the growth of a dense forest.
Low diversity under the canopy
The denser the tree stand in the taiga, the lower the diversity of the lower layers. When growth in the dark conifer forests is very dense, a significant part of the area shaded by the canopy is totally devoid of plants. In principle, the presence of these areas without vegetation should create conditions for the arrival of alien species, but dark conifer ecosystems are so dense, and the conditions in them so unusual, that this does not occur. Other plants simply cannot live there.
In general, the taiga has a shrub layer under the tree layer. It is usually 3-13 ft (1-4 m) tall, and its height depends directly on the density of the tree layer. The shrub layer is poor in the central and northern taiga, and is most developed in the southern taiga. The shrub layer consists of just a few shade-resistant species: junipers (Juniperus), alders (Alnus spp.), wild roses (Rosa), and a few others. The shrub layer may play an important role in the life of the forest, for example as a nesting site. Many taiga animals feed on the fruit of wild roses (Rosa), honeysuckles (Lonicera), and raspberries (Rubus idaeus) or on the pine nuts of the Japanese stone pine (Pinus pumila). The soils are also enriched in nitrates, as the roots of alders bear nodules that contain nitrogen-fixing actinomycete fungi.
Below the shrub layer is a herbaceous layer, though this could be called a subshrub layer due to the presence (and sometimes dominance) of low (up to 24 in [60 cm]) perennial plants, which are woody and branched and very typical of boreal forests. Almost all the herbaceous plants of the taiga are perennial, including the cowberry (Vaccinium vitisidaea), the bilberry (V. myrtillus), and the twinflower (Linnaea borealis), which are all characteristic of other colder areas, such as the tundra and high mountain, and also of the bog areas in the taiga. Like the shrub layer, the growth of the herbaceous layer is also proportional to the density of the tree stand.
The lowest layer consists of lichens and mosses growing on the soil surface. Lichens generally require a lot of light, while most boreal species of moss are shade-tolerant. This shade-tolerance means that the moss layer is thick and lush in most boreal forests. It is unusual to find a complete cover of lichens in the taiga, because they tend to be restricted to well-lit sites and open forests growing on dry, poor, sandy soils, such as pine forests on sand dunes. These lichens become so fragile in the dry season that walking on them causes them to crunch underfoot.
In the boreal forests, unlike the tropical forests, there is almost no other vegetation, such as climbers. A tropical rainforest is usually full of climbing plants, but there are almost none in the taiga, almost the only exception being some species of clematis (Clematis). Epiphytic flowering plants are very abundant in tropical rainforests (see vol. 2, pp. 78-79 and 348-352), but only mosses, lichens, and algae grow as epiphytes (plants that grow on other plants instead of soil but are not parasitic) in the taiga. Because of their high osmotic pressure, they obtain almost all the water they need from the atmosphere. These epiphytes, especially the fruticose lichens (lichens with a shrubby appearance) that hang from the branches of trees, are very sensitive to air pollution, especially sulfur dioxide, and this makes them good indicators of atmospheric pollution. Recording the species of lichen present and their condition allows a preliminary assessment of local air pollution. The total absence of epiphytic lichens indicates that the air is heavily polluted.
The mosaic distribution of the lower layers
Sometimes, the subshrub and moss layers in the taiga forest are distributed in patches, in a mosaic pattern. Some sites have almost no plant cover, while other places may be densely covered with just a single species. Thus a spruce forest may contain relatively large patches of May lily (Maianthemum bifolium), starflowers (Trientalis europaea and T. borealis), common wood sorrel (Oxalis acetosella), and other plants. These mosaics are largely a reflection of the heterogeneity of environmental conditions, the reason why the floristic composition of the populations of wet microdepressions is different from that of the flat areas.
These microdepressions ("craters") are formed when trees are blown over and uprooted by the wind. Trees falling are largely responsible for the redistribution of the plants of the lower layers. A crater is left where moisture-loving plants grow. The pile of earth adhering to the roots when the tree falls is loose and well drained, and for a short time conditions are excellent for herbaceous plants and shrubs. The rotting trunk shelters a range of mosses and lichens. In many taiga ecosystems, it has been shown that almost the only place where tree seeds germinate successfully is on fallen logs and trunks, as leaf litter prevents successful germination elsewhere.
The intensity of sunlight at ground level also plays an important role in the creation of mosaics. Sunlight reaches the ground level in gaps, and this is where conditions are most favorable for the growth of herbaceous and shrubby species, which in turn hamper the growth of the moss and lichen layer. In contrast, where the trees are dense and the soil is in deep shade, there are few herbaceous plants and shrubs, and the moss layer is very well developed. This mosaic structure is very clear in open woodlands, such as the larch forests of Yakuty, for example, where mosses are dominant under the tree cover and lichens dominate the unsheltered surface.
Another factor contributing to this mosaic distribution is the model of growth of some plants. Many boreal herbaceous plants and small shrubs can spread laterally by producing underground rhizomes (rootstocks) or above-ground stolons (creeping stems), and this leads to the formation of new patches. The activity of some animals is also involved in this mosaic distribution, especially that of burrowing mammals and social insects (like ants) that build their nests in the soil. These animals are scarce in the taiga, however, and their contribution to the formation of mosaics is almost irrelevant.
1.2 Primary production and biomass
The boreal conifer forests differ from the temperate deciduous forests and the tropical rainforests in that most of the organic matter accumulates in the soil and leaf-litter layer as dead undecomposed plant remains, not as above-ground biomass (like in the tropics) or as roots. In other words, there is more undecomposed dead plant material than living biomass.
The accumulation of biomass
The plant biomass varies greatly from one forest to another. Broadly speaking, plant biomass per unit area increases from north to south, and along a gradient from highly continental climates to coastal ones. Thus, the spruce forests of the plains of eastern Europe and Siberia typically show a variation in plant biomass from 45-89 short tons/acre (100-200 metric tons/ha) in the northern taiga, 89-120 short tons/acre (200-270 metric tons/ha) in the central taiga, and 120-156 short tons/acre (270-350 metric tons/ha) in the southern taiga. This is many times higher than the plant biomass in the tundra, but considerably lower than in other types of forest, such as the deciduous forests. In the extremely continental climate of central Yakuty, the plant biomass may fall to values of 22-54 short tons/acre (50-120 metric tons/ha), 22-76 short tons/acre (50-170 metric tons/ha) in flooded forests, or as low as 4-8 short tons/acre (9-17 metric tons/ha) in open bogs. Most of the plant biomass, 60-80%, is in the trunks of the trees; mature stands in the central and southern taiga may yield 212-265 yd3/acre (400-500 m3/ha). The photosynthetic parts (the green leaves) represent 5-10% and the roots account for the remaining 20-30%.
In the boreal forests, the animal biomass is far smaller than the plant biomass, but it too increases from north to south and from continental climates to coastal ones. Animal biomass in the European taiga increases from 89 lb/acre (100 kg/ha) in the northern subzone to 268 lb/acre (300 kg/ha) in the southern subzone. About 90% of the animal biomass consists of saprophytes (decay-eaters) living in the leaf litter and upper soil horizons. Phytophagous (plant-eating) invertebrates living in the canopy represent another 9 lb/acre (10 kg/ha), while vertebrates as a whole only contribute 1.8 lb/acre (2 kg/ha)-and even less in central Yakuty, only 0.39 lb/acre (0.44 kg/ha).
Nutrients and productivity
In the northern European and western Siberian taiga, the annual growth of the plant biomass is 1.8-2.7 short tons/acre (4-6 metric tons/ha), and less than 0.9 short ton/acre (2 metric tons/ha) in eastern Siberia; the figures reach 2.7-3.6 short tons/acre (6-8 metric tons/ha) in the central taiga, 3.6-4.5 short tons/acre (8-10 metric tons/ha) in the southern taiga, and even higher in some places. More or less half of this increase is in the form of the needles of the conifers. In the peat bogs in the taiga, the annual increase in plant biomass is 1.3-3.1 short tons/acre (3-7 metric tons/ha), 80% of it corresponding to sphagnum mosses, while the yield of fruits from the wood and peat bogs is between 0.09 and 0.45 short ton/acre (0.2 and 1 metric ton/ha). A large part of the increase in plant biomass (1.3-3.1 short tons/acre [3-7 metric tons/ha]) is shed each year, and forms part of the dead material in the leaf litter. Fifty percent of the litter consists of conifer needles, 20-30% is above-ground perennial organs, and 10-20% is root debris. The fallen parts decompose very slowly, and so a thick layer of leaf litter accumulates, whose mass of 922 short tons/acre (20-50 metric tons/ha) or more is several times greater than the plant biomass that falls down over the course of a year (or over 10 years, in the case of swamp forests).
In terrestrial communities, the net yield of photosynthesis does not usually exceed 3%, but the conifer forests are one of the most photosynthetically efficient communities. They reach values of 1-3% efficiency, two or three times greater than in deciduous forests, and tens or hundreds of times greater than in desert communities. Because most of the plants in boreal forests are evergreen, photosynthesis can take place even during the colder periods of the year, despite the harsh climatic conditions, and this is why the level of production in the taiga is very high. It is precisely for this reason that the coniferous forests have been able to occupy 10% of all dry land, and become the dominant vegetation where conditions are unfavorable for most other vascular plants.
The abundant herbivores and detritivores
The vertebrates of the taiga are mainly herbivores. Ungulates, numerous rodents, grouse, and many others survive on the vegetative parts of trees and shrubs. Fruit and seeds are the main food of mammals like squirrels (Sciurus), chipmunks or burunduks (Tamias), woodmice (Apodemus), and birds, such as the nutcracker (Nucifraga) and crossbills (Loxia). Birds, together with some rodents and insects, eat most of the seed production in the taiga, and many different types of animal eat the fruit and leaves of herbaceous plants and small shrubs, as well as lichens and to a lesser extent mosses. The plant biomass they consume represents less than 1% of the annual increase in plant biomass, but the percentage of seed production that is consumed is much higher.
Most invertebrates are detritivorous, living in the leaf-litter layer and the upper soil horizons. The most abundant groups are arthropods (insects, especially springtails), soil mites, centipedes, wood lice, different worms (nematodes, enchytraeids, and earthworms), tardigrades, etc.; there are also many protists. The number of these animals and protists may reach values of millions per square yard. The mites (oribatid, gamasid, and acarid mites) may reach densities of about 84 million per sq yd (100 million per sq m) in the leaf litter and soil of the boreal forests. The larger soil invertebrates, especially earthworms, burrow through the soil, forming galleries that aerate the soil and increase oxygen levels, in addition to increasing its ability to absorb water.
Saprophagous organisms feed on the dead organs of plants, on animal corpses (necrophages), or on excrement (coprophages). They either directly mineralize these materials or create favorable conditions for the later activities of bacteria and saprophytic fungi, which complete mineralization. Taiga invertebrates can barely digest recently fallen leaves, but they become much easier to digest once rain has washed out a part of the phenolic substances they contain. After this the leaf litter is shredded into smaller pieces, greatly increasing its surface area. This allows saprophytic microorganisms to attack the fragments, making it easier for new groups of invertebrates to make use of them. Depending on the species diversity and abundance and the environmental conditions, the invertebrates speed up the rate of mineralization of leaf litter and soil plant remains by three to eight times.
There are two different types of saprophytic invertebrate: those that release nitrogen and those that release carbon. The nitrogen-releasing invertebrates, including earthworms, enchytraeids, fly larvae (dipterans), and springtails, mainly participate in the decomposition and mineralization of nitrogen-containing substances, some of them also participating in humus formation. The second group consists of millipedes, wood lice, and beetle larvae and adults; they mainly participate in the decomposition of materials that are very poor in nitrogen. Symbiotic microorganisms living in the digestive tract play an active role in the digestion of the food that these invertebrates eat. These gut symbionts often include soil microorganisms, which can break down organic matter in the favorable conditions within the gut (abundant food, relatively constant temperature and humidity, and a chemically neutral environment). In return they break down the plant matter the animal has eaten, releasing vitamins and other substances that stimulate the host's growth. In the guts of invertebrates, and even in their excreta, ammonification is intense, humus formation takes place, and even atmospheric nitrogen is fixed.
Saprophytes and other consumers of dead biomass
One of the most distinctive features of the taiga is the large number of saprophytic and semisaprophytic plants that grow there. This is very clearly shown by the members of the Indian pipe family (Monotropaceae); by some orchids, such as dwarf rattlesnake plantain (Goodyera repens), Microstylis [=Malaxis] monophylla, Listera cordata, Epipogium aphyllum, etc.; and by the gametophytes of club mosses (Lycopodium). A semisaprophytic way of life is very characteristic of some members of the wintergreen family (Pyrolaceae). The saprophytic way of life allows the boreal plants to make use of the large quantities of slowly decomposing organic matter in the leaf-litter layer. When there is little light, a plant may find it more advantageous to make use of this organic matter than to be autotrophic (i.e., produce its own food by photosynthesis). The next step in the evolution of saprophytism in flowering plants might be to develop parasitism, but there are few parasitic plants in the boreal forest. Perhaps the only one is Boschniakia rossica (Orobanchaceae), a parasite of the roots of the alder that is very frequent in the Siberian taiga.
Bacteria and decomposer fungi complete the process of decomposition and mineralization of dead organic remains. Detritivores and decomposers play basically the same role in the ecosystem. Both break the complex organic molecules in detritus down into simpler molecules and absorb them as their food source. The decomposer community can be as diverse in its activity and composition as any other community, or even more so. The lower the temperature, the lower the population of decomposers and the lower their activity. Their activity may also be greatly slowed down in highly acidic conditions. In the cold boreal forests on acid soils, their activity is relatively low, and so the decomposition of organic remains is slow, a thick layer of leaf litter accumulates, and soils rich in organic matter form.
Fungal and bacterial spores are present everywhere. They are often present on the surface or within the bodies of plants and animals when they die. These opportunist decomposers try to exploit energy-rich organic compounds, such as amino acids and sugars; they include molds (Penicillium, Mucor, Rhizopus) and yeasts (Saccharomyces, etc.). As the sugars are broken down, the number of decomposers increases. The first are those specialized in breaking down starch, followed by those that decompose hemicelluloses, then those that work on pectins and proteins. Cellulose (plant cell-wall fiber) is very resistant to decomposition, lignin (a cellulose-binder) is even more resistant, and suberin and cutin (waxy, resinous plant coverings) are among the most resistant organic materials known. Most decomposer species can only break down a restricted number of types of compound. Fungi that attack wood, for example, can be divided into two main types of specialized decomposers. One group destroys cellulose but leaves lignin intact (dry rot). The other group mainly destroys the lignin but leaves the cellulose intact (white root rot). In the taiga, dry rot is mainly due to species of Coniophora, Coriolus, Gloeophyllum, etc., while white root rot is mainly owing to species of Fomitopsis, Stereum, Armillaria, and others.
The essential role of the mycorrhiza
Fungi are important in taiga ecosystems because in addition to decomposing organic matter, they perform another vital function: forming mycorrhizae (a symbiotic relationship) with the roots of vascular plants. This is the most common form of symbiosis between autotrophs and heterotrophs. Between 70% and 90% of taiga plants form mycorrhizae, usually with zygomycete fungi, but conifers usually form mycorrhizae with basidiomycete fungi (and occasionally with ascomycetes). Discomycete fungi are the most common symbiotic partners of members of the heath family (Ericaceae). There are two types of mycorrhizae: endomycorrhiza, in which the fungus penetrates the cells of the root, and ectomycorrhiza, in which the fungus grows between the cells and covers the root. In both cases, the relation is beneficial to both partners. Some fungi that form ectomycorrhiza on the roots of conifers are unable to produce their fruiting bodies (i.e., mushrooms) or disperse their spores unless they are in contact with these roots. The fungus basically obtains carbohydrates from the plant, and also some vitamins. The plants obtain macronutrients from the fungus (especially phosphorus and nitrogen), and some micronutrients (zinc, sulfur, strontium), as well as vitamins and some growth substances. The well-developed mycelium (vegetative part of the fungus) growing on the roots makes the plants more effective at taking up nutrients, as the surface area of the mycelium may be several hundred times larger than the root on which it is growing. The outer mantle of hyphae (the threadlike parts that comprise the mycelium) also acts as a reserve organ and is also involved in water storage. The outer mantle also protects the roots from adverse environmental factors and from attacks by virulent agents, both physically (by enveloping the root) and biochemically (by impregnating the surrounding soil with antibiotics).
Mycorrhizae play a very important role in the taiga. Taiga soils are nutrient-poor, and the ions needed by plants often cannot be taken up because they are unavailable, as they are in forms that are immobile. The immobile ions are depleted in a zone about 1-1.5 mm in radius from around the roots. However, by forming a mycorrhiza the roots can obtain ions from a distance of around 1 in (20-30 mm), and possibly as far as 3 in (80 mm). The orchids that grow in the taiga, as well as some members of the wintergreen family (Pyrolaceae), are closely dependent on symbiotic fungi. The seeds of many of these orchids do not germinate unless the fungus is present. Orchids form a mycorrhiza with a fungus that can consume the cellulose and lignin in the soil, and the fungus supplies the plant with carbohydrates while it remains saprophytic. The underground saprophytic stage of these taiga orchids and members of the Pyrolaceae may last two to 15 years in different species. When the plant starts its autotrophic stage, the mycorrhiza may disappear. The embryos of these plants, which grow from tiny seeds, could not survive without these mycorrhizae.
Mycorrhizal symbiosis is generally closely related to a saprophytic way of life, and is present in all plants that lack chlorophyll. A series of experiments showed beyond all doubt that Monotropa hypopitys obtains its carbohydrates from a fungus. This species is a flowering plant that lacks chlorophyll and grows in fir groves in the taiga. The fungus in fact obtains the carbohydrates from a conifer, usually a pine. Thus Monotropa is not a true saprophyte, but a parasite of the tree, obtaining carbohydrates indirectly from the tree through its symbiotic relation with the fungus.
1.3 Adaptations to the boreal environment
Boreal forests are restricted to areas with a humid climate, but most of the taiga conifers are evergreens and highly xeromorphic, i.e., they have adaptations to water shortage. Xeromorphism and several other adaptations are discussed below.
The xeromorphic adaptations of the conifers
Xeromorphic conifers have leaves that are reduced to needles, with a relatively large number of epidermal cells, sunken stomata (pores), and a thick cuticle that reduces transpiration. Xeromorphic evergreen leaves are typical of many other vascular plants of the boreal forests, including pteridophytes such as club mosses (Lycopodium); rhododendrons (Rhododendron, Ericaceae); blueberries, cranberries, and cowberries (Vaccinium); marsh rosemary (Ledum palustre, Ericaceae); bog rosemary (Andromeda, Ericaceae); bearberry (Arctostaphylos uva-ursi, Ericaceae); Scotch heather or ling (Calluna vulgaris, Ericaceae); wintergreen (Pyrola, Pyrolaceae); pipsissewa (Chimaphila, Pyrolaceae); one-flowered shinleaf (Moneses, Pyrolaceae); crowberry (Empetrum nigrum, Empetraceae); twinflower (Linnaea borealis, Caprifoliaceae); holly (Ilex rugosa, Aquifoliaceae); and skimmias (Skimmia repens, Rutaceae). Ever-green leaves often represent an ancestral feature. Among the conifers, the deciduous habit of larches is considered to be acquired relatively recently in evolution.
Boreal plants are xeromorphic for a series of reasons that are not necessarily related to dry conditions. Many experiments have shown that xeromorphism is induced by nitrogen shortage and poor soils, conditions found in both taiga and raised bogs, which explains the presence of many xeromorphic plants in these sites. Application of nitrogen-containing fertilizers reduces the expression of xeromorphic features in plants.
The idea of "physiological drought" has also been suggested, in which the cold acid soils hamper the plant's uptake of water. In the taiga, xeromorphic growth is linked to the evergreen habit, which allows the plants to increase the duration of the period of photosynthesis. Net photosynthetic assimilation by conifer leaves during the cold season is very low, but in comparison with deciduous trees at the same latitudes, the period of photosynthesis for evergreens starts earlier in spring and finishes later in autumn. Yet it is precisely in winter when water transport within the vascular tissues is hardest, so the plant has to reduce water loss by transpiration; this is why so many trees of the taiga produce xeromorphic leaves.
A liking for shady woodland habitats
Taiga plants are well adapted to shady conditions. Blueberries (Vaccinium) can grow in forest environments where light intensity is only about 2% (1/50) of that in open environments, and the common wood sorrel (Oxalis acetosella) can survive on a light intensity only 1/70 of that of open areas. Conifer seedlings, especially those of spruces (Picea), firs (Abies), and Douglas fir (Pseudotsuga menziesii), are highly adapted to shade. Many plants of the dark conifer forests are also adapted to the low intensity sunlight throughout the growing period.
The leaves of some taiga plants, such as common wood sorrel, alpine enchanter's nightshade (Circaea alpina), and the fern Gymnocarpium dryopteris, are very fragile and only thrive in the shade. Many boreal species adapt to the shade by increasing the area of their leaves, as do baneberries (Actaea (Ranunculaceae); some boreal ferns, such as the lady fern (Athyrium filix-femina) and Diplazium sibiricum; and some grasses (Cinna and Hystrix). The poor light is one of the reasons why so many taiga plants have adopted a saprophytic way of life.
Nearly all the trees of the boreal forests are wind-pollinated. However most dicotyledonous herbaceous plants are pollinated by insects, as are some small trees and many shrubs, such as willows (Salix), wild roses (Rosa, Rosaceae), white beams (Sorbus, Rosaceae), cherries and other stone fruits (Prunus), and heathers (Erica, Ericaceae). The plants of the herbaceous and subshrub layer in the taiga usually bear white flowers, which are more conspicuous in the gloomy forest. White flowers make it easier for insects to locate flowers and thus pollinate them. White flowers are produced by the cowberry (Vaccinium vitis-idaea), wintergreens (Pyrolaceae), the May lily (Maianthemum bifolium), the European and American starflowers (Trientalis europaea and T. borealis), the Canadian dogwood or dwarf cornel (Cornus [=Chaemaepericlymenum] canadensis), and many other plants.
Animals not only pollinate plant flowers, they also help to disperse their seeds. The nutcracker (Nucifraga) and the burunduk chipmunk (Tamias [=Eutamias] sibiricus), for example, contribute to the regeneration of forests of the Swiss stone pine (Pinus cembra) that have been logged or burned. The brown bear (Ursus arctos) and the capercaillie (Tetrao urogallus) both help spread berry-bearing shrubs. Insects transport spores of mosses and lichen propagules, and dung beetles and earthworms help to bury seeds in the soil. Ants, birds, and mammals are the most important dispersal agents, and there are three main ways of dispersal: epizoochory, dyszoochory, and endozoochory.
Epizoochory is when fruit and seeds are passively transported, adhering to the body of animals (especially large mammals). This method of seed dispersal is not very widespread in the taiga, as few large animals live there. Almost the only exception is the twinflower (Linnaea borealis), whose tiny dry fruit are covered with small hooks that are easily caught in the fur of many animals.
Dyszoochory is more common in boreal forests and occurs when ants, rodents, or birds collect seeds for storage but never get around to eating them, thus leaving the seeds to germinate. The most common seed dispersal mechanism in the taiga, however, is endozoochory, in which the seeds retain their ability to germinate after they have passed through the animal's digestive system. The seeds are deposited in the animal's dung, which is an excellent fertilizer for the seedlings. The many taiga species that disperse their seeds in this way include the Siberian pine (Pinus sibirica), the Japanese stone pine (P. pumila), juniper (Juniperus), the cowberry (Vaccinium vitis-idaea), the bilberry (V. myrtillus), the bearberry (Arctostaphylos uva-ursi), the baneberry (Actaea), the May lily (Maianthemum), the wild rose (Rosa), and mountain ashes or white beams (Sorbus). Birds are the main dispersal agents. In the central part of the European Russian taiga alone, there are 23 species of bird that feed on the fruit of the bearberry (Arctostaphylos uva-ursi), 20 that feed on crowberry (Empetrum nigrum), 14 that feed on mountain ash (Sorbus aucuparia), 13 that feed on dog rose (Rosa canina), 10 on the bog bilberry (Vaccinium uliginosum), etc. It is interesting to note that when the insect population declines in autumn, insectivorous birds such as thrushes (Turdus), robins (Erithacus), and warblers (Sylvia) switch to eating fleshy fruits.
Some mammals also eat these fleshy fruits and help to disperse them, including the brown bear (Ursus arctos), the pine marten (Martes martes), the sable (M. zibellina), the stoat (Mustela erminea), some lagomorphs such as the hare (Lepus), and some rodents, such as the red squirrel (Sciurus vulgaris) and the burunduk chipmunk (Tamias [=Eutamias] sibiricus). Some plants have to be dispersed this way, as their seeds do not germinate unless they pass through an animal's digestive tract.
It is also worth pointing out that because of the harsh climate in the boreal forests, plants do not flower every year, and bad weather may sometimes destroy all the flowers. This has led a very wide range of herbaceous and shrub species to develop special methods of vegetative reproduction. Almost all the herbaceous plants growing in the taiga are perennial, and they and the subshrub species have long creeping rhizomes (rootstocks) in the upper soil horizons, or aerial shoots that spread over the soil surface or within the leaf-litter layer, allowing them to grow laterally to new areas. This means that in the taiga, there may be large patches of vegetation that are in fact made up of a single plant.
1.4 Changes and disturbances
The taiga is uniform in space but not in time. Its appearance changes greatly with the passing of the seasons. Fire and other disturbances also introduce important changes into the boreal landscape.
The seasonal cycle
In winter, the soils are usually frozen solid, meaning that the plants have to withstand these conditions and remain inactive, or freeze solid or die of exposure or desiccation. The thick cuticles of the conifer needles have a small surface area and lose little water during the winter, and so most conifers retain their leaves through the winter, unlike the broad-leaved trees and larches. The trees with tough, leathery leaves shed their leaves as one strategy to conserve water, but also toughen their tissues to prevent tissue breakdown by concentrating so much sugar in the sap that it acts as an antifreeze. Every-thing is, of course, totally covered by snow.
In spring, the conifers usually shed the leaves that have died during the winter, and the shoots that have already formed within the vegetative buds start developing until the leaves open and the bud bursts. Spruces, firs, and American and Eurasian pines (Pinus resinosa, P. sylvestris) are typically dark green in early spring, while the jack pine (P. banksiana) is redder because it has more dead needles, although once the new leaves have sprouted the shoots are light green. This dark green contrasts strongly with the pale green of the new needles produced on the deciduous larches. This is also the time when the xeromorphic species of the understory start their growth, because the shade increasingly limits their growth as the summer advances. The female cones of the conifers, produced in the previous year, are pollinated in the spring, and the shoot primordia (the first embryonic parts) start to produce new lateral buds, which will be the following spring's branches and cones. On rocky outcrops, in the clearings in the boreal forest, and in recently burned sites, the summer shrub cover provides a variety of fruits.
As the mosses grow and the ericaceous shrubs produce their leaves and flowers, even the sphagnum bogs cease to be so dark and gloomy, turning reddish, yellow-green, or brownish. The banks of the lakes and wetlands become sites of great activity, as ducks, geese, herons, pelicans, and other migratory birds all arrive to feed and breed. This is the time when the muskrat (Ondatra zibethicus) collects bulrushes to build or extend its burrows in the middle of marshes, and the beaver (Castor) fells the trees that it will later feed on or use to build dams (see p. 326).
In the autumn, millions of water birds stop by the taiga on their migration south from the tundra and the subarctic. This is when the beavers are especially active in felling trees, dragging the branches along muddy paths to their canals, where they submerge them and bury them in the mud upstream from their dams, as food reserves for the approaching winter.
The broad-leaved trees prepare for the winter by finishing their bud formation, toughening their tissues for winter, and shedding their leaves. In autumn, the conifers also finish preparing their buds for the following year, and many shed their seeds. Pines are unusual in this respect, as the cones pollinated in one year do not produce their seeds until the following year.
Fire in the taiga
In regions with a continental climate, forests fires are usually ground fires, in the understory. Ground fires are one of the natural factors that most influence the taiga's appearance. In these continental sites, little snow falls in winter and most of the precipitation falls as water in the summer. And so in May and June, after the snow has melted and before the rains have started, conditions favor ground fires. The slow decomposition of organic materials typical of taiga forests also favors these fires, and in spring, when the understory is extremely dry, a single spark can start a fire that spreads in all directions.
Fires in the taiga forest can affect immense areas. In 1915, a catastrophic fire in western and central Siberia burned an area roughly five times the size of the Iberian Peninsula. In 1915, precipitation was only about one-third of normal, and the fire, which started in May, did not burn itself out till August. It burned an area from the Sayanskiy Khrebet to the lower stretches of the Yenisey River and from the Ob River to the source of the Lower Tunguska. Dense smoke spread for dozens of kilometers. River transport had to be suspended and many regions could not receive vital necessities. Domesticated livestock could not eat ash-covered grass, and the herds started to die. The hot air and smoke pushed the clouds away from the sites of fires and totally halted the condensation of water vapor. Forest fires are as frequent in North America as in Siberia. In the large area of taiga in North America, there is probably not a single area where there has not been a forest fire in the last 500 years.
Crown fires (which affect the tree canopy) are equally destructive for all the different species of taiga trees, but the effect of ground fires depends on the species. In the light taiga, conifers such as larches and pines have thick bark and deep roots, and can withstand fires in the understory relatively well. The jack pine (Pinus banksiana) retains its seeds within the pine cone for many years, until a fire, when the pine cone explodes, releasing the seeds for germination. In the dark coniferous taiga, on the other hand, fire causes much more damage to the firs and spruces, which have thin bark and surface roots, and die after weak ground fires. Larches can tolerate understory fires well, and also tolerate low temperatures and frozen soil conditions better than firs and spruces, and this is why there are large areas of light coniferous taiga in eastern Siberia, where the climate is very continental.
Succession and maturity
One of the best examples of secondary succession is the recovery of the taiga after a fire. This often includes an intermediate stage of deciduous forest with light-demanding species, such as birches (Betula) and the trembling aspen (Populus tremula) in Eurasia and quaking aspen (P. tremuloides) in North America, which are as cold-resistant as conifers and grow faster. Once a gap has formed in the taiga forest, it fills with deciduous species within a few years. Sites destroyed by fire are also colonized by deciduous species, but these clearings tend to be much larger. Within a relatively short time, the birches and aspens reach their full height; meanwhile, the shade-resistant conifers, which cannot compete with the deciduous trees in the earliest stages of succession, grow beneath the deciduous canopy.
Yet once the birches and poplars have formed a dense canopy, deciduous saplings stop growing because the light is insufficient, and the conifer seedlings are at a distinct advantage. After a few decades, when the conifers grow taller than the birches and poplars, they start to overshadow them. The light-demanding shrubs and herbaceous plants that have grown in gaps of the deciduous forest are replaced by shrubs and herbaceous species typical of the taiga.
Within the taiga zone, conifer forests are considered primary, or mature, while deciduous forests are considered secondary. If conditions are not extreme (i.e. the soils are not too wet or too dry), firs and spruces outcompete larches and pines. Thus, the light taiga can also be considered a secondary forest. When ground fires occur with some degree of regularity, the dark taiga does not have time to displace the larches and pines, and domination by the light taiga can continue indefinitely.
The invertebrates of the taiga are not restricted to decomposing dead organic matter. Phytophagous (plant-eating) insects, for example, may occasionally cause severe damage in boreal forests. The caterpillars of many phytophagous lepidopterans, such as the Siberian pine moth (Dendrolimus sibiricus) and the pine-tree lappet (D. pini, Lasiocampidae), as well as the green Scotch pine caterpillar or cankerworm (Buparius piniara, Geometridae), eat the leaves of conifers, and the caterpillars of some leafroller moths eat the buds on the young shoots. Significant damage may also be caused by some groups of hymenopterans whose larvae develop within the tissues of the tree, such as sawflies (Tenthredinidae) and horntails (Siricidae), or by the chrysomelid larvae of leaf beetles (family Chrysomelidae).
What human beings may perceive as damaging, however, is not necessarily harmful to the ecosystem. It has long been known that in boreal forests (and especially in pure stands), outbreaks of defoliating insects seem to show periodicity. The populations of the bordered white moth caterpillar (Bupalus piniara) may vary by five orders of magnitude over a period of four to 10 years, increasing from around 1 individual per sq yd (or sq m) to 8,361 per sq yd (10,000/m2). Yet, as was shown by studies of the North American spruce budworm (Choristoneura fumiferana, Tortricidae), these outbreaks are ecosystem-level phenomena, as the defoliating insects, their parasites and predators, and the trees affected (mainly the white spruce, Picea glauca, and the balsam fir, Abies balsamea) are all evolving together.
As the tree biomass increases, the largest and oldest trees become more susceptible to the action of spruce budworm larvae, and many of them die as a result of this regular defoliation. The decomposition of dead wood and insect droppings and remains all return nutrients to the forest soil. Young trees are more resistant to insect attack after the cover is removed, and can grow rapidly and reach the upper layer of the canopy within a few years. At the same time parasites and predators of the budworms soon reduce their numbers. If boreal forests are observed over long periods of time, it is clear that budworms act as a factor ensuring regular stand rejuvenation in taiga communities. Budworm attack is a natural part of the ecosystem, and by no means the catastrophe one might think when looking at the dead and injured trees at the peak of the outbreaks.
Another major group of forest pests are the larvae of the cerambycid longhorn beetles (family Cerambycidae), a group that includes pine borers, longicorn beetles, sawyers, and longhorn beetles. Their caterpillars, such as the sawyers (Monochamus sutor, M. galloprovincialis, and Tetropium), cause huge losses to the forestry industry, as they attack living trees as well as feeding on dead ones. Bark beetles (family Scolytidae) can also cause severe damage, for example causing lodging of young stands in mountain forests of North America. But in this case the bark beetles also contribute to increasing the forest's productivity by decreasing its density, as budworms do. This is why some forestry experts even recommend provoking regular and controlled increases in the population of bark beetles in some types of forest.
2. The plant life.
2.1 The same trees everywhere
Travelling a few kilometers through a tropical rainforest, one may see dozens of species of tree, and hundreds, or even thousands, of other species of plant. The taiga, in comparison, contains many immense areas, hundreds or even thousands of square kilometers, covered by remarkably uniform, monotonous forests. A single tree species may be totally dominant, while the herbaceous and shrub layer may be totally dominated by one or two species. Excluding introduced species, there are fewer than 2,000 species of vascular plants in the Eurasian taiga.
Low floristic diversity
This low floristic diversity is largely due to the harsh climate. Few plants can grow normally in sites where the winter may last for nine months and where soils are very acidic and nutrient-poor. There is not a single family of flowering plants that is endemic to the taiga, and there are only a few endemic genera. Only a few relatively large families or subfamilies and some genera are represented in the taiga by a relatively high number of species.
The families that are abundant include the Betulaceae, Salicaceae, Pyrolaceae, Juncaceae, and the Ericaceae. The genera with many species that grow in the taiga include sedges (Carex, Cyperaceae), cotton grass (Eriophorum, Cyperaceae), Calamagrostis and Elymus (Poaceae), Potentilla, Rosa, and Rubus (Rosaceae), Saxifraga (Saxifragaceae), and a few more. The wood sorrel family (Oxalidaceae) has about 900 species, most of them tropical, but only a single species, common wood sorrel (Oxalis acetosella), grows in the taiga. The birthwort family (Aristolochiaceae) contains more than 400 species, most of them tropical and subtropical, but only has a single species that grows in the southern taiga, asarabacca (Asarum europaeum). The largest family of flowering plants, the orchids, has more than 30,000 species, but fewer than 50 species of orchid occur in the taiga, and most of them are rare.
About a dozen species of tree, most of them evergreen needle-leaved conifers, dominate the entire taiga biome, largely because of the harsh growing conditions. The conifers can be divided into two types: dark conifers, such as spruces (Picea), firs (Abies), and the hemlock spruce (Tsuga) and Douglas fir (Pseudotsuga spp.) in North America; and light conifers, such as larches (Larix) and pines (Pinus). Surprisingly, some broad-leaved deciduous trees appear to thrive in these conditions, such as the birches (Betula) and poplars and aspens (Populus), especially in the southern subhumid taiga; they are, however, often only present in the initial stages of succession or in wet plains, and are not present in the mature forests.
The species of these genera are not always clearly defined. Some specialists consider that the European taiga contains dozens of species of white birch (Betula pendula), but only a real expert can tell them apart. The taxonomy of larch species is also tricky. Some specialists consider that the Far Eastern taiga contains more than ten species of larch, but others only recognize four or five species, and argue that they are difficult to classify because they hybridize with each other. Other specialists consider that all the different larches in the Far Eastern taiga are just forms of a single, highly polymorphic species. Even in Europe, where the flora has been studied for centuries, botanists have trouble distinguishing the Norway spruce (Picea abies) from the Siberian spruce (P. obovata). Throughout almost the entire taiga area of European Russia there are spruces of intermediate appearance that some authors consider a single species of hybrid origin, the Finnish spruce.
The dark conifers: spruces and first
The typical forest cover of the European taiga is dark conifer forests dominated by the Norway spruce (Picea abies). In the northeast of European Russia, it is replaced by the Siberian spruce (P. obovata), a related species that covers an immense area crossing northern Siberia to the Pacific coastline. In some areas of Japan and the Korean Peninsula, another related species, the Koyama spruce (P. koyamai), also occupies some small areas. The yeddo spruce (P. jezoensis) has flat needles. It forms forests in the coastal regions of the Russian Far East, the Korean Peninsula, and the northern islands of Japan.
All Eurasian spruces are very similar in their habits. Like other dark conifers, spruces do not shed their needles in autumn, and so in winter their water loss by evaporation and transpiration is considerably greater than that of deciduous trees. This constant need for water is the reason why the dark conifers cannot tolerate dry soils, low air humidity, or extreme frosts. Spruces are highly shade-tolerant and slow-growing, occurring mainly on clay and loamy soils. The deep shade of the high canopy means spruce forests are always dark and cool. Decomposition of plant remains is very slow, and so the forest floor is covered with dead branches and trunks. This all makes old-growth spruce forests look like something out of a fairy tale. For a long time Nordic folklore populated these spruce forests with wood-goblins, gnomes, trolls, and all sorts of evil spirits. The spruce forest is at its most attractive in winter, when the white snow contrasts with the dark green foliage.
The plains of the North American taiga contain two further species of spruce, the black spruce (Picea mariana, also known as the bog spruce) and the white spruce (P. glauca), which grow from Alaska in the west to the Labrador in the east, and as far south as the northern Rocky Mountains, the Great Lakes, and New England. The white spruce is a handsome tree, with a strong trunk up to 150 ft (45 m) tall, large long branches, and a lanceolate (narrow and tapering), light bluish green crown. The elegance of the white spruce is in stark contrast to the unattractive black spruce, which is not very tall (rarely exceeding 33-39 ft [10-12 m]) and is badly proportioned, with a slender, crooked trunk, the upper part of which is molded by the wind into a broomlike tuft. In ecological terms, the white spruce is similar to the Eurasian species, preferring relatively moist but not waterlogged habitats with relatively rich soils. The black spruce, in contrast, is often found in soils on top of permanent ice (permafrost), in peat bogs, and even on floating islets of peat, where they manage to grow without any mineral soil at all.
Habitats like this are typical of the Mackenzie Basin and other regions of the north and northwest of North America, where an immense area is covered by dismal forests of black spruce. These conditions are not the most favorable for the black spruce, and it grows there because of its poor competitive ability: in the well-drained rich soils of central Saskatchewan, where it is not in competition with other species, the black spruce can reach a height of 100 ft (30 m), and the stands are very different in appearance from those in the harsh climate of northern Canada. These two species of spruce also vary greatly in their life span, the white spruce living up to 600 years, while the black spruce rarely exceeds 350 years. Forests of white spruce are more vulnerable to disturbance (including anthropogenic disturbance) than forests of black spruce.
Firs (Abies) are the other genus of dark conifers with a widespread distribution. The firs did not advance far to the north, where they gave way to spruces, pines, and larches. The most common fir in Europe is the silver fir (Abies alba), mainly found in the mountain regions of central and southern Europe, from the Pyrenees to the Carpathians, but they do not grow into the taiga zone of northern Europe. In Europe, the only fir that is really typical of the taiga biome is the Siberian fir (A. sibirica), which lives in the northeastern tip of Europe, in the foothills of the Urals; its range roughly coincides with that of the Siberian spruce (Picea obovata). Both species show great ecological similarities, and often form mixed spruce-fir forests. Similar mixed forests in the Far East consist of yeddo spruce (P. jezoensis) and the Siberian white fir (Abies nephrolepsis). Firs have a narrower range of ecological tolerances than spruces, and despite tolerating shade better than spruces, firs are less resistant to cold and temperature fluctuations, and need more moisture and more fertile soils. This is why spruces grow at higher elevations on mountains than firs, and why they grow further north.
Some Mexican and Mesoamerican fir forests at high elevations can be considered as the southernmost boreal forests. In the center of Mexico patches of the oyamel fir (Abies religiosa) grow at an altitude of 13,123 ft (4,000 m) at 20[degrees]N, and A. guatamalensis grows even further south. Most of the North American firs, like the Eurasian firs, are present in mountainous regions. The North American taiga zone is characterized by the presence of the balsam fir (A. balsamea), a relatively small tree that is famous for its oleoresin ("Canada balsam"), which is widely used to mount medical and biological specimens on microscope slides. In the boreal forests of North America, other species of dark conifer are known as firs, the most important of which is the Douglas fir (Pseudotsuga menziesii), one of the world's largest trees--it can reach a height of 330 ft (100 m) and a diameter of 16 ft (5 m) at the base of the trunk. It is mainly restricted to the Rocky Mountains and is rarely found in the lowland boreal forests. It is one of the main species in all the temperate forests of the North American Pacific coastline (see vol. 6).
The light conifers: pines and larches
The most common light conifer of the European taiga is the Scotch pine (Pinus sylvestris). No other forest-forming conifer has such a large range, from Scotland to the Pacific Ocean and from northern Norway in the north (at 70[degrees]N) to the mountain areas of the Iberian Peninsula and Asia Minor in the south. The environmental tolerances of the Scotch pine are much broader than those of the dark conifers, and it occupies a wider range of habitats throughout its range, in both the plains and mountains of Europe. The environmental optimums for pines coincide almost completely with those for spruces, but if spruces grow together with pines, the spruces outcompete the pines, and if there is no external disturbance, the spruce will eventually displace the pine. Their broad ecological tolerances allow pines to grow on sites where spruces cannot grow, or where spruces cannot compete with pines. In the European taiga, pines outcompete spruces on the poor, dry, and often sandy soils, where the pine forests have a ground cover of the lichen Cladonia; in bogs, where pines can survive despite the harsh environmental conditions; and in sites where there are frequent fires in the understory.
The range of the Siberian pine (Pinus sibirica), usually considered a dark conifer, runs from the Dvina Basin (in European Russia) to the basin of the Lena River (in eastern Siberia), as far north as the Arctic Circle, and as far south as northern Mongolia. When the first Russian explorers traveled east in the fifteenth century they discovered this conifer with soft needles and aromatic wood. As it did not resemble the pines they were familiar with, they thought it must be a cedar, a tree they had never seen but had read of in the Bible, and since then it has been known as "cedar" in Russian. It is usually classified within the subgenus Cembra, together with the Korean pine (Pinus koraiensis), which grows in the Far East, and the Swiss stone pine (P. cembra). All three species share similar habits and bear their leaves in clusters of five, rather than in clusters of two, like most other pines. The Siberian pine and the Korean pine are tall (up to 140 ft [43 m]), with a very dense conical crown. The Siberian pine rarely grows in pure stands, but is frequent in the mixed forests of conifers of the central taiga, while the Korean pine is one of the dominant trees in the mixed conifer and broad-leaved deciduous forests of eastern Asia (see vol. 7, pp. 120-121).
The dwarf Siberian pine (Pinus pumila) also belongs to the subgenus Cembra. It is a shrub 13-16 ft (4-5 m) tall that forms dense thickets at the timberline in the mountains of eastern Siberia and the Far East. Its timber anatomy provides the dwarf Siberian pine with a unique manner of trapping snow at the onset of frost. The snow thus protects it, allowing it to spread further to the north and higher up mountains. In the altitudinal belt of dwarf Siberian pine in the mountains of northeast Asia, the snow layer may be very thick, partly because the trunks and branches form a sort of structure that traps most of the snow and prevents it being blown away. The seeds of the dwarf pine are edible, but are much smaller than those of the Siberian pine, and are mainly eaten by birds, bears, and burunduk chipmunks.
In North America there are two main species of pine that deserve mention: the jack pine (Pinus banksiana) and the ponderosa pine (P. ponderosa). The jack pine is typical of the southern plain taiga, and in the north it mainly colonizes sandy soils, where it forms pine forests with a lichen ground cover, like those of the Scotch pine in Eurasia. Regular fires favor its spread. The ponderosa pine is not strictly a taiga species, and is more typical of the pine forests of the Rocky Mountains, of which it has become a symbol. Frequently used as a backdrop in western films and television series, it has become familiar to people all over the world. It grows on slopes facing in any direction at altitudes from sea level in Washington State to 9,843 ft (3,000 m) in Arizona.
The larches (Larix) are the last group of light conifers typical of the taiga. The larches follow a seasonal pattern of growth that includes shedding their needles in the winter, unlike all the other members of the pine family (Pinaceae). The ecology of larches is similar to that of pines, and the interactions between larches and dark conifers are the same as those between pines and dark conifers. Their main difference from pines is that larches are extremely cold-resistant, and they can grow further north than any other tree species. The range of larches in the Eurasian taiga is similar to that of the Siberian spruce (Picea obovata) and the Siberian fir (Abies sibirica), stretching from the north of European Russia to the Pacific Ocean. Over the Eurasian taiga zone, the genus Larix is represented by several closely related species that hybridize very easily with each other. From west to east, the main species of larch are: the Siberian larch (Larix sibirica [=L. russica]) in northern Europe; the Dahurian larch or Gmelin larch (L. gmelinii), which grows throughout central Siberia, northern Mongolia, and northeast China; and the Kamchatka larch (L. kamtchatica), which grows on the Kamchatka Peninsula, Sakhalin Island, the Kuril Islands, etc. In Russia, larches are the most widespread genus of forest trees; almost half of the Russian taiga, approximately 642 million acres (260 million ha), corresponds to larch forests.
In North America, the most widespread larch is the tamarack (Larix laricina), whose range includes almost all of Canada and part of Alaska. The western larch (L. occidentalis) can grow to a height of 260 ft [80 m], and mainly grows to the north of the Rocky Mountains, where it forms large, highly productive forests that are maintained by regular ground fires.
The boreal deciduous trees: birches, aspens, and poplars
There are also some broad-leaved trees in the taiga, either as minor components in the conifer forests or as pure stands in the early stages of succession after a fire, such as the European white birch (Betula pendula) and trembling aspen (Populus tremula) in Eurasia, and the paper birch (B. papyrifera) and quaking aspen (Populus tremuloides) in North America. The Siberian poplar (P. suaveolens) in Eurasia and the balsam poplar or tacamahac (P. balsamifera) in North America are very typical of riverbanks and flood plains, where they form almost pure stands in the first stages of growth on alluvial deposits, but are later displaced by conifers.
2.2 The sparse acidophilic flora of the understory
The understory of the taiga supports a low diversity of vascular plants. The diversity of species in this biome is directly related to the climate, as the further north and the further inland, the lower the diversity. However, the number of species of herbaceous species in the boreal forests is considerably higher than the number of woody species. Most taiga forests grow on acidic rocks, and so calcicolous plants are almost absent or are highly localized. The herbaceous plants, bushes, and shrubs typical of the taiga may have a very extensive range. Some of these species are present throughout the boreal forest zone in Eurasia and North America and have a circumpolar distribution, including the gray alder (Alnus incana), rose (Rosa acicularis, Rosaceae), raspberry (Rubus idaeus, Rosaceae), cowberry (Vaccinium vitis-idaea, Rosaceae), blueberry (V. uliginosum), crowberry (Empetrum nigrum, Empetraceae), twinflower (Linnaea borealis, Caprifoliaceae), one-flowered shinleaf (Moneses uniflora, Pyrolaceae), dwarf rattlesnake plantain (Goodyera repens, Orchidaceae), club mosses such as the stiff club moss (Lycopodium annotinum, Lycopodiaceae), and horsetails such as Equisetum sylvaticum (Equisetaceae).
Small trees and large shrubs
In the boreal forests, junipers are the most common shrubby conifers. In the European taiga, junipers are represented by the common juniper (Juniperus communis), which also grows in North America. In Siberia, they are represented by the Siberian juniper (J. sibirica), which is very similar to the common juniper. In the far east of Russia, the junipers are represented by a series of less well-known species. Junipers may sometimes grow upright to a height of 16 to 20 ft (5-6 m), but they are normally shrubby or even prostrate or creeping, and occur high up on mountains. The juniper is light-demanding and is most typical of light conifer forests. When spruces appear in a pine or larch forest and then spread, the junipers cannot get enough light and die.
Though they are usually shrubs, some alders may also show an arborescent growth form, most frequently in the white alder (Alnus incana) and A. fruticosa. In Alaska, where glaciers are retreating, thickets of gray alder (A. crispa) are one of the first stages of succession, and are followed later by the Sitka spruce (Picea sitchensis). In addition to the alders, there may be other arborescent species below the canopy, such as the dwarf Siberian pine (Pinus pumila) in eastern Siberia and in the Far East. In North America and at the southern edge of the taiga in the Far East, in the lower stories of the boreal forests, an important role is also played by some shrubby species of maple (Acer), such as A. ukurunduense in Asia and mountain maple (A. spicatum) in North America.
In the American taiga, at the edges of the forest and in patches in the early stages of succession, there are some large shrubs and short trees, such as the wild red cherry (Prunus pennsylvanica), the Virginia cherry (P. virginiana), the Saskatoon serviceberry (Amelanchier alnifolia) and other species of the same genus, the Canadian cherry (P. nigra), and the beaked hazel (Corylus cornuta). The red-osier dogwood (Cornus stolonifera) is very frequent in the Canadian taiga in open forests, while poison ivy (Rhus radicans) is present in the patches of open forest where soils are not too acidic, and in mixed forests. This plant is poisonous when touched, as it produces urushiol, a phenolic compound used in lacquer that produces severe inflammation of the skin in most people, raising large blisters.
The small shrubs and clumps
In the lowest layers of the boreal forests there are many species of wild rose (Rosa, the most widespread being R. acicularis), blackberries and raspberries (Rubus), currants (Ribes, especially R. triste, from eastern Siberia and North America), small stone fruits such as bird cherry (P. padus), honeysuckles (Lonicera edulis, L. dioica, etc.), willows (Salix), and some other shrubs.
The most characteristic shrubs of the taiga zone are the evergreen cowberry (Vaccinium vitis-idaea) and the deciduous bilberry (V. myrtillus). Both species occur in Eurasia, and the cowberry also occurs in the North American taiga, where the bilberry is replaced by its closest relative, the sourtop blueberry (V. myrtilloides). In the European taiga, the cowberry usually grows on dry, loamy sand soils, while the bilberry prefers wetter loamy soils.
The bilberry is more cold-sensitive than the cowberry, and so in the European taiga it can grow where there is a lot of snow. The lower cold resistance of the bilberry explains why it is not abundant in eastern Siberia, where the climate is extremely continental and the winters are extremely cold, though with little snow, and where the herbaceous and subshrub layers are totally dominated by the cowberry. Peat bogs and open wet forests in both Eurasia and North America may contain the small cranberry (V. oxycoccos), the American cranberry (V. macrocarpon) in North America, and the lowbush blueberry (V. angustifolium) in some not very wet forests in Canada.
The boreal forests include many species of the genus Ledum (Ericaceae). In the European taiga they usually occur in bogs, although in Siberia they are particularly frequent in larch forests on relatively dry soils. In the Eurasian taiga, the most common species is the wild rosemary (L. palustre), while in North America it is Labrador tea (L. groenlandicum). Ling (Calluna vulgaris, Ericaceae) is very typical of the pine forests of northern Europe. Ling avoids competition with other plants by growing on very poor, acidic soils. In these conditions, it is very important for the plant to reduce water loss from its leaves as much as possible. The leaves of ling are rolled up like a tube, reducing the air flow over the stomata and thus evaporation.
Open dry sites with acidic soils support other members of the heath family (Ericaceae), such as the bearberry (Arctostaphylos uva-ursi) and different species of wintergreen (Gaultheria), especially the checkerberry (G. procumbens) in the American taiga. The twinflower (Linnaea borealis, Caprifoli-aceae) is another widespread Holarctic plant of the northern taiga forests. The bunchberry or dwarf cornel (Cornus [=Chamaepericlymenum] canadensis, Cornaceae) and the sand cherry (Prunus pumila, Rosaceae) are only present in the North American taiga.
The herbaceous plants
The herbaceous evergreens of the shinleaf family (Pyrolaceae)--Pyrola rotundifolia, P. asarifolia, Chimaphila umbellata, Orthilia secunda, O. obtusata and Moneses uniflora--play an important role in taiga ecosystems. The common wood sorrel (Oxalis acetosella) and May lily (Maianthemum, Liliaceae) are also very common. Studies of these species have shown that deep shade and other characteristics of the boreal forests, especially the dark coniferous taiga, has led them to adopt similar adaptations in their habits and morphology. Among these species vegetative propagation is at least as important as dispersal by seeds, and cross-pollination has in many cases been replaced by self-pollination, although they still continue to produce the conspicuous flowers typical of insect-pollinated plants, and look attractive to insects.
The common wood sorrel (Oxalis acetosella) is the most widespread herbaceous plant in the southern and central taiga, especially in the dark conifer forests of Eurasia and North America. Yet it cannot really be considered a boreal plant, as it grows best in relatively warm mixed coniferous and deciduous forests rather than in the cold taiga. The range of the starflower (Trientalis europaea, Primulaceae) includes the entire Eurasian taiga, while the American starflower (T. borealis) occurs in North America. Unlike the common wood sorrel, this plant never becomes dominant in the herbaceous and subshrub layer.
The boreal May lily (Maianthemum, Liliaceae), like the wood sorrel, is more common in the southern taiga and mixed conifer and broad-leaved forests. Its area of distribution includes all the Eurasian taiga, while a closely related species, the wild lily of the valley (M. canadensis), grows in North America. Northern May lilies can tolerate poor soils and are very shade-resistant. They usually spread vegetatively by rhizomes, and though they produce pleasantly scented flowers to attract insects, they seem to be predominantly self-pollinating.
The herbaceous and subshrub layer of the taiga contains many vascular cryptogams (plants that reproduce via spores). The most important are the ferns, such as bracken (Pteridium aquilinum), lady fern (Athyrium filix-femina), male fern (Dryopteris filix-mas), and many more (D. austriaca, Gymnocarpium dryopteris, G. jessoense, Diplazium sibiricum, etc.), whose broad dissected leaves are well adapted to the dense shade of the taiga forest. The boreal forests also contain many horsetails (Equisetum), such as E. pratense, E. arvense, and E. sylvaticum. The usual inhabitants of the taiga forest include several club mosses, such as Lycopodium annotinum, L. clavatum, and L. obscurum. The ranges of most of these cryptogams are very large, and they also tend to propagate vegetatively.
2.3 The rich flora of mosses and lichens
Both the climate and the geomorphology of the taiga stimulate direct competition for resources between the trees and the bryophytes (mosses) in regions where decomposition is slow. The bryophytes are so successful that huge areas of wet and waterlogged taiga are covered in quaking bogs and peat bogs dominated by mosses, which sequester enormous quantities of carbon. It appears that the dynamics of the wet boreal forest largely depend on the relative success of trees and mosses in dominating the local water balance in the soil.
In fact, the trees in the taiga often reflect the success of the mosses, in a way unlike any other forest. Sphagnum moss can thus be considered to be totally dominant in sphagnum bogs. When walking through the boreal forest, the success of lichens and mosses is much clearer if one does not restrict oneself to the ground cover of terricolous (ground-dwelling) species, but also observes the abundance of epiphytic species growing on the trunks or hanging from the dead or dying branches.
Mosses and other bryophytes
Drepanoclados uncinatus is a hydrophytic (water-adapted) moss that is almost ubiquitous, as it can grow in lowland taiga and subalpine forests as well as in bogs. It can also grow on the bark of living trees, on rotten wood, on the soil surface, on stones, and on rocks. Other epigeal (low-growing) mosses, such as Hylocomium splendens, Pleurozium schreberi, Ptilium crista-castrensis, Rhytidiadelphus triquetrus, Dicranum scoparium, and many more--often species with a circumpolar distribution--form moss carpets, and though they grow mainly on the soil, like Drepanoclados uncinatus, they also occur on a wide range of substrates.
Many other mosses, however, only grow on a particular substrate. The mosses of the family Splachnaceae only grow in the presence of breakdown products of rotting plants and animal remains. Some species, such as those of the genus Splachnum, only grow on the dung of herbivores, while others, such as Tetraplodon spp., only grow on the dung of predators and on the corpses of small animals. Some boreal mosses that grow on rocks (epilithic species) are calcicolous (preferring calcium-rich soils), such as Timmia megapolitana and Seligeria diversifoliata. However, most of the epilithic species of their family, the Grimmiaceae, are obligate acidophiles, i.e., limited to acidic soils. In general, most taiga mosses are acidophilous or can at least tolerate acidic substrates. Epiphytic mosses are often closely associated with a specific habitat. For example, Neckera pennata is a widespread epiphyte of spruces, climbing up the trunk as cylindrical ribbons to the crown.
There is a community of mosses specific to each stage of wood decomposition. In the Far Eastern taiga, the first mosses to grow on a fallen tree are the epiphytic mosses, which were growing on the tree when it was still alive. They are replaced by mosses characteristic of the base of the trunks, such as Homalia trichomanoides; the epiphytic species disappear in the middle and late stages of decomposition of wood, but the species characteristic of the base of the trunk continue growing. Then the mosses typical of rotten wood start to appear, such as Oncophorus wahlenbergii, Eurhynchium pulchellum, Dicranum congestum, and Tetraphis pellucida. Finally, when the wood is completely rotten, species typical of the soil arrive, such as Rhytidiadelphus triquetrus. In Canada, decomposing trunks on the forest floor are an ideal habitat for Brachythecium salebrosum and Eurhynchium pulchellum, which also forms thin carpets around tree stumps, and for lichens of the genus Cladonia. Other carpet-forming epiphytic mosses include Orthotrichum and Pylaisiella polyantha, which grows on quaking aspens in open mixed forests.
Some of the most characteristic mosses of the taiga are the epigeal species Hylocomium splendens, Pleurozium schreberi, Dicranum scoparium, and Ptilium crista-castrensis. They can be found in almost any ecosystem in the Eurasian taiga, from the Atlantic to the Pacific, and in the North American taiga. In the European part of the boreal forests there are two main groups of mosses, those of the northern taiga and those of the central and southern taiga. Thus, in the northern taiga the moss layer of the conifer forests on the interfluvial plains (those between rivers) is dominated by Hylocomium splendens, but this is rare in the southern taiga, where it is usually replaced by Pleurozium schreberi and by the Brachythecium oedipodium and Rhytidiadelphus subpinnatus (plus R. triquetrus and Cirriphyllum piliferum) complex. In the northern taiga, this set of species can only be found in riverside sites in spruce forests. The epiphytic moss flora of the southern taiga necessarily contains Hypnum pallescens, Callicladium haldanium, Platygyrium repens, and Orthodicranum montanum, all of which are absent or very rare in the northern taiga. Dicranum fuscescens and D. fragilifolium are very common on tree stumps and rotten wood in the northern taiga, but are rare in the southern taiga. Some liverworts, such as Blepharostoma trichophyllum, Lophozia incisa, L. ventricosa, and Lepidozia reptans, are rare in the southern taiga but abundant in the northern taiga. The moss flora soil of soil outcrops also shows large differences, consisting in the southern taiga of Atrichum undulatum and Dicranella heteromalla, while in the northern taiga they are replaced by Dicranella subulata, D. crispa, Pohlia cruda, and Distichium capillaceum. P. cruda and D. capillaceum are present in the southern taiga, where they grow on slopes but not in the forests on the interfluvial plains.
In the most continental regions of the boreal forests, in addition to these northern and central southern complexes, there is another associated with larchwoods; there the moss layer consists almost entirely of Rhytidium rugosum, epiphytic mosses are almost completely absent, and the epixylous (growing on wood) moss flora is also very poor and similar to that of the northern taiga. This layer is characterized by a combination of what might be called steppe tundra, containing elements of the steppe (Tortula ruralis) and of the bog tundra (Tomenthyopnum nitens, Aulacomium palustre, A. turgidum, Dicranum elongatum), which are absent from the moss cover in the northern taiga.
In the North American taiga, the soils of a typical closed spruce forest also usually contain a carpet of plumose (feathered) mosses that grow fast enough to cover the branches, cones, and needles that fall from the trees. In northern Ontario, these carpets are usually dominated by Pleurozium schreberi in the open spaces between the crowns and near the trunks of the trees, while Hylacomium splendens is dominant at the edges of the forest, and Ptilium crista-castrensis is present on both the trunks and at the edges of the forests. Other frequent species include Polytrichum commune, club mosses (Lycopodium), and lichens such as Peltigera aphthosa, Stereocaulon tomentosum, and, in open sites, Cladonia.
Some mosses, especially the sphagnum mosses and some species of Aulacomnium, Drepanocladus, etc., play a vital role in the formation of oligotrophic (nutrient-poor) or mesotrophic (having moderate levels of nutrients) bogs. In minerotrophic bog-bogs with a high pH but almost no nitrogen, and thus not strictly eutrophic--grow mosses like Tomenthypnum nitens, Paludella squarrosa, Meesia triqueta, etc.
Sometimes the moss layer contains lichens, which almost never play a major role. Lichens, like mosses, show poikilohydry, becoming dormant in the dry season after losing most of the water content of the cytoplasm without being damaged. They renew growth as soon as they are wetted. The sphagnum mosses and other nonpoikilohydrous mosses need a continuous supply of water, and this is why they grow best in very humid sites. On the other hand, although species able to withstand drying out are not at an advantage where water is abundant, they are favored in sites where wet and dry conditions alternate, such as rocky outcrops or dry exposed mineral soils. They can also grow as epiphytes on tree trunks or branches.
The rocky outcrops of the Canadian Shield are one of the distinctive characteristics of the taiga, a landscape feature distinguished by its abundant crustose (forming a thin, flat crust strongly attached to the surface on which it is growing), foliose (leafy), and fruticose lichens and mosses. Bare rocky outcrops are inhospitable sites until crustose lichens, such as Dimelaena oreina and D. carpon, or even foliose species such as Arctoparmelia centrifuga, colonize them, creating a thin organic layer that retains water. This layer accumulates because the acidity of the granite rock and the acidophilous nature of the lichen combine to slow down the decomposition of the dead plant matter.
Crustose lichens tend to be replaced by foliose lichens, such as the rock tripe Umbilicaria dilenii, and even mosses, and soon the underlying organic substrate gets thicker and thicker. Fruticose lichens, which prefer deeper organic layers with a better water supply, then start to appear. Soil formation is slow, and may be interrupted by fire or, as happens near some metalworks, by rain and snow containing high levels of sulfur dioxide, which is very toxic to mosses and lichens. As trees start to colonize these incipient soils, a carpet of fruticose lichens forms, including species of Cladonia such as reindeer moss (the lichen Cladonia mitis, C. rangiferina, and C. stellaris), and further north Cetraria cucullata, C. nivalis, and "Iceland moss" (the lichen C. islandica) appear. The covering of thick folisols usually includes mosses like Hedwigia ciliata, Schistidium apocarpum, and Orthotrichum, as well as herbaceous plants, junipers (Juniperus communis), and even the occasional pine.
The habitats where lichens really thrive are on bark and fallen wood. Some authors believe that epiphytic lichens protect the trees they grow on, for they are at less risk from wood-rotting fungi because the lichens produce a series of specific acids, such as usnic acid and pulvinic acid, which show antibiotic activity against wood-destroying fungi. Lichens like Parmelia sulcata and Hypogymnia physodes grow very well on conifer trunks and they may totally cover dead or dying branches. In the western Canadian taiga, for example, the beard lichens Bryoria (especially B. fremontii and B. tortuosa) usually grow hanging from the dead branches of conifers that are already covered with foliose species that are light in color.
2.4 The different taiga landscapes
There are many different approaches to classifying vegetation in general, including boreal forests. Using the Sigmatist phytosociological method (describing associations on the basis of species that are considered dominant) or describing them on the basis of the dominant species in each layer give very similar results, showing the presence of two main types of plant community: the conifer forests and the boreal bogs.
The dark taiga
At the northern limits of the northern taiga of North America, the typical vegetation is a low, open scrub with lichens or an open forest of spruces. They are open forests dominated by white spruce (Picea glauca), black spruce (P. mariana), and tamarack (Larix laricina), with a layer of lichens and mosses on the soil surface and often a low shrub layer of, for example, Labrador tea (Ledum groenlandicum). The dominant trees are the same but these forests are very different from the northern conifer forest (which it borders to the south), because the better-drained soils are dominated by a thin carpet of reindeer moss (Cladonia) and the spruces do not form a closed canopy but grow well separated from each other. The cover is denser near the edges of the watercourses, while few trees grow in the large waterlogged spaces, except for a few isolated larches. On well-drained sites, the white spruce tends to be dominant in the less acid areas, while the black spruce is dominant in the more acid areas of the Canadian Shield. The jack pine (Pinus banksiana) only competes successfully with spruces on shallow organic soils on rocky outcrops, or on sandy moraine soils and alluvial deposits, while the quaking aspen (Populus tremuloides) and the paper birch (Betula papyrifera) are present along the rivers, where water and nutrients are available. These trees are very important for beavers, because their wood provides the material for the beavers' dams and lodges, and the bark of the quaking aspen is their main food source in winter.
The forests of northeast Russia and western Siberia, which are dominated by Siberian spruce (Picea obovata) and Siberian larch (Larix sibirica [=L. russica]), are also open dark tundra. Further east, in central and eastern Siberia, larches are the dominant trees, mainly the Siberian larch, the Dahurian larch L. gmelini), and the Kamchatka larch (L. kamtchatica), and cover large areas. All these species may form arii-mas ("patches of trees" in Yakut) far to the north within the tundra. Birches are dominant in the more maritime regions of Scandinavia and northern Asia; the common birch (Betula pendula [=B. verrucosa]) is found in Scandinavia and Karelia, where the Norway spruce (Picea abies) also grows; and the birch Betula ermanii is in northeastern Asia.
Spruces are the dominant trees in the most typical North American dark taiga. From Alaska to Newfoundland, the white spruce (Picea glauca) and black spruce (P. mariana) usually form dense stands with a closed canopy. The variety of soils and climates means that tamarack (Larix laricina), balsam fir (Abies balsamea), paper birch (Betula papyrifera), jack pine (Pinus banksiana), quaking aspen (Populus tremuloides), and balsam poplar (P. balsamifera) are all present and form a significant part of the plant cover. In central and eastern Canada, the closed forests are usually dominated by advanced stages of succession with balsam fir, while in the southern bog regions the American arborvitae (Thuja occidentalis) accompanies the tamarack. The jack pine is also common, but it is not restricted to rocky outcrops and sandy soils, as in the open forest with lichens. Here it is often an early stage of succession in sites that have recently suffered a forest fire, especially in subhumid central Canada. On the better-drained outcrops and sandy soils bordering the mixed forest ecotone to the southeast, the white pine (Pinus strobus) and red pine (P. resinosa) appear. Along the foothills in Alberta and in the Yukon Territory, the dominant species of the closed forest may include some of the species of the subalpine story of the Rocky Mountains, such as lodgepole pine (P. contorta), Engelmann's spruce (Picea engelmannii), Douglas fir (Pseudotsuga menziesii), and alpine fir (Abies lasiocarpa). The nonclimax species include broad-leaved trees such as paper birch, quaking aspen, and balsam poplar. In the subhumid northern conifer forests of Saskatchewan and Alberta, the quaking aspen and balsam poplar, together with the spruces, are such important features of the landscape that the region has become known as "northern mixed wood boreal forest" or "mixed wood." In this area, the paper birch and quaking aspen are also important elements of the gallery forest, and further south they are accompanied by other broad-leaved species, such as oaks, ashes, lindens, and maples, species typical of the mixed forests of southeast Canada and the northeastern United States.
The typical European taiga in Scandinavia and northwest Russia is the characteristic forest of Norway spruce (Picea abies), with many mosses but almost no other tree species, an understory of young spruce, a subshrub layer dominated by bilberry (Vaccinium myrtillus), and a moss layer with Hylocomium splendens, Pleurozium schreberi, and several species of Dicranum, etc. The Norway spruce may be accompanied by the silver birch (Betula pendula) and occasionally by the trembling aspen (Populus tremula). As soils become poorer, some subtypes of vegetation are replaced by others. Rich soils are characterized by a herbaceous layer dominated by common wood sorrel (Oxalis acetosella), while on poor soils the bilberry is replaced in the subshrub layer by the cowberry (Vaccinium vitis-idaea), and reindeer moss (Cladonia) replaces the mosses. If drainage is poor, the soil becomes waterlogged and boggy and the mosses are first replaced by more hydrophilous mosses, mainly from the Polytrichales, such as Polytrichum commune, and then by different species of sphagnum moss. In wet but not boggy habitats, there are spruce forests with a rich herbaceous layer and some broad-leaved trees. The Scotch pine (Pinus sylvestris) plays a similar role in the European and Siberian taiga to that of the jack pine (P. banksiana) in the North American taiga, and it is present in the early stages of succession and is particularly widespread in areas that suffer fires.
The most typical taiga is the Siberian dark taiga, which runs from the northeast of European Russia to the Yenisey River and as far as Lake Baikal in the south of central Siberia. It is characterized by a tree layer dominated by Siberian spruce (Picea obovata), although it is accompanied by other trees in variable proportions, such as the Siberian fir (Abies sibirica), which may become dominant in the best soils in the mountain areas, the Siberian pine (Pinus sibirica), and the Siberian larch (Larix sibirica [=L. russica]). The Siberian pine, like the Scotch pine, mainly grows in the early stages of succession, after clear-cutting or fires. In the true dark taiga the understory is very poor, while the moss layer is very important.
The light taiga
The light taiga is found in the regions of central and eastern Siberia with highly continental climates, and has an almost pure tree layer of the Gmelin larch (Larix gmelinii) with a few scattered specimens of the Siberian spruce (Picea obovata). Unlike the dark taiga forests, the herbaceous, subshrub, and shrub layers of the light taiga forests are very rich.
At the boundary of the taiga with the deciduous forests, in eastern North America, the variable conditions mean that the best sites contain stages of the succession to deciduous forests, with broad-leaved species such as oaks, maples, and beeches, while in waterlogged or nutrient-poor sites succession usually leads to forests of conifers, such as the white pine (Pinus strobus), the red pine (P. resinosa), the Canada hemlock (Tsuga canadensis), various spruces (Picea), and the tamarack (Larix laricina). In the maritime Canadian provinces, the red spruce (P. rubens) is dominant. When the Europeans arrived, these forests contained abundant white pine and red pine, but after two centuries of intensive forestry exploitation their appearance has changed dramatically. The American chestnut (Castanea dentata) has almost totally disappeared since the arrival of the chestnut blight (the fungus Endothia parasitica) in 1909 (see vol. 7, p. 117). The climate of this mixed forest is milder, and this is responsible for the dominance of different trees and also makes this climate more suitable for agriculture. Because of this, many broad-leaved forests growing on the more fertile cambisols and luvisols have been felled, and many waterlogged areas with conifers have been drained and plowed. The typical vegetation of southeast Canada is pine trees, but some broad-leaved species are also important, such as yellow birch (Betula lutea), sugar maple (Acer saccharum), American beech (Fagus grandifolia), red ash (Fraxinus pennsylvanica), red oak (Quercus rubra), American white elm (Ulmus americana), and American linden (Tilia americana).
Mixed forests also occur in the Far East. They are very rich in species (see vol. 7, pp. 120-122) and are more diverse than the European mixed forests. In the forests of central Russia, there is frequently a transition through spruce forests with two tree layers: the upper tree layer is dominated by the Norway spruce (Picea abies) and trembling aspen (Populus tremula), while the lower layer is dominated by the small-leaved linden (Tilia cordata), Norway maple (Acer platanoides), and European white elm (Ulmus laevis). The shrub layer is relatively poor where the tree canopy is dense and continuous, but is very dense in the gaps. It contains many species typical of deciduous forests, such as the hazel (Corylus avellana), the European fly honeysuckle (Lonicera xylosteum), and the guelderrose or snowball (Viburnum opulus).
In western Siberia, the transition between the southern taiga and the steppe takes the form of open forests of birches and trembling aspens. In the northern areas, the understory is mainly Altai honeysuckle (Lonicera altaica), Tatarian dogwood (Cornus alba), black currant (Ribes nigrum), and red currant (R. rubrum). In the southern areas, the understory is made of spirea (Spiraea hypericifolia), Tatarian honeysuckle (Lonicera tatarica), and cotoneaster (Cotoneaster multiflora). East of the Yenisey River, the climate becomes excessively continental for birches and aspens and the transition takes the form of a steppe forest, dominated by the Scotch pine (Pinus sylvestris; see pp. 295-297).
The boreal peat bogs
Bogs are an integral part of the plant cover of taiga landscape. Bog ecosystems may develop in other biomes, from the tropics to the polar regions, but bogs are most abundant in the taiga (see vol. 9, pp. 47-53). A variety of circumstances may lead to the formation of quaking bogs and peat bogs in the boreal forests. The most important factor is a wet, cold environment, followed by the abundance of acid, nutrient-poor soils. Permafrost also plays an important role, as does the very flat relief and the occurrence of forest fires.
Low bogs form in more or less flat areas because the water table is rich in nutrients and near the surface. The herbaceous vegetation (sedges and grasses) is very abundant, and trees (birches, willows, and alders) may also be abundant. The moss layer is mainly represented by normal mosses rather than sphagnum mosses. Low bogs often form on the banks of lakes and in river valleys, on relatively nutrient-rich soils often considered to be eutrophic (so nutrient-rich there is little oxygen). Raised bogs of sphagnum mosses form where the growth of moss has raised the level of the bog.
The sphagnums, mainly Sphagnum angustifolium, S. balticum, and S. recurvum, form a dense cover in depressions. They always grow upwards, as unlimited growth occurs at the tip of the shoot, while the lower part dies due to lack of light and turns into peat. As the peat accumulates, the sphagnum and all the plants living in the bog are raised further and further above the nutrients of the substrate. Raised bogs, which only receive nutrients in rainfall, can only support oligotrophic plants (those adapted for nutrient-poor areas).
Ridges are better drained, and normally small pines may grow, as well as several different characteristic shrubs, and a dense carpet of sphagnum mosses usually forms in the depressions. Sphagnum fuscum, S. rubellum, S. acutifolium, and S. magellanicum can grow on hummocks 20 in (50 cm) above the water table. In depressions, however, where mainly the above species grow, they cannot raise water so high and an extreme example is S. cuspidatum, which can only live in water.
Thus the peat in raised bogs contains more nutrients than the plants that grow there. To the contrary, in low bogs or fens and transition bogs, the peat contains more nutrients than in raised bogs, because of the input from ground water.
The West Siberian Plain contains the largest area of bogs in the Eurasian taiga, and probably in the entire world. These bogs occupy an immense area, approximately half the West Siberian Plain, about 247 million acres (100 million ha). The largest peat bog in western Siberia, in the basin of the Vasyugan River, covers about 13.3 million acres (5.4 million ha), about half the size of Switzerland. In eastern Siberia, the permafrost stimulates the spread of bogs and boggy forests. Peat conducts heat very poorly, and the soil hardly thaws in summer where it is covered by sphagnum. The permafrost acts as an impermeable horizon, and peat bogs form where precipitation in summer is sufficient, even on sites that slope slightly. Open forests of larches grow on the bogs in river valleys on top of the permafrost, and represent the final stage of succession. They are similar in structure and composition to the upland bogs of Europe and western Siberia.
In North America, there are many large areas of waterlogged soil, both open and forested, in the wet and poorly drained areas of the northern taiga. Fifty-one to 75% of the area of northern Alberta, northeast Manitoba, and central Ontario is waterlogged, and at the northern tip of Lake Winnipeg and on the southern shorelines of Hudson Bay and James Bay, more than 75% of the area is bog. Proportionally, the regions of southern taiga and mixed woodland contain less waterlogged ground, as they are better drained and do not receive excess precipitation.
Eutrophic bogs (fens) form if the water emerging at ground level is eutrophic or mesotrophic (containing high to medium levels of nutrients), while true bogs form when the water table is oligotrophic, that is, highly acidic and containing few nutrients. Fens rich in nutrients are dominated by sedges and dark mosses: in calcareous fens the most abundant species are Scorpidium scorpioides, Aulacomnium turgidum, and Drepanocladus revolvens; less basic fens are dominated by D. vernicosus and Helodium blandowii.
Many present-day peat bogs were formerly basic fens that later became acid peat bogs as a consequence of paludification: this involves, on the one hand, a raising in height of the peat, and as a consequence the surface of the bog rises above the water table, and thus away from it; on the other hand, it means that the peat at the edge of the bog rises above the fen and even the surrounding dry ground. The input of water and nutrients into sphagnum bogs thus comes mainly from the rainfall, and a sort of ridge or dome forms in the center, perhaps a few meters above the edges of the bog, which is rich in nutrients. The surface of the typical waterlogged environments of the taiga is frequently dotted with brightly colored small rises of sphagnum, with the greenish yellow Sphagnum angustifolium in the depressions, the reddish S. magellanicum on the sides, and the brownish S. fuscum covering the highest part. S. fuscum is the sphagnum best adapted to the oligotrophic environments, and the one that releases most acids. It is also the most important bog-forming species in the entire circumpolar zone, and it appears to be the peat most resistant to decomposition.
3. The fauna and animal life
3.1 Animal life in the taiga
Some factors are of crucial importance to animal life in the taiga, such as the long winter and short summer, and surviving the long, harsh winters is particularly critical. The weather is cold and wet all year round, but in winter the thick layer of snow makes movement extremely difficult and renders it impossible for many taiga animals to get access to food. In some regions of Quebec, for example, up to 16 ft (5 m) of snow may fall in winter.
The origin of the animal population
The taiga occupies a huge area in Eurasia and North America, but the animal species are usually the same, or very similar, in both areas; for example, large taiga animals such as the brown bear (Ursus arctos), the moose or elk (Alces alces), the wolverine or glutton (Gulo gulo), and the wolf (Canis lupus) occur from Scandinavia to Canada. This can be explained by the fact that the biota of the taiga is one of the most recent on the planet.
Most scientists believe that the fauna of the taiga developed during the last glaciations or in the postglacial periods, i.e. about 500,000-600,000 years ago, in what is now eastern Siberia. Before the ice ages, this area was covered by typical temperate broad-leaved forests, and the other regions of northern Eurasia and North America were covered in subtropical vegetation. It is thus not surprising that the animals living in the broad-leaved forests of eastern Siberia were better prepared for the cold of the following millennia. The taiga fauna spread from eastern Siberia through the whole of northern Eurasia, and during the following glacial and interglacial periods many species reached North America over the Bering land bridge, just as humans did.
The taiga in each continent received locally isolated species from the south, and it is these species that are responsible for the regional differences between the faunas of the taigas of North America and Eurasia. For example, the mountain taiga of eastern Siberia contains the musk deer (Moschus moschiferus), a small hornless deer originally from southeast Asia. The striped skunk (Mephitis mephitis) and the rufous hummingbird (Selasphorus rufus) are characteristic of the North American taiga, although their closest relatives live in the tropical forests of South America.
The large herbivores and carnivores
The species Rangifer tarandus, called the caribou in North America and the reindeer in Eurasia, occurs in the northern taiga, although it moves between the taiga forests and the tundra. But the principal herbivore in the taiga is Alces alces, called the elk in Eurasia and the moose in North America; it is the world's largest deer, and the only one that can walk without any problems on a thick layer of snow. When the snow is thicker than 39 in (1 m), especially in late winter, not even the moose's strong legs can cope, however, and it retreats to willow groves or aspen thickets that grow on burnt sites, where they may stay for weeks feeding on bark, leaves, and their favorite food, young willow and poplar shoots. Moose consume large amounts of plant foods, and an adult eats 26-33 lb (12-15 kg) daily in winter, and can eat up to 77 lb (35 kg) a day in summer. A large population of moose can seriously damage the forest, and even kill the trees.
Surviving the harsh winter is also hard for the large carnivores, such as the wolf (Canis lupus), the lynx (Felis [=Lynx] lynx), and the Gulo gulo, called the glutton in Eurasia and the wolverine or carcajou in North America. The wolverine is better able to survive winter in the taiga than the wolf and lynx, and occurs in the Arctic and subarctic tundra of North America and Eurasia as well as in the boreal forest (see pp. 314-317). The lynx is mainly found in the mountain taiga regions, where many of its main prey also live, such as the roe deer and musk deer in Siberia and cottontails in North America. The wolf finds it hard to move over the soft snow in the flat taiga, and so it avoids dense forests; it prefers to move along paths near rivers, where the snow is much harder. Hunting in packs means wolves can hunt animals as large as caribou and moose, though they also catch smaller mammals, such as deer, hares, and beavers, especially in summer.
The brown bear (Ursus arctos) is one of the most typical animals of the taiga. The black bear (Ursus americanus) is smaller and also lives in the boreal forests, leading a very similar life to the brown bear. Both species overwinter in a den under the snow in a state of semihibernation; their heartbeat slows and their body temperature falls a few degrees, thus saving energy. Despite this, they can rouse themselves rapidly, and they may even leave their den temporarily on warm days.
The small mammals
The beaver perhaps best symbolizes the mammals of the taiga. The Old World beaver (Castor fiber) lives in Eurasia and the New World beaver (C. canadensis) lives in North America. Both build lodges and dams on rivers (see pp. 326-329). In the boreal forest there is a wide range of small arboreal mammals that basically eat pine nuts. Some species, such as the chickaree or North American red squirrel (Tamiasciurus hudsonicus), accumulate cones in summer to eat in winter.
The smallest carnivores of the taiga are the least weasel (Mustela nivalis) and the stoat (M. erminea). Both are distributed throughout the forests of the taiga of Europe, Asia, and North America, and both have ranges spreading north and south beyond the taiga biome. They spend part of their time in the winter buried in the snow, where they hunt their small prey, usually voles. The stoat is larger than the least weasel, and prefers the easily flooded biotopes of flood plains, where it hunts large Nordic voles (Microtus oeconomus). The least weasel frequents the interfluvial forest areas in the taiga, where it mostly feeds on the red-backed vole (Clethrionomys rutilus), which is smaller.
The taiga forests support a high diversity of shrews, because the animals can choose to live in all the biotopes in the taiga; thus, each species can choose its preferred habitat, and even share it. In effect, species that differ greatly in size can occupy the same habitat, meaning they can feed on a wide range of different invertebrates. The larger shrews mainly eat earthworms or other relatively large prey, while the smaller ones catch arachnids and insects hibernating in the forest leaf litter.
In addition to the shrews, voles are also active under the winter snow cover. Voles are the smallest of all rodents, and adult species of the genus Clethrionomys only weigh 0.7-1.4 oz (20-40 g). They dig tunnels in the snow, where they are insulated from the cold, to find the buried fruit of shrubs. There are only four species of vole in the taiga, the most common being the red-backed vole (Clethrionomys rutilus), which lives in the boreal forests of Eurasia and North America. Voles eat a wide range of foods, such as the seeds of trees and herbaceous plants, fruits, mushrooms, lichens, the bark of young trees and shrubs, and insects. In years of abundance, 2.5 acres (1 ha) of taiga may support more than 100 shrews and voles.
Birds, amphibians, and reptiles
About two-thirds of the bird species in the taiga are passerine birds, distinguishing this biome from the neighboring steppe and tundra biomes; this is linked to the fact that the passerines evolved in temperate forest environments.
It is worth mentioning that most of the birds that spend the winter in the Eurasian or North American taiga are represented either by the same species on the two continents, or by different species of the same genus, such as the willow tit (Parus montanus) in Eurasia and the black-capped chickadee (P. atricapillus) in North America. However, the taiga areas of North America and Eurasia share almost no species of bird that migrate to the south every year. The ecologically similar habitats of Eurasia and North America may be occupied by different genera of the same family, such as the European thrushes (Turdus, Turdidae) and the North American nightingale thrushes (Catharus, Turdidae), or by unrelated birds that have become morphologically similar as result of living in similar conditions and habitats, as in the case of the Old World warblers, such as Phylloscopus (family Sylviidae), and the New World wood warblers, such as Dendroica (family Parulidae).
Flycatchers (Muscicapidae) also frequent the taiga forests of Asia and live in the crowns of trees. Warblers find food on leaves and conifer needles, but flycatchers catch flying insects. They spend less time in the temperate latitudes than the warblers. Typical flycatchers of the Siberian taiga include the European red-breasted flycatcher (Ficedula parva) and the Siberian dark-sided flycatcher (Muscicapa sibirica).
There are few species of amphibians and reptiles in the boreal forest, as they are limited by the short summer. The most abundant amphibians can maintain their activity at low temperatures. In experimental conditions, for example, the salamander Salamandrella [=Hynobius] keyserlingi survived temperatures of 21[degrees]F (-6C[degrees]), and was active and able to move at temperatures of 36[degrees]-39[degrees]F (2[degrees]-4[degrees]C), and even at 32[degrees]F (0[degrees]C). During the short taiga summer, warm periods are interrupted by intense cold ones, and so eggs laid on the soil surface would die. It is thus not surprising that the reptiles whose range reaches as far north as the boreal forests, the lacertid lizard (Lacerta vivipara) and the European viper (Vipera berus), are viviparous (live-bearing).
The ever-present mosquitoes and other insects
Anyone who has visited the taiga in the summertime will vividly remember the abundance of bloodsucking insects. They hatch in early June, and make life miserable for warm-blooded mammals almost until the first snow falls. They are most abundant in late June and early July, so abundant that the traveler is confronted by a solid wall of mosquitoes that may cover the sun and sometimes makes it impossible to see the landscape. Even the people living in villages try not to leave them unless absolutely necessary.
The term "mosquitoes" broadly refers to insects with widely differing ways of life. To begin with, there are the most aggressive true mosquitoes and gnats (family Culicidae, order Diptera), especially the genus Aedes, which are spectacularly abundant in summer and attack people and other large mammals until well into August. Many of these insects are active throughout almost the entire day, and bite both in the open and in enclosed places, which they enter through cracks and even down chimneys.
The bite of Aedes is painful. They pierce the skin with an elongated proboscis similar to a stylet that is normally sheathed in the lower labium (lip). Only female mosquitoes suck blood, and the males generally only suck nectar from flowers. The female requires blood for her eggs to mature, and may suck more than her own body weight of blood. The eggs are laid in small pools in the taiga. The larvae hatch the following year, and feed on plant remains or tiny water organisms. After several molts, the larvae grow eight to 10 times in volume and turn into the curved pupae that are active and free-swimming. Eventually, the skin of the pupa's back splits open, and the adult mosquito (the imago) emerges on the surface of the water.
In years with hot, rainy summers, mosquitoes are extremely abundant. If early summer is hot, the pools of water dry out rapidly before the adult mosquitoes can emerge from the pupa and fly off. Intense cold in early summer, which is by no means uncommon in the taiga, also kills many mosquitoes, because the pools are covered with ice and this prevents the respiration of the larvae and especially the pupae.
There are other bloodsucking insects, such as black flies or buffalo gnats (family Simuliidae, order Diptera), which are only 4-5 mm long, but their bite is much more painful than that of mosquitoes. Black flies (or simuliid flies), like mosquitoes, inject an anticoagulant to keep the blood flowing. Their saliva is also highly irritating to human beings and causes inflammation. If the bites are abundant, they may induce an allergic reaction consisting of generalized swelling and a rise in body temperature. Black flies are very abundant in the second half of summer, and some species may still be active when the first snows fall. They are sensitive to changes in the weather, and are normally inactive at night and do not enter closed spaces.
Black fly larvae develop in fast-flowing water and in rivers, where they attach themselves to aquatic plants and rocks, and have a way of life more similar to plants than animals. As in mosquitoes, only the females are bloodsuckers, and the males feed on nectar. It is worth pointing out that one or two species of black fly are active bloodsuckers in the taiga, but only feed on nectar in the tree steppe and prairie. This is because the adult female only needs to feed on blood when its larval stage has developed in unfavorable circumstances and she has been unable to accumulate sufficient nutrient reserves to mature her eggs.
Among the lush green of the taiga in summer, the traveler may spot patches of dead forest that appear to have been burned. This is not the result of a fire, but of the attack of the Siberian pine moth (Dendrolimus sibiricus, family Lasiocampidae, order Lepidoptera), a light greenish moth. Massive swarms of these moths occur in late July, often around sunset. The females lay their eggs directly on conifer needles, on which the caterpillars start to feed, later moving on to attack the young shoots. They usually take two years to develop completely, though this depends on how far north they are, as the life cycle takes longer in the north. If there are very many pine moth caterpillars, they may kill the tree.
The caterpillars do not attack all the trees, but only the weakest. The tree may be weakened by growing in poor soil, as a result of unfavorable environmental conditions for one or more seasons, or due to industrial pollution. After attack by pine moth caterpillars, the weakened trees are open to attack by the larvae of the longhorn beetle Monochamus urussovi (family Cerambycidae, order Coleoptera), which destroy their wood. Taken together, the activity of the pine moth and the longhorn lead, in central Siberia, to the destruction of the trees of the dark taiga and their replacement by broadleaved forests.
3.2 Seasonal rhythms and periodic fluctuations
When talking about the conditions of life of the animals of the taiga, it should be remembered that the taiga occupies a huge area. Cone and seed production by pines and other dominant conifers varies greatly from one year to the next, and may vary greatly between different regions in a single year. Most of the animals of the taiga depend on the seeds of conifers directly or indirectly, so it is not surprising that many birds and mammals migrate, either periodically or seasonally, to sites where food is more abundant.
How birds overwinter
Local migrations in search of food are especially typical of birds that are specialized feeders, such as crossbills (Loxia, family Fringillidae). For example, the parrot crossbill (L. pytyopsittacus) eats pine seeds, the red crossbill (L. curvirostra) eats fir seeds, and the two-barred crossbill (L. leucoptera) eats larch seeds.
These are the only taiga birds that reproduce in the winter, and inside the nest the growing chicks survive the intense cold. This is not surprising if it is taken into account that their reproductive success depends entirely on the production of cones, which are present on the branches throughout the winter, meaning that the winter is the best period for the chicks to grow.
The large seeds of the Siberian pine (Pinus sibirica) are too tough for small passerine birds, though one species of the crow family (Corvidae) has specialized in eating the seeds of the Siberian pine. This is the nutcracker (Nucifraga caryocatactes), whose long sharp bill is ideal for extracting the seeds. There is a related species in the North American taiga, Clark's nutcracker (N. columbiana). Nutcrackers do not breed in winter, as crossbills do, but in mid-spring and early summer, and they mainly feed their chicks on Siberian pine seeds and insects. The reason why they do not breed in winter is probably that the cones of the Siberian pine are usually shed in autumn, and rarely stay on the tree throughout the entire cold season, unlike those of other conifers. As a result, nutcrackers suffer some degree of food shortage in the winter. To get through the winter, in autumn they cache cones near stumps or under fallen branches, or simply bury them. They often cache their supplies far from sites where the Siberian pine grows, which helps Siberian pines to regenerate in sites that have been logged or burned.
In addition to conifers, almost all the taiga forests contain birches and different species of alder. These produce small seeds that are greatly appreciated by several birds, such as the common redpoll (Acanthis [=Carduelis] flammea), a very small bird that breeds in spring and feeds its chicks on insects. The golden-crowned kinglet (Regulus satrapa) is another small insectivorous bird, found in the boreal forests of North America, and which remains after larger birds have gone south. Light green in color, it has a shorter tail than other kinglets. It builds a spherical nest, with an entry hole at the top, among the highest branches of the conifers.
Silence reigns in the taiga throughout the winter, but the arrival of spring fills it with birdsong. Many of the birds that arrive are passerines returning to their nests after overwintering in the south. Most of them feed on the insects of the tree crowns or eat a variety of foods from the soil surface. Though this food is only abundant in summer, there is enough for them to raise their chicks.
In autumn, the young birds accompany their parents on their migration thousands of kilometers south to overwinter. Some of the taiga birds overwinter in southern Africa, others in southeast Asia, and others in South America. The following spring they return to the site where they bred the previous season, and sometimes build their nest in the very same tree as the previous year.
The populations of many species of bird in the taiga show large variations from one year to another, and these variations depend on many different factors, which vary from species to species. The number of birds that spend the winter in the taiga is normally conditioned by the crop of the seeds on which they feed, but the number of migrant birds is largely determined by conditions in their overwintering sites. The area of boreal forests where these birds spend the summer is huge in comparison with the area of the tropics and subtropics where they can overwinter.
Furthermore, the areas where they overwinter may already be occupied by other birds that eat the same foods. When the density of individuals in these sites is very high, the birds arriving from elsewhere find it very difficult to occupy the most suitable habitats. As a result, even a slight deterioration in feeding and climatic conditions in these overwintering sites can cause a substantial reduction in the boreal bird populations of the taiga.
Some birds remain in the taiga throughout the winter, such as the capercaillie (Tetrao urogallus), a large bird that can reach a weight of 13 lb (6 kg). In summer and autumn it mainly feeds on different wild fruit, but in winter, when this is insufficient, it eats pine needles. This large bird spends the day high up in the trees eating the needle leaves, and in the evening it buries itself in the snow to spend the night.
A closely related species, the black-billed capercaillie (T. parvirostris) lives in the larch forests of Siberia, and feeds on larch shoots throughout the winter. The spruce grouse (Canachites canadensis) is one of the most common birds in the boreal forests in North America. Like other grouse, this species eats conifer needles and shoots during the winter, but it also eats a wide range of wild fruit whenever it can. It is a very docile bird that never shows fear of human beings, and for this reason it is known as the fool hen. This group of grouse, pheasants, and partridges (family Tetraonidae) are the only birds that can overwinter on such a coarse and low-calorie foodstuff as conifer leaves. They are like specialized machines that are highly efficient at processing the cellulose of conifer needles and shoots.
The small birds that remain in the taiga through the winter form mixed flocks of woodpeckers and other birds, for example the great spotted woodpecker (Dendrocopos major) or sometimes the northern three-toed woodpecker (Picoides tridactylus) is found together with the Eurasian nuthatch (Sitta europaea) and several species of titmouse, especially the willow tit (Parus montanus). Each species has its favorite foods; the woodpeckers search the trunks of dead trees for hibernating insects, or look for cones.
The blue nuthatches carefully search the surface of the branches and the titmice search for food on the branches. This suggests that these mixed flocks form because the smaller birds prefer to accompany the woodpeckers. When the woodpeckers have broken open the trunks and cones, some of the insects or seeds are more accessible to the small birds. The titmice that form part of these mixed flocks stay near the sites where they nest, even in winter, but in fact these birds are characterized by their seasonal migrations, especially those of the juveniles.
These birds usually migrate to their overwintering sites in the first week of September, and they return to nest in the spring. The migrating flocks are especially large in the years when the summer breeding season is successful, and the population of birds is thus much greater.
The mammals of the taiga also migrate, depending on the availability of food. This is shown by the red squirrel (Sciurus vulgaris), which is very abundant throughout the forests of the Eurasian taiga, and the North American red squirrel (Tamiasciurus hudsonicus), which is common throughout the forests of North America. Squirrels feed on the seeds of almost all species of conifer, and because they are active all year round they are forced to undertake long migrations to find sites where seeds are abundant. Squirrels can move quite fast on these journeys in search of food, at a speed of 2-2.5 mph (3-4 km/h), crossing major obstacles such as the Yenisey River, which is roughly 1 mi (2 km) wide. Squirrels may even undertake mass migrations: for example, in 1917, there was a large migration of squirrels that took 45 days to cross the Northern Dvina River.
In late autumn, reindeer (Rangifer tarandus) move from the tundra to the northern taiga forests. They avoid dense forests, and in the winter they go to the large treeless wetlands where they eat the lichens below the snow layer. Elk also perform seasonal migrations in winter to regions where there is less snow, making journeys of 124-186 mi (200-300 km). They range widely in summer and may travel dozens of kilometers in a single day, grazing on the grass in the meadows, the water plants in pools and shallow lakes, or the juicy shoots that they find in open areas of forest, especially in areas that have suffered fires or that have been logged.
The martens (Martes) also travel in search of food. The sable (Martes zibellina), whose warm glossy fur is highly valued, only lives in the Asiatic taiga. A member of the weasel family (Mustelidae), the sable can climb up trees in search of food, but in winter, when the branches are covered with a thick layer of snow, it has to travel long distances on foot. Its varied diet includes pine nuts, small mammals, and squirrels, which it complements, especially in summer, with wild fruit, honeycombs, and small birds. Sables usually travel at the end of autumn and early winter. Its tracks clearly reveal that the sable can travel distances of 75-93 mi (120-150 km), and can even cross high mountains in its path.
The pine marten (Martes martes) is found in the European taiga, while the American marten (M. americana) and the fisher (M. pennanti) live in the North American taiga. The American marten mainly feeds on small rodents (especially mice), squirrels, rabbits, and birds, as well as carrion and wild fruit when they are abundant. The American marten is smaller than the sable, weighing 1-3 lb (0.5-1.5 kg), compared to 1.5-4 lb (0.7-1.8 kg) for the sable, and it is a rather solitary animal that defends its territory from competitors of the same sex. The fisher, which reaches 4.5-11 lb (2-5 kg), is also an opportunist hunter, but it shows a great preference for the North American porcupine (Erethizon dorsatum). The decline in populations of the fisher as a result of hunting for its fur has led to a great increase in the number of North American porcupines, and in many regions of eastern North America it has now become a pest.
The populations of many species of taiga mammals--including shrews, voles, hares, and some carnivores--sometimes show dramatic rises and falls. These fluctuations are clearly cyclical, and occur every three or four years (eight to 11 years in the case of hares). After peaking, the population falls rapidly, to one-tenth or one-hundredth of the peak level. The maximum density of the snowshoe hare (Lepus americanus), for example, may reach 6,215 individuals per sq mi (2,400 per sq km), 100 times greater than the minimum value. After the crash, the population starts to grow again until it reaches a new peak, which will be followed by another crash.
There are more than a dozen different hypotheses to explain these fluctuations. Some authors attribute the population changes to external factors, such as the weather, the abundance of food, epidemics, or even the cycles of solar activity. Other authors attach great importance to internal factors within the population, especially the stress derived from aggression due to the high population density. For example, in regions where the snow is not very thick, the decrease in the number of small mammals is irregular and is observed in years with very cold winters and little snow. In the regions where the snow cover is thick and long-lasting, the cyclical nature of population dynamics is very clear and density-dependent factors play a key role. In the taiga, where there are large areas with similar habitats and the snow layer protects the small mammals from the unfavorable climate, these cyclical fluctuations in population density are very clear. This is because living conditions differ greatly from one place to another, even in the uniform taiga. After the population crash, the surviving animals are concentrated in the most favorable biotopes. They breed fast, and the younger individuals start to disperse. Within a short time, all the other biotopes are occupied, even those where the species can only survive in the summer. Yet during the survival phase, the animals continue breeding, as the emigration of young animals prevents overpopulation. In autumn and early winter, when conditions get harsh, the animals start moving to the mildest and most favorable habitats. Thus in the most favorable biotopes, overpopulation, with the consequent destruction of the food supply and the mass death of the animals, does not occur as a result of the rapid reproduction rate, but because of the immigration of animals from other biotopes.
One of the most widely accepted hypotheses to explain these cyclical fluctuations is the influence of carnivores, whose numbers increase greatly when prey are abundant. Pressure from the carnivores leads to a large fall in the number of prey, and the number of carnivores then falls as a result. An alternative point of view suggests the number of carnivores is totally dependent on the abundance of food, and thus follows fluctuations in the prey population. The great gray owl (Strix nebulosa), one of the world's largest nocturnal raptors, is widely distributed throughout the boreal forests of Europe, Asia, and North America. Its populations fluctuate depending on changes in the vole population. In the years when the number of voles is lowest, the great gray owl does not even attempt to breed. Another typical bird of prey of the boreal forests is Tengmalm's owl (Aegolius funereus), which feeds on small mammals and birds and sometimes eats insects. It normally breeds in small holes in trees, often in abandoned woodpecker nests, and it lays a clutch of three to six eggs. The number of surviving chicks depends on the availability of food in that year. Unlike these two owls, the northern hawk owl (Surnia ulula), a diurnal species, waits on the high branches of trees for prey to pass below.
4. Life in rivers and lakes
4.1 The waters of the taiga
The Eurasian and North American taiga contain many water masses, which is not surprising, as precipitation is abundant and evaporation is low because temperatures are low. A water deficit is virtually impossible, except in mountain regions. In the typical lowland taiga, evaporation does not usually exceed 50-70% of the total precipitation, the rest flowing into streams and rivers; but if the soil is poorly drained, it leads to the formation of marshes and bogs.
Rivers and lakes
As winter lasts at least half the year in the taiga, most of the precipitation falls as snow. In spring, in April and May, the sun rises higher above the horizon and shines for longer, and the rapid increase in insolation causes the snow to thaw quickly, saturating the soil and feeding the streams that flow into larger rivers. Rivers as large as the Ob, Yenisey, and Lena in Eurasia and the Mackenzie in North America have most of their basins in the taiga.
There are very many lakes in the taiga, especially where Precambrian crystalline sedimentary rocks are close to the surface, as this prevents the water from percolating into the subsoil. For example, lakes occupy much of the area of the immense Canadian Shield in North America and the Baltic (Fennoscandian) Shield in northern Europe. In Karelia in northwest Russia, for example, an area of 66,410 sq mi (172,000 sq km) contains 41,789 lakes that cover more than 25 acres (0.1 sq km) each. Some lakes in the taiga biome are of tectonic origin, such as Lake Baikal (maximum depth 5,371 ft [1,637 m]). Thermokarstic lakes, formed in sites where large blocks of ice melted, are very common in the permafrost regions. Yet most lakes in the taiga are of glacial origin. Some deep lakes formed as a result of the combination of tectonic processes and later glaciation. This is the probable origin of the Great Slave Lake, the deepest lake in North America (with a maximum depth of 2,014 ft [614 m]).
Apart from the large number of small lakes, the taiga biome also contains immense lakes that are usually located on the edges of the crystalline shields. In Europe, these include Lake Ladoga (6,825 sq mi [17,677 sq km]) and Lake Onega (3,738 sq mi [9,682 sq km]), on the easternmost edge of the Baltic crystalline shield. In North America, they are exemplified by Great Bear Lake (12,028 sq mi [31,153 sq km]), Great Slave Lake (10,502 sq mi [27,200 sq km]), and Lake Winnipeg (9,382 sq mi [24,300 sq km]), all of them on the western or southwestern edge of the Canadian Shield. On the southern edge of the Canadian Shield are the Great Lakes, the headwaters of the St. Lawrence River, but they are outside the taiga biome.
The water regime
The area occupied by the taiga biome is very large and the geological, soil, and weather conditions vary greatly, so it is not surprising that the bodies of water are also very diverse. There are, however, common factors affecting all the organisms living in water bodies in the taiga.
The first of these factors is the short growing season: for much of the year, many lakes and rivers are covered by a thick layer of ice that transmits little light. The second and third most important factors are the low temperatures and the water's low content of mineral salts (including nutrient ions). All these factors seriously limit photosynthesis by photoautotrophs, mainly microscopic planktonic algae and cyanobacteria. This is why most lakes in the taiga are oligotrophic, i.e. their nutrient levels are low, primary production is low, and the water is usually very clear. In regions with many peat bogs, such as western Siberia, many lakes are dystrophic. The water in dystrophic lakes is very acidic and yellowish or brown in color, due to the high levels of humic acids; they are not very transparent, and photosynthesis is low.
4.2 Life in the lakes
The seasonal cycles that have such a major influence on life on dry land in the taiga also influence life in the lakes and rivers. During the long winter, biological activity under the thick ice layer is extremely slow. The first reason is that the temperature of the entire water column is very low, around 39[degrees]F (4[degrees]C). The second is that light levels are very low: light may be reflected by the snow cover, transmission through the thick ice layer is poor, the day is short, and the sun is low above the horizon.
Ice formation and vertical mixing
Winter is a very difficult time for many plants and animals inhabiting the waters of the taiga. Shallow lakes often freeze solid, right to the bottom, and the only organisms that can survive this are those with resting stages adapted for these conditions. Even lakes that do not freeze to the very bottom lack fish, due to oxygen depletion in the winter.
Spring begins in the lakes in the taiga well before the ice breaks up, when enough light is transmitted through the ice for photosynthesis to take place. In some cases, a sharp increase in photosynthetic activity occurs in February, and in others this happens in March and April. Photosynthesis under the ice cover is generally higher in large lakes, where the strong winds blow the snow layer away. In Lake Baikal, for example, photosynthesis sometimes starts in late January under the recently formed ice, which transmits light well and is not yet covered in snow. The populations of these diatoms that grow under the ice (Melosira baicalensis, Cyclotella baicalensis, Melosira islandica helvetica, etc.) peak in April. When the lake melts these species are replaced by "summer" species whose populations do not usually reach such high levels.
After the ice has melted (usually in April in the southern taiga, and in May-June in the northern taiga), the water in the lakes undergoes vertical mixing. At this time, the whole water column is at the same temperature (39[degrees]-41[degrees]F [4[degrees]-5[degrees]C]), in what is called the homothermy period. The subsequent mixing may follow two models. If the lake is subject to the influence of strong winds and warming is weak, the period of mixing lasts until the autumn cooling; lakes like this, with only a single period of free circulation or mixing, are known as monomictic. The large water masses in the more northerly areas of the taiga are usually monomictic. Great Bear Lake, for example, is monomictic and in the ice-free period (mid-July to December) the temperature rarely exceeds 39[degrees]F (4[degrees]C). Events after the spring melt are different in dimictic lakes, where free circulation or mixing of the water occurs twice a year, in spring and in autumn, and the water column is thermally stratified in the period between the two mixes. The top layer (epilimnion) is warmed and mixed by the wind, while the mass of deep water (hypolimnion) remains at a temperature of 39[degrees]F (4[degrees]C, the temperature at which water is densest). At the transition between the epilimnion and hypolimnion, the temperature of the water falls sharply (sometimes several degrees), and this transition is called the thermocline. Small deep forest lakes in the southern taiga show particularly clear thermal stratification. Stratification is less clear in the large lakes, which take longer to become free of ice, and are subject to stronger mixing by the wind.
The phytoplankton bloom in the spring and early summer, and this is usually followed by an increase in the zooplankton. In the small water bodies that freeze solid in winter, and thus lack fish, the zooplankton controls the phytoplankton population because there are no predators to limit the zooplankton's population growth. Yet when the autumn cooling starts, biological processes slow down. The shortage of light drastically reduces photosynthesis, but decomposition of organic matter continues under the ice. The length of time the lake is frozen varies greatly, depending on the geographical latitude, and to an even greater extent on the size of the water mass and the local climate. Thus, the Great Bear Lake freezes over as late as December, despite its northerly location. The largest lake in Europe, Lake Ladoga, which is influenced by the warm Atlantic Ocean, almost never freezes over completely, and Lake Onega, a little further north, freezes in January, but only in very cold winters is it completely covered by ice.
The primary producers
A given lake may contain many species of phytoplankton, and large lakes may contain a total of 200 or 300 species, but only a few species are dominant at any given moment. In the northern taiga the most abundant populations are chrysophytes (Chrysophyceae, especially Dinobryon), dinoflagellates (Dinophyceae), and cryptomonads (Cryptophyceae). In the southern taiga, there is an increase in the share of chlorophytes (Chlorophyceae), diatoms (Baccilario-phyceae), and cyanobacteria. The species composition of the phytoplankton may be very similar in different lakes, even if they are geographically distant, or very different. For example, Melostra islandica, Asterionella formosa, and Dinobryon divergens are present in large numbers in the Great Slave Lake (North America) and in Lake Ladoga and Lake Onega (in northern Europe). Melostra islandica and Asterionella formosa are also abundant in Lake Baikal.
The phytoplankton biomass in the lakes of the European taiga is between 1 and 4 g dry weight per sq yd (1-5 g/[m.sup.2]) on average, and annual production is about 42 g of C/[yd.sup.2] (50 g of C/[m.sup.2]), which is roughly twice as high as in the lakes of the forest-tundra zone, but is only one-third of levels in the lakes in the mixed conifer and broad-leaved forests to the south. In the shallow waters there is a rich algal periphyton flora, including many diatoms, which sometimes account for a considerable part of the entire primary production of the water mass. Much of the primary production in the northern oligotrophic lakes is by the picoplankton, consisting of very small cyanobacteria (0.5-2.5 mm), such as Synechococcus, and small chlorophytes.
The consumers: from zooplankton to seals
In most large oligotrophic lakes the zooplankton is usually represented by two to three species of copepods (Limnocalanus macrurus, Epischura, Eudiaptomus) and several rotifers (Kellicottia longispina, Filinia longiseta, Keratella cochlearis, etc.). Cladocerans, especially Holopedium gibberum and Daphnia spp., mainly occur in the more productive lakes in the south of the biome, as well as in the small lakes present throughout the taiga that freeze solid. Copepods are dominant in ultraoligotrophic lakes because, unlike cladocerans, they can locate and catch separate algal cells, while cladocerans require a much greater concentration of food particles to feed effectively. Furthermore, the copepods can accumulate lipid reserves that allow them to survive long periods without feeding.
The benthic community generally consists of a few species, such as bloodworms (chironomid larvae, Chironomidae), oligochaete worms, and bivalve mollusks of the Sphaeriidae family. The insects found in the littoral (intertidal) zone include mayflies (Ephemoptera), stone flies (Plecoptera), caddis flies (Trichoptera), and others. The amphipod Pontoporeia affinis and the mysidshrimp Mysis relicta are present in the near-bottom layer, often in large numbers, both in northern Europe and in the subarctic areas of North America. Both these species are considered glacial relicts (i.e., remnants from the Ice Age), as are the copepod Limnocalanus macrurus and the amphipods Pallasea quadrispinosa and Gammara-canthus lacustris.
The waters of the taiga zone also contain species of recent marine origin, such as some sponges, the polychaete Manayunka baicalensis (in the basin of the Yenisey), the fish called the fourhorn sculpin Myoxocephalus quadricornis, as well as two subspecies of the ringed Baltic seal, the Ladoga ringed seal (Phoca hispidas ladogensis) and the Saimaa ringed seal (P. h. saimensis).
4.3 Life in rivers
Almost all the large rivers flowing into the Arctic Ocean and the North Pacific cross through the taiga. Most of the area of their watershed is in the lowland taiga, though many have their headwaters in the mountains. The water in these rivers comes almost entirely from melting snow, and to a lesser extent from rains. They are covered by ice in winter, and an oxygen deficit under the ice causes a "winter kill" of fish, as occurs occur in the middle stretches of the Ob, which receives a huge input of organic matter in the outflow from bogs.
Nutrient-poor waters with few primary producers
There are few higher plants, but the periphyton is abundant. In small rivers with rapids, attached algae account for most of the production, though many organisms consume the organic compounds discharged from the terrestrial surroundings (mainly as leaf litter). Despite fast currents and rolling stones, some animals, mainly insect larvae--such as stone flies (Plecoptera), mayflies (Ephemeroptera), bloodworms (Chironomidae), and caddis flies (Trichoptera)--may attain high population densities. Carnivorous caddis flies dominate, such as Rhyacophila and Hydropsyche, which do not build the typical case associated with caddis flies but cast nets of silk threads to catch their prey. In some places with very fast currents, entire surfaces of stones are covered with firmly attached black fly larvae (and later pupae).
The large rivers have true phytoplankton and zooplankton. The benthos includes bloodworms, oligochaetes, mollusks, and sometimes freshwater sponges. The large isopod crustacean Mesidotea entomon, whose closest relatives are marine, occurs in the lower reaches and estuaries of many Eurasian rivers (and some North American ones) flowing into the Arctic Ocean.
The consumers: salmonids and other fish
The Siberian sturgeon (Acipenser baeri, Acipenseridae) is present in all the large rivers of Siberia, from the Ob in the west to the Kolyma in the east, though not in large numbers. This large fish (reaching a length of 10 ft [3 m] and a weight of 220 lb [100 kg]) occurs as a semianadromous (migrating to estuaries) form in the Ob and Yenisey; as a river form in the Lena, the Yana, and the Kolyma; and as a lacustrine-fluvial (lake and river) form in Lake Baikal and Lake Zaysan. The kaluga Huso dauricus is an even larger sturgeon that lives in the basin of the Amur River and can reach a length of 13 ft (4 m) and a weight of 2,205 lb (1 metric ton). Many lakes and rivers in the taiga zone contain several widely distributed fish, such as the common perch (Perca fluviatilis), the northern pike (Esox lucius), the roach (Rutilus rutilus), the European dace (Leuciscus leuciscus), and the burbot (Lota lota).
Yet by far the most typical fish of the taiga biome are the salmonids (family Salmonidae), including whitefish (subfamily Coregonidae) and graylings (subfamily Thymallidae), which are sometimes considered separate families. These fish are common even in small rivers; brown trout (Salmo trutta) is present near the rapids, and may be represented by exclusively freshwater forms, while the common grayling (Thymallus thymallus, see fig. 209) and the Arctic grayling (T. arcticus) are present in the calmer stretches. The Arctic char (Salvelinus alpinus) is present in the northern taiga, on the edge of the tundra; it has a circumpolar distribution and occurs in anadromous, lacustrine, and lacustrine-fluvial forms. Many lakes also contain whitefish, such as Coregonus clupeaformis, C. nasus, C. lavaretus, C. peled, C. muksun, and others, and in North America the lake trout (Salvelinus namaycush) occurs.
Almost all the rivers that flow into the Arctic Ocean contain the inconnu (Stenodus leucichthys, Salmonidae), which can reach a length of more than 39 in (1 m) and a weight of 66-110 lb (30-50 kg). It is a semianadromous species that migrates far upstream to spawn, traveling 932-1,181 mi (1,500-1,900 km) up the Yenisey, and as much as 2,175 mi (3,500 km) in the Ob. The taimen (Hucho taimen) is another large salmonid fish that occurs in the waters of central Eurasia (in the basins of the Ob, Yenisey, and Lena), but it prefers fast-flowing mountain streams and deep cold lakes (such as Lake Baikal and Lake Teletskoye in the Altai Mountains). The taimen spawns in spring in small streams with gravel bottoms. In the autumn they migrate downstream and spend the winter in large lakes or rivers. The taimen can reach a length of 39 in (1 m) and a weight of 132 lb (60 kg), and is an angler's dream. Yet the most widespread and commercially important salmonid fish are the Pacific salmon (Oncorhynchus) and the Atlantic salmon (Salmo), which occur in the north of the Pacific and Atlantic Oceans respectively. Most species of these genera are anadromous, feeding in the sea but returning to spawn in the river where they hatched. Some species, however, may have forms that live permanently in rivers or ones that feed in lakes but spawn in rivers.
The Atlantic salmon (Salmo salar) is a very large fish that can reach a weight of more than 22 lb (10 kg); the heaviest on record was 86 lb (39 kg). The Atlantic salmon feeds in the North Atlantic and spawns in the rivers of Europe (from the Iberian Peninsula in the southwest to the basin of the White Sea and the Kara River in the northeast) and along the coastline of North America (from the Connecticut River in the south to Greenland in the north). This highly valued fish used to be so abundant in all the rivers of Europe that the Scottish writer Sir Walter Scott (1771-1832) wrote that Scottish farm laborers in search of work complained about having to eat salmon too often. The young salmon, known as parr, is a lively fish with variegated coloring, with eight to 11 dark blotches that resemble thumbprints and small red spots. In Russian, the parr are known as pestriatki (derived from piostri, which means "mottled"), and are so different from the adult salmon that until the mid-nineteenth century they were thought to belong to different species. When they reach a length of 3.5-7 in (9-18 cm), the parr change color, as the scales turn shiny and silvery due to guanine deposition, and are known as smolt. The smolt then go downriver to the sea, where they grow quickly, and after one to five years they return to spawn in their home rivers. Some Atlantic salmon die after spawning, but others return to the sea and spawn again one or two years later. One individual female (identified by marks on scales) is known to have returned five times to the same river to spawn, but most salmon only spawn once or twice before dying.
Pacific salmon (Oncorhynchus) spawn in the rivers of the Pacific coastlines of northeastern Eurasia and North America. All the Pacific salmon spawn a single time and then die. The length of time the young remain in the river (and the sea) varies from species to species. The pink salmon (O. gorbuscha), the most abundant pacific salmon, spends 18 months in the sea, where it reaches a length of 16-20 in (40-50 cm), and it migrates upstream to spawn in its second year of life. The sockeye salmon (O. nerka) does not reach maturity for five to six years, and only then does it return to its home river to spawn.
169 The Canadian taiga is a jigsaw puzzle of forests, wetlands, and lakes, as shown by this photo of the Yukon Territory. The Yukon River is more than 1,550 mi (2,500 km) long and has a volume of flow greater than 1 million gal/s (4,000 [m.sup.3]/s), making it one of the largest rivers flowing through the conifer forests of North America. It emerges from Lake Tagish in the southeast of Yukon Territory (Canada), crosses Alaska from west to east, and flows into the Bering Sea at Norton Gulf. It crosses the landscape "of a thousand lakes," which is typical of the Canadian taiga (and the Finnish taiga), where areas of forest alternate with large areas covered with water, often connected to each other. There are more than 250,000 lakes like this in Canada, occupying depressions scoured by huge glaciers during the last Pleistocene glaciation. The Yukon Territory was largely covered by herbaceous tundra, and the spruces and other conifers that now form large forests in this region had retreated far to the south. At the end of the glaciation they moved back north as the ice retreated.
[Photo: Stephen Krasseman / NHPA]
170 The herbaceous layer in the North American taiga in Alaska (USA), showing the devil's club (Oplopanax horridus, Araliaceae). In some places, the taiga understory may be lush, for example in moist sites or where a tree falls and creates a clearing where the sunlight can reach the lower layers. Herbaceous plants and shrubby species from a variety of families can then grow. Many of them, such as bilberries and their relatives (Vaccinium spp.) and junipers (Juniperus), produce berries that are eaten by the resident animals and by the many animals that arrive in the taiga from the south when the days start getting longer. When these clearings are covered by the canopy, these plants do not die out, but their ground cover is greatly reduced. The growth of the herbaceous and shrub layer is not only limited by the shade of the tree canopy, but also by many other factors, such as the soil type, the leaf-litter layer, and especially competition with the tree roots, which absorb most of the nitrogen and mineral nutrients from the soil, thus limiting the growth of shrubs, herbaceous plants, and mosses.
[Photo: Antoni Agelet]
171 The roots of the taiga conifers are well adapted to extract water from the driest sites and tolerate waterlogging, highly acid conditions, and low nutrient availability. It even seems that many conifers growing on podzols with a low nutrient content live almost saprophytically on the thick layer of acid leaf litter (mor-type humus) that they have formed. They can do this because most of their fibrous root system is restricted to the lower layers of leaf litter, and perhaps to the Ah horizon, if one is present. Though the mor humus of podzols is an inhospitable environment for most decomposers, the mycelia (vegetative parts) of free-living and symbiotic fungi can grow there without any problems.
[Photo: Vadim Gippenreiter]
172 The burunduk chipmunk (Tamias [=Eutamias] sibiricus) is one of the many taiga vertebrates that lives on conifer seeds all year round. As this photo shows, the burunduk eats the seeds of conifers, in this case those of the Siberian pine (Pinus sibirica). The animal peels off the scales, removes the pine nuts, and fills the pouches in its cheeks, in the same way as hamsters (Cricetidae) do. It then takes the pine nuts to its burrow, where it places them in a special storage chamber. The burunduk spends the winter in its burrow in dormancy, waking occasionally to eat some of the stored pine nuts. It does not, however, enter true dormancy. The burunduk is not restricted to Siberia, as it occurs in a wide area of Eurasian conifer forests.
[Photo: Vadim Gippenreiter]
173 The orchid Epipogium aphyllum is one of the many saprophytic (decay-eating) plants that live in the taiga. Many green plants that grow in the temperate latitudes are able, to a greater or lesser extent, to nourish themselves heterotrophically--i.e., from organic material rather than photosynthesis--and those that have developed this ability were able to grow under the dense canopy of the taiga forest. The basic requirement for saprophytism to develop is insufficient light for photosynthesis. When the light levels within the forest are lower than 1% of those in open spaces, green vascular plants cannot grow. Below 1% "forest shade death" starts, and only saprophytic plants can grow. The large amount of dead organic matter accumulated in the leaf litter in the conifer forests favors the development of these plants.
[Photo: Tom Leach / Oxford Scientific Films]
174 Fungi are very important in the conifer forest eco-system. Some fungi decompose the wide range of dead organic matter. Other fungi, such as the honey fungus (Armillaria mellea, shown in the photo), grow as hyphae (threadlike mycelium parts) under the bark of trees and decompose the lignin of the wood. The large quantity of undecomposed acidic leaves that accumulate on the forest soil could end up containing all the soil nutrients--but this does not happen because of the symbiosis that forms between the roots of conifers (and many other vascular plants) and mycorrhizal fungi, without which the trees could not survive. These fungi grow in the leaf-litter layer and help transfer nutrients to the trees they are associated with, before the nutrients are washed from the soil and become unavailable to the plant. This relationship is symbiotic, and in return for the nutrients they supply to the plant, fungi obtain carbohydrates, which they cannot produce.
[Photo: Kim Taylor / Bruce Coleman Collection]
175 The xeromorphic evergreen leaves of conifers are the trees' main adaptation to the harsh boreal climate. They can resist low temperatures, snow, wind, and especially dry conditions. The needle-like shape of the leaves and the waxy covering of the epidermis prevent excessive transpiration, and help to retain the little water available during the winter months, when the external environment is frozen solid. The thick cuticle also plays a mechanical role, acting as an external skeleton, preventing the leaf tissues from collapsing and dying when the water deficit is extreme. The leaves of most conifers are long-lived, and remain on the branches for several years. The only exceptions are the larches (Larix) and some other deciduous species that shed their leaves when the temperature is very cold, and then produce new ones in spring (see figure 178). The photo shows a branch of the Scotch pine (Pinus sylvestris), the most common conifer in the Eurasian taiga. The photo shows one female cone in the center and four male cones, which are yellow and borne around the previous year's shoots. In pines, the leaves are borne on short shoots, the number of leaves in each group varying from species (two in a group in the Scotch pine). In cedars and larches, the leaves are numerous and are borne in clusters.
[Photo: John Lythgoe / Planet Earth Pictures]
176 Many of the flowers in the taiga are white. This makes them more visible to pollinating insects within the gloomy forest. This is exemplified by the common wood sorrel (Oxalis acetosella, above) and the starflower (Trientalis borealis, below). The common wood sorrel shows very interesting flowering and pollination adaptations. It produces normal flowers that open so that the flowers can be pollinated by pollen from other flowers (i.e. chasmogamous flowers), but it also produces cleistogamous flowers, which do not open and are self-pollinated. The normal flowers open during the day and close by night, and show the typical features of insect-pollinated flowers: they are white, large (up to 1 in [2.5 cm] in diameter), with yellow nectar glands on the petals--but almost no insects visit them, and the seeds are formed by self-pollination. The cleistogamous flowers are much smaller (about 3 mm) and are borne, often hidden among the leaf litter, until the end of summer, when the seeds in the chasmogamous flowers start to ripen. The petals of the cleistogamous flowers are reduced to small scales, and their anthers do not open, meaning that the pollen matures within the anther; it then grows out through the wall, towards the stigma and then to the ovaries. Cross-pollination is not very important in the starflower, although its white flowers are visited by butterflies, bees, and nectar-eating flies. It typically reproduces by self-pollination and vegetatively by stolons, which root and produce new shoots at their tips, with a tuber, a new bud, and secondary roots. In autumn, the plant and the stolons die, but the rooted tubers will sprout the following year.
[Photos: P. Clement / Bruce Coleman Collection and Dick Scott / Natural Science Photos]
177 Ants are one of the main seed-dispersing animals, as they always lose some seeds when they transport them to the nest. The photo shows the first activity in spring of an ant nest in the Siberian taiga in the region of the Taz River, before the snow layer has melted. Dispersal by ants, myrmecochory, is a special form of dyszoochory and is very important in many herbaceous plants, such as louseworts (Pedicularis), woodrushes (Luzula), asarabacca (Asarum europaeaum), and Galeobdolon [=Lamiastrum] luteum. As a result of their coevolution with the ant, the seeds of plants dispersed by ants have developed special structures, oil bodies (elaiosomes), that are the favorite food of these animals. Once they have eaten the oil body, the ants remove the seed from the nest, thus helping to disperse it. The seeds of some plants dispersed by ants lack oil bodies. Many seeds of the common wood sorrel (Oxalis acetosella, see fig. 176) are dispersed by ants. For example, in the forests of Sweden a colony of red ants (Formica rufa) was observed to transport 36,480 seeds over the course of the summer (but how many were lost en route?).
[Photo: Andrey Zvoznikov / The Hutchison Library]
178 In autumn, the forests of larch turn yellow, like this mixed forest of spruce (Picea) and European larch (Larix decidua). Larches are almost the only deciduous conifers. Their leaves are a bright light green in spring, turning darker green in summer and orange or dark yellow in autumn, before they are shed. In winter, the tree has no leaves, and the bare, knotty branches are clearly visible. The tender new leaves borne in the spring are softer than those of most other conifers. In addition to being deciduous, larches differ from other conifers in producing their cones in very harsh climatic conditions, where no other tree could reproduce. Like most conifers, larches produce a straight trunk with a conical crown that is narrow and symmetrical. The branch tips frequently curve upwards, and old specimens may have very long lower branches.
[Photo: Sandro Prato / Bruce Coleman Collection]
179 The fireweed (Epilobium [=Chaemaenerion] angustifolium, Onagraceae), seen here growing after a fire in a boreal forest in southern Alaska (USA). Also known as great willowherb, its purplish pink flowers are very common in clearings caused by fires, and it is usually the dominant herbaceous plant in the early stages of succession. Within a few years, fireweed and other invasive herbaceous perennials are replaced by shrub species (especially Acer, Rubus, Berberis, Rhododendron, and Ceanothus), whose seeds have been waiting patiently in the soil. In the first stages of succession, the floristic composition varies greatly, depending on the type of clearing and the intensity of the fire. After this, small trees begin to grow, and as the tree cover closes, the diversity and biomass of herbaceous plants and shrubs decline rapidly. If no fire occurs within the following 200 years, and the forest is not cut, the trees will eventually become totally dominant, forming a dense continuous cover, but these conditions are uncommon.
[Photo: Kenneth W. Fink / Ardea London]
180 Wood-boring long-horned beetle larvae (Cerambycidae) can totally destroy wood and play a very important role in the decomposition of dead tree trunks and branches, thus fulfilling an important role in the functioning of taiga ecosystems. As with some saprophagous beetles, many cerambycid larvae contain bodies in the intestinal wall or in the fat body, called mycetomes, which house saprophytic fungi that fix atmospheric nitrogen and turn it into proteins. These symbionts allow cerambycid larvae to feed on pure cellulose. The species best adapted to living on wood can assimilate up to 20% of the amount of food they consume. The digestive juices of some beetles contain an enzyme, cellulase, that is unusual in other animals. Cellulase can break down cellulose, one of the most resistant components of wood, into simple sugars. Species that do not produce this enzyme, such as Xystocera globosa, can only grow in living wood, which contains about 10% starch and easily digested sugars. The cerambycid beetle shown in the photo belongs to the genus Monochamus, and is infested by gamarid mites, frequent parasites of invertebrates.
[Photo: Scott Camazine / Oxford Scientific Films]
181 The firs (Abies) and spruces (Picea) are the "dark conifers." They are tall evergreen trees with a dense, symmetrical, conical crown. The cones are borne upright on the branches, but when spruce cones are ripe, they hang downwards, as do the cones of the Douglas fir (Pseudot-suga menziesii). The Douglas fir can reach a height of 330 ft (100 m) and a trunk circumference of 16 ft (5 m), and is one of the most typical trees of the moist temperate rainforests of the northwest coastline of North America. Spruces bear cones with scales large enough to conceal the short bracts, but some firs have protruding bracts when ripe, such as the Siberian white fir (Abies nephrolepsis). The Douglas fir also bears cones with long pointed bracts between the scales. In some species, such as the balsam fir (A. balsamea), the scales and bracts of the mature cones disintegrate and fall, leaving a rigid axis that remains on the branches for a long time. The leaves of many firs and spruces release an odor when crushed. These include the Siberian fir (A. sibirica), the black spruce (Picea mariana), and the Siberian spruce (P. obovata), which smell of menthol; the balsam fir, which smells of balsam; while others smell unpleasant, such as the white spruce (P. glauca).
[Drawing: Jordi Corbera]
182 The cones of the silver fir (Abies alba) are cylindrical, 6 in (15 cm) long, and grow upright in groups on the highest branches of the tree, unlike the cones of the spruces (Picea), which hang downwards when mature. The flat scales are covered with pointed bracts, which often curve downwards and drip resin. Each scale bears two seeds with a large triangular wing. The young cones are light green and only turn the dark brown color seen in the photo when they are ripe. The cones of the silver fir are not shed whole, but are shed gradually. In early autumn, the scales and bracts are shed one by one, leaving the bare central axis that remains on the branches during the entire winter.
[Photo: Antoni Agelet]
183 The taiga pines (Pinus) and larches (Larix) are known as "light conifers." Pines have an oval-conical crown when they are young that becomes irregular when they are adult. Pine leaves are rigid needles, and are borne singly or in groups of up to five. The jack pine (Pinus banksiana), the lodgepole pine (P. contorta), and the Scotch pine (P. sylvestris) bear their needles in groups of two, while the western yellow pine (P. ponderosa) bears its leaves in groups of three. The white pine (P. strobus) and the Siberian pine (P. sibirica) bear their leaves in clusters of five. Pines do not shed all their leaves during the winter, although Scotch pines (which are also very widespread outside the taiga) turn yellow in very cold winters. Larches typically have a conical crown, but have thinner and softer leaves than pines, which are borne off the branches in dense clusters. The cones of larches and pines are also different. Larch cones are always solitary and are borne upright, while pine cones may be borne in groups, may hang downwards when ripe, and are cylindrical, narrow, and resinous, such as those of the white pine. The cones of the jack pine are conic-oblong and are usually borne in pairs, while those of the ponderosa pine are ovoid-oblong and not perfectly symmetrical, and are borne singly or in groups of two to four. Some cones open when they are mature and others remain closed. The resin-covered cones of the Siberian pine remain closed, and when ripe the seeds are dispersed when the cones are opened by small rodents or birds. The cones of some larches, such as the western larch (Larix occidentalis) and the Gmelin larch (L. gmelinii), have exserted (protruding) bracts.
[Drawing: Jordi Corbera]
184 The ponderosa pine (Pinus ponderosa) accounts for about one-third of all the conifer forests in the United States. The photo shows ponderosa pines in the deschutes National Forest, Oregon (USA). Forests of ponderosa pine, like forests of lodgepole pine, are not primary forests but are permanent communities that have developed in habitats subject to regular and repeated fires in the understory. In Arizona, there are fires in forests of ponderosa pine almost every year, while in Colorado fires occur every 25 to 40 years. The measures taken to prevent fires, when these forests started to be managed in the early twentieth century, have had unexpected consequences. Because the understory is not burned, conditions favor the growth of young trees, which increase in density. If there is a fire, it does not just burn the dead parts, but burns almost the entire tree. This may have disastrous consequences, as the fire may reach the crowns, where it spreads rapidly and is very difficult to bring under control.
[Photo: Charlie Ott / Bruce Coleman Collection]
185 The female flowers of the European larch (Larix decidua) are sometimes known as "larch roses" because of their color and shape. The cones are about 1 cm across with crimson bract-scales, and are borne at the tip of strong branches five to 10 years old, on a scaly stalk 0.5 cm long. The young branches bear one to six female cones, but the old branches may bear many more. The female cones open about 15 days before the small, yellowish white male cones, which are borne underneath the tips of the smaller branches. As they mature, the female cones turn reddish, then green, and when fully ripe they form an ovoid cone that is darkish in color (see fig. 183).
[Photo: Jan Tove Johansson / Planet Earth Pictures]
186 The mixed taiga of conifers and deciduous trees occupies the central region of the Ural Mountains. The dominant conifers are spruces (Picea obovata), firs (Abies sibirica), and pines (Pinus sibirica). The Scotch pine (P. sylvestris) dominates the lower eastern slopes while the Siberian larch (Larix sibirica [=L. russica]) dominates the higher eastern slopes. The photograph was taken in autumn, when the yellow autumn colors of the different species of deciduous tree stand out against the evergreen conifers. In the higher parts of the Ural Mountains, the plant cover is grassland with a few scattered trees, and tundra in the highest areas. A thick layer of grass prevents tree seeds from germinating, except under the crown of a spreading tree, where the herbaceous cover is much less dense because of the low light levels. In deep wet soils, the thick impenetrable mat of roots produced by the herbaceous plants is a further obstacle to seedling establishment.
[Photo: Konrad Wothe / Survival Anglia / Oxford Scientific Films]
187 The fruit of the white alder (Alnus incana), on a specimen in the Lofoten Islands (Norway). The conelike fruits are 1.0 by 0.8 cm, light green in color, and turn woody and open in autumn. They are not true cones, like those of pines and taxodiums, as the alder is not a coniferous gymnosperm but an angiosperm of the birch family (Betulaceae). Alders appear to be the only plants in the boreal forests that can fix nitrogen, a role occupied by legumes in other biomes. Alders produce nodules on their roots, but the organisms within them are not nitrogen-fixing bacteria, but actinobacteria (also known as actinomycetes because they form filaments and mobile spores, zoospores, like those of fungi). These nitrogen-fixing actinobacteria give the alders a competitive advantage, and they spread readily under the canopy of spruces (Picea), even in the darkest forests. They also bear their leaves before other deciduous trees and shed them later.
[Photo: Xavier Font]
188 The cowberry (Vaccinium vitis-idaea) is a prostrate evergreen subshrub that is characterized by its broad ecological tolerances. It grows in open, well-lit forests and in open spaces (meadows, heaths, and even peat bogs), as it needs a lot of light to produce fruit. In extremely shady conditions, it can grow and flower, but produces almost no seeds. The cowberry has thin underground rhizomes, but maintains its territory and does not invade new areas. The plant is constantly being renewed as the old parts die and new ones grow. Data show that the plant as a whole can live for 100 to 200 years, but this does not refer to the individual clumps (which only live for five to seven years), but to the clone as a whole, that is, all the clumps joined together by a single underground network of roots. The photo shows a fruiting plant among moss and lichen in Katmai National Park, Alaska (USA).
[Photo: Jeff Foot / Auscape International]
189 In spring the solitary flowers of Moneses uniflora (Pyrolaceae) add a touch of color to the understory in wet sites in the taiga. Its flowers and those of other members of the shinleaf family go through two distinct stages over the course of their development. In the first stage, the flowers can only be cross-pollinated by insects (in Pyrola, Chimaphila, and Moneses) or by the wind (in Orthilia). The second stage only occurs if the flower is not pollinated: the stamen and stigma move towards each other and the flower is self-pollinated. This adaptation is useful in taiga conditions, where pollinating insects are unreliable because climatic conditions may make flight impossible some years and the wind speed is not very great.
[Photo: John Mason / Area London]
190 The stair-step moss (Hylocomium splendens ) may sometimes be the totally dominant moss in boreal forests. The photo was taken in a forest in the United States, and also shows a wood sorrel leaf (Oxalis, see fig. 176). Hylocomium splendens requires a lot of moisture, but in favorable conditions it easily displaces the other bryophytes. This species is easily identified by its reddish, flat, pinnate (featherlike) caulids (stems) with phyllids (leaves) that are green, transparent, and oval with toothed edges and an elongated tip. Among the phyllids there are many small squamiform (scaly) structures, the paraphylls. This moss is abundant in conifer forests, but it also forms thick carpets in moist shady sites in many other parts of the world. There are reasons to believe that its broad distribution is the result of the spread of the spruce forests about 6,000 years ago.
[Photo: Deni Brown / Oxford Scientific Films]
191 Polytrichum and Sphagnum moss covering the soil of a forest in Quebec (Canada). Polytrichum is the dark green moss in the photo, and grows upright (acrocarpous) to a height of a few centimeters. Then it produces a very distinctive capsule (sporangium), as can be seen in the photo. The sporangium is usually covered by a hairy calyptra (caplike spore case covering) that is yellowish or light brown. The top of the sporangium is surrounded by a ring of teeth (the peristome). When the lid (operculum) falls off the capsule, it looks like a small version of the capsule of a poppy. The capsules are borne at the tip of a relatively long seta (stalk) and are usually visible in summer. In the photo, Sphagnum is the light green moss. Sphagnum mosses may be acrocarpous or pleurocarpous (bearing fruit at the end of a stalk or on the side), in which case they do not usually grow so high. The capsules of Sphagnum mosses open violently, expelling the operculum (see fig. 195) and launching the spores several centimeters. The spores cannot germinate in the absence of certain fungi, with which they form a symbiosis. Both Polytrichum and Sphagnum mosses grow on wet or waterlogged soils, usually acidic: they both grow outside the taiga biome, in sites with a cold or temperate and wet climate.
[Photo: Xavier Font]
192 The terricolous (ground-lying) lichens of the genus Cladonia, like these covering the soil in the Siberian taiga in the Taz River region, live mainly on bare soil or on rotting wood. They have two type of thallus (nonvascular plant body), one (the primary thallus) that grows on surface of the substrate, and one (the podetium) that grows upright, as can be seen in the photo. The podetia are a few centimeters tall and are usually hollow and may be branched. If they are fertile, they produce bright or dull red apothecia (cuplike spore sacs) at the tips. These lichens are the only members of their family, the Cladoniaceae, that live in the cold and temperate biomes, where they have diversified greatly. The other members of the group live in tropical or subtropical areas.
[Photo: Andrey Zvoznikov / The Hutchison Library]
193 The straight upright trunks of these conifers in a forest in Sweden add rhythm and symmetry to this part of the taiga landscape. Their straight upright trunks and the almost geometric arrangement of their branches make conifers excellent sources of wood. Not all forests are as uniform as this, only the oldest ones that consist of trees of the same age. In the taiga, the upper canopy is regularly renewed. In an old forest, it is very hard for a sapling to grow tall, as they are vulnerable to all the potential adverse conditions and have to compete with the adult trees and with the herbaceous plants and mosses of the understory. Often, not a single sapling survives out of an entire cohort, indicating that the forest has reached stability, and then it resembles the forest in the photograph. Only when there is some catastrophic event, such as a windstorm that blows down many of the older trees, can the saplings make it to a gap in the canopy.
[Photo: Ake Lindau / Ardea London]
194 Terricolous lichens play an important role in the taiga biome. They usually require a lot of light, and preferentially grow in clearings, where they sometimes completely cover the soil, as in this photo showing Cladonia alpestris in a grove of Scotch pine (Pinus sylvestris) in northern Karelia. Some other lichens can grow in poorly lit sites and live in the densest areas of forest, generally on tree trunks (lignicolous lichens) or on mosses.
[Photo: Vadim Gippenreiter]
195 Sphagnum mosses and insectivorous plants are frequent in peat bogs and other wet and waterlogged sites, where they often grow together. The upper photo shows a sphagnum moss with its developing capsule. The sphagnums are an ancient and specialized group of mosses that grow upright, form a soft dense carpet, and are usually light green. They are well adapted to absorbing and retaining water by means of special cells in the caulids (stems) and phyllids (leaves), called hyalocysts. The capsules are clearly visible in this photo (taken in Michigan, USA), and are dark, round, and lack a calyptra. When they are ripe, the capsule walls slowly contract, increasing the pressure inside until the operculum (spore case lid) is blown off and the spores are ejected. The lower photo shows a purple pitcher plant (Sarracenia purpurea, Sarraceniaceae) in an acidic bog in southeast Manitoba (Canada). Insectivorous plants have evolved as a response to the shortage of nutrients in the acidic soils where they grow. Pitcher plants perform photosynthesis, but their leaves have been modified into pitchers that are full of digestive juices, forming a very effective trap for insects and other small invertebrates.
[Photos: John Shaw / Bruce Coleman Collection and Geoffrey Scott]
196 Peat bogs are frequent within the taiga conifer forests, as can be seen in this aerial photograph of a forest in British Columbia (Canada). The bog vegetation consists exclusively of mosses and hydrophytic plants. Because the environment is anaerobic, the plant remains do not decompose completely, and accumulate as peat. The bogs in the taiga are raised bogs that form on acidic oligotrophic soils, and they consist essentially of Sphagnum mosses (see fig. 197).
[Photo: Steve Pridgeon / Natural Science Photos]
197 Transformation of a bog into a forest. Bogs are very common in boreal conifer forests. They form when the lakes occupying depressions left by glaciers gradually fill up with peat, which is partially decomposed plant remains, mainly derived from sphagnum mosses. In the beginning, the peat is only present in the bottom of the lake, which still contains a large amount of water (top drawing). Sphagnum mosses gradually colonize the edges of the lakes and start to cover it, reducing the size of the sheet of water (middle drawing), which is often divided into smaller pools of water (see fig. 196). As more soil forms in the bog, rushes advance behind the mosses, until eventually shrubs and trees appear (lower drawing). The large conifers are the last to establish themselves in the bog, when the bog has almost disappeared. As a result of this process, many large areas of boreal conifer forest are now growing on former bogs.
[Drawing: Jordi Corbera, based on Ricciuti, 1990]
198 The moose (Alces alces) is the lord of the taiga. Tall and majestic, it grazes in the muddy sites with low vegetation on the edges of the forests, where the forest merges into the tundra. The photo shows an adult male (a bull), which can reach a height of 6.6 ft (2 m) at the withers, a length of 10 ft (3 m), and a weight of 1,100 lb (500 kg), meaning the moose is the largest member of the deer family (Cervidae). Their very large antlers can be 79 in (2 m) wide and weigh 44-55 lb (20-25 kg). A healthy adult male can repel wolves, and its surprisingly sharp hooves can seriously wound any animal attacking it. But moose are very peaceful and shy animals that move away as soon as their well-developed senses of smell (note the large nose) and hearing detect the presence of intruders. In summer they eat water plants and shoots of willows (Salix), birches (Betula), and aspens (Populus), building up reserves for the winter. In the winter everything is covered in snow, and they can only strip trees of their bark. Moose are especially fond of sites with water, where they submerge for hours to escape the bloodsucking insects that are so abundant in the taiga forests.
[Photo: Martin Grosnick / Ardea London]
199 The stoat (Mustela erminea) is present in throughout the boreal conifer forests of Eurasia and North America. It feeds on small rodents, birds, eggs, and insects, and makes its den in cracks in rocks, in tree stumps, or using an abandoned rodent burrow. The stoat lines its den with dry grass and the skin and feathers of its prey. Though it can be active at any hour of the day, the stoat is mainly nocturnal. It varies greatly in size, and Eurasian stoats are larger than North American ones. Their body size also varies with latitude, in accordance with Bergmann's Rule. The male stoats from northern North America are 9 in (24 cm) long, including the tail, and weigh about 7 oz (200 g), while those from more southerly regions of North America are only 7 in (17 cm) long and only weigh 2 oz (60 g). The stoat is perhaps best known for its winter coat, which is completely white except for the black tip of the tail, and is thicker and longer than the summer coat. Known as ermine, its winter coat has long been highly valued, and was formerly used to make and decorate ceremonial gowns for important people (see p. 397). Judges in Britain used to use ermine in their gowns. In 1937, Canada exported 50,000 ermine pelts to Great Britain for the coronation of King George VI, for use in the robes of judges and court dignitaries. Demand has fallen since then, as about 300 pelts are necessary to make a gown, and labor costs make this very expensive.
[Photo: Danegger Manfred / Jacana / Auscape International]
200 A male spruce grouse (Canachites (=Falcipennis) canadensis) displaying its colorful plumage to attract females in a forest in Alaska (USA). This member of the grouse family (Tetraonidae) is one of the few birds that can be considered to be typical of the taiga and restricted to it, as its range coincides closely with the belt of conifer forests running across North America. It is mainly found in forests of pines (Pinus banksiana, P. contorta), but it also occurs in forests of spruces (Picea mariana, P. glauca, P. rubens), and in forests of firs (Abies balsamea, etc.). It prefers young, very dense forests with a well-developed shrub layer, although in summer it descends to the understory to look for the berries of Vaccinium or other small shrubs. It is very agile as it moves among the compact branches of these dense forests. Many authors consider the spruce grouse to be one of the most primitive members of the grouse family, and closest to the ancestor from which the other members of the family have evolved (see fig. 203).
[Photo: Dicon Joseph / Natural Science Photos]
201 The salamander Sala-mandrella [=Hynobius] keyserlingi is one of the few amphibians that live in the taiga forests. It lives out of the water, and is mainly active in the early morning and late afternoon. During the short breeding season (May-June) it moves to a small pool of water. The mating of these animals is of great interest. The males choose a pool where the eggs can thrive, and with their mouth open and moving their tail, they release their pheromones into the water to attract the females. The females lay two groups of eggs (one from each oviduct) in the chosen site, and the male fertilizes them with his sperm. The clumps of eggs remain on the submerged vegetation until the larvae hatch. This species is present in the tundra as well as the taiga, making it the most northerly amphibian. It is very resistant to cold, and remains active and can move with agility at temperatures around 32[degrees]F (0[degrees]C). During the cold months of winter, when it is hibernating, it can resist temperatures as low as -31[degrees]F (-35[degrees]C).
[Photo: Milos Andera / Natural Science Photos]
202 The two-barred crossbill (Loxia leucoptera) is distinguished by the two white stripes on the wings of the males. It only eats conifer seeds and is so dependent on them that its breeding cycle is determined by the ripening of their cones, and it regularly makes local migrations in search of food. Crossbills, members of the finch family (Fringillidae), are called crossbills because the mandibles of the bill are crossed, a feeding specialization. The bill functions as a lever to open the scales of the cones, and as tweezers to remove the seeds. The bills of the different species of crossbill vary in their size and shape, depending on which conifers occur in their range. The two-barred crossbill feeds on larch (Larix) cones, and its bill is thinner than that of the parrot crossbill (Loxia pytyopsittacus), which specializes in removing pine nuts from pine (Pinus) cones and has a thicker, rounded bill. All the crossbills also make use of their bills to move between the branches, in a similar way to parrots.
[Photo: Edgar Jones / Ardea London]
203 The capercaillie (Tetrao urogallus) is one of the members of the grouse family (Tetraonidae) that is found furthest north (see fig. 200). But it is not exclusive to the taiga biome, as it also occurs in other conifer forests and mixed forests. The male, which has dark shiny plumage, is highly territorial and it defends its territory with cries and spectacular displays. It normally begins with warning calls from high up on a branch, and then descends noisily to the ground, adopting the posture seen in the photo, with the neck stretched upward and the throat feathers raised, the tail upright and displayed in a fan, and the wings lowered to show the white patch on the shoulders. Then it starts to make its distinctive call. If a rival male ignores these warning calls and enters the occupied territory, a ritualized struggle may take place, in which both threaten to peck the other without actually doing so. This makes the intruder aware that he is in another male's territory. Even so, he may not go away, and then a genuine struggle begins, in which each bird tries to immobilize the other by using its bill to grasp the other's neck or neck feathers. These struggles rarely end in the death of the loser, but they are sometimes so wounded that they die shortly afterward.
[Photo: Sylvain Cordier / Jacana]
204 The Eurasian nuthatch (Sitta europaea, family Sittidae) is one of the most active small birds that lives permanently in the conifer forests. It runs up and down tree trunks, searching the bark for insects, in irregular bursts in any direction (even headfirst downward), and using only its strong legs without needing to support itself with its tail, as woodpeckers (family Picidae) do. This nuthatch nests in holes in old trees, preferably deciduous trees. Before settling in, it seals any cracks with mud, and if the entrance is too large, the bird makes it narrower with mud. If it cannot find a suitable hole, it adapts a nest abandoned by another bird. Like other nuthatches, the Eurasian nuthatch does not show sexual dimorphism, and the male and female are identical.
[Photo: Peter Laub / Ardea London]
205 A North American porcupine (Erethizon dorsatum) high up in a spruce (Picea) tree. It is a terrestrial species, but in autumn and winter, when it cannot find any fruit, leaves or shoots at ground level, it climbs up the trees (to almost 66 ft/20 m) in search of food (mainly the tender needle leaves of conifers). The upper part of the North American porcupine's body is covered with long thick quills, about 0.1 in (2 mm) in diameter and 3 in (75 mm) long, amid other long rigid hairs. Its legs are well adapted to arboreal life, and have four digits on the forelegs and five on the hind legs, all of them bearing strong curved claws. Though it prefers mixed forests with deciduous trees, it is adaptable and is at ease in other habitats, including conifer forests and even the tundra.
[Photo: Tom Kitchin and Vicki Hurst]
206 The relationship between hares (Lepus) and their predators, lynxes (Felis [=Lynx] lynx), is one of the most widely used examples to explain how predator populations fluctuate in response to the changes in the number of their prey. Populations of the blue hare (Lepus timidus) show cyclic fluctuations lasting eight to 11 years. The population may rise or fall several hundredfold, and in some regions of Yakuty in eastern Siberia it may fluctuate by a factor of a thousand. Analysis of the number of animals caught every year since 1800, recorded in the accounts of the Hudson Bay Company, has revealed the existence of correlated cyclic fluctuations in the populations of the snowshoe hare (L. americanus) and the lynx. It has also shown that the peaks in the lynx population generally occur one or two years after the hare population peaks. The number of carnivores is closely related to the number of their prey, though the role that the lynx plays in reducing the hare population is not so clear. In regions where there are no lynxes, the same cyclic fluctuations occur in the hare population as in regions where both are present. Furthermore, in the years when the hare population peaks, in the winter the young vegetation of the forests (under the snow layer) is almost totally destroyed, and this only food available. As a result, in the years when the hare population peaks, hunters often find specimens that have starved to death.
[Photo: Tom Ulrich / Oxford Scientific Films]
207 Streams with dystrophic waters flow through the conifer forests of the peat bog region of northern Karelia, when the snow and the ice in lakes and bogs starts to melt. Humic acids from peat bogs darken the water and lower its pH. Dystrophic waters are very typical of the taiga, and are unfavorable for phytoplankton growth, as they contain few nutrients, and so zooplankton and small benthic (bottom-dwelling) organisms are not common.
[Photo: Vadim Gippenreiter]
208 The middle stretch of the Lena River, between its tributaries the Vitim and the Aldan, is about 935 mi (1,500 km) long and flows through dense, swampy conifer forests. The Lena River, in eastern Siberia, is one of the largest rivers crossing the taiga. It is 2,734 mi (4,400 km) long, and its average annual discharge of water at its mouth exceeds 4.2 million gal/s (16,000 [m.sup.3]/s). Its basin covers some 1 million sq mi (almost 2.6 million sq km), most of it covered by conifer forests. Like most of the rivers that cross the taiga, the Lena's headwaters are in mountains, in a small lake on the western slope of the Baikal Mountains. It then crosses Irkutsk and Yakuty-Sakha and flows into the Laptev Sea in the Arctic Ocean, where it forms a large delta. Its final stretches emerge from the taiga and flow through the tundra landscape. The Lena's volume of flow is low in winter and high in summer, when it bears the water released by the spring melt and from the high precipitation. The weather is so cold that many stretches of the Lena freeze from October to June, and some stretches are permanently frozen, which is a severe problem for the fauna and flora living there. Even so, the river supports a rich plant and animal life, including a wide range of fish.
[Photo: Vadim Gippenreiter]
209 The salmonids (family Salmonidae) are the most abundant fish in the waters of the taiga. Salmonids vary in their morphology, but are easily identified as they have a small additional fin, the adipose fin (a flap of fatty tissue), on their back, located between the dorsal fin and the pelvic fin (usually closer to the pelvic fin). The life cycles of salmonids are also very variable. Some species have anadromous forms, and migrate upstream from the sea to spawn (see fig. 210). Some salmonids, such as the grayling (Thymallus thymallus), which is shown in the photograph, take lacustrine forms that live permanently in lakes or rivers, while other species live permanently in lakes and spawn in rivers. Sometimes a single body of water may contain forms with very different life cycles, and this makes the details of their taxonomy difficult but interesting.
[Photo: Lutra / NHPA]
210 Salmonids are a good food source for brown bears (Ursus arctos) and other animals in the taiga forest, as they are the most abundant fish in the waters of this biome (see fig. 209). The brown bear from Alaska shown in the photograph has caught a female Pacific salmon (Oncorhynchus) full of eggs that was trying to swim upstream to spawn. The salmonids do not need to eat while migrating upstream, as they accumulate reserves before setting off, and this makes them a rich foodstuff for the bears. The salmonids may have to jump rapids or even waterfalls and swim long distances against the current--as much as 2,485 mi (4,000 km) in the Yukon River and its territories in the case of the chinook salmon (Oncorhynchus tschawytscha)--and so the fish need to lay down large energy reserves. After spawning the exhausted fish die and sink to the bottom or wash up on the banks of the rivers. These weakened and dead fish have lost up to 40% of their weight, but are still a very desirable catch for bears and other animals that eat them.
[Photo: Tom Walker / Planet Earth Pictures]