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Chapter 3: Biomes.

Ecosystems occur in a different geographical setting where latitudinal and elevation differences combine with proximity to mountain ranges and coastline to produce four unique sets of indigenous plant and animal species. Although different, these organisms are all typical of deserts in general. The Chihuahuan, Sonoran, Mojave, and Great Basin deserts of North America, the Sahara, Gobi, Namib, Patagonian, and all the other deserts of the world make up the desert biome. The plant types and the abiotic environmental factors common to all these otherwise unique ecosystems are the basis for describing this biome.

Objectives

After completing this chapter, you should be able to:

* Understand the biomes in the worldwide ecosystems

* Identify the plants and their relationship to tropical rain forests, savannas, deserts, grasslands, temperate deciduous forests, coniferous forests, tundras, or aquatic biomes

Key Terms

biomes

terrestrial biomes

tropical rain forest

canopy

epiphytes

holdfast

lianas

thorn forest

savannas

succulents

spines

ephemeral

desertification

mesic

grasslands

xeric

temperate deciduous forest

chaparral

taiga (snow forests)

tundra

permafrost

marine biome

littoral zone

limnetic zone

profundal zone

thermal stratification

eutrophic

algal blooms

eutrophication

Biomes are worldwide groups of similar ecosystems. An ecosystem is a balanced and self-perpetuating assemblage of all the living organisms and the nonliving environmental factors in a given area. Ecosystems differ in size according to the geographic limits and climate ranges that control the types of living organisms within them (see Figure 3-1). The two broad categories of biomes include the terrestrial biomes and the aquatic biomes.

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Terrestrial Biomes

Only one-fourth of the earth's surface is land, and yet the vast majority of all plant and animal species are found there. Continents range from equatorial to polar latitudes and from sea level to more than 9,000 m in elevation. Climatic and soil differences, combined with ranges in latitude and elevation, result in a phenomenal number of ecological settings for plants and animals to inhabit. Even though this variation results in many different plant types, reasonably accurate and useful assemblages or categories according to vegetation can still be recognized. The number of distinct terrestrial biomes varies according to the authority, but most commonly seven are described.
BIOSPHERE EXPERIMENT

Biosphere 2 is a three-acre structure built to be an artificial
closed ecological system in Oracle, Arizona, near Tucson.
Constructed between 1987 and 1989, it was used to test if and how
people could live and work in a closed biosphere, while carrying
out scientific experiments. It explored the possible use of closed
biospheres in space colonization, and also allowed the study and
manipulation of a biosphere without harming the earth's biosphere.
The name comes from the idea that it is modeled on the first
biosphere--the earth. Funding for the $200 million project came
from Edward Bass, a Texas oil and investments businessman.

The project conducted two sealed missions: the first from September
26, 1991, to September 26, 1993, and the second for six months in
1994. In 1995, the Biosphere 2 owners transferred management to
Columbia University. Since 1996, hundreds of college students have
spent at least one semester at the Biosphere 2 Center campus. The
site has its own hotel and conference center. Columbia has since
divested itself of all Biosphere-related responsibilities. While
the structure is no longer maintained in an airtight state,
Biosphere 2 is still open for tours and plans to be open in the
future.

Biosphere 2 was an achievement of engineering more than science.
The above-ground physical structure of Biosphere 2 was made of
steel tubing and high-performance glass and steel frames. The
window seals and structures had to be designed to be almost
perfectly airtight, such that the air exchange would be extremely
slow, to avoid damage to the experimental results. To deal with
atmospheric expansion during the daytime and contraction during the
nighttime, diaphragms permitted the building to grow larger during
the day and shrink at night, thus, keeping the atmospheric pressure
inside constant.

Whether considered a success or a failure, an interesting
consequence of the experiment is that it showed the difficulty of
copying the functions of the earth's biosphere with human knowledge
and technology. Despite an expenditure of over $200 million, this
attempt at a new biosphere did not sustain eight humans for even a
relatively short period of time. Biosphere 2 developers and
experimenters learned that small, closed ecosystems are complex and
susceptible to unplanned events, and that such events occur
unexpectedly. Perhaps a lesson for future use!

The property, which is outside of Tucson, Arizona, is scheduled to
be redeveloped for a planned community.


Tropical Rain Forest

Found predominantly at or near the equator, a tropical rain forest, as shown in Figure 3-2, is characterized by having 200 to 500 cm of precipitation per year with some areas occasionally having over 1,000 cm in a year. Because of the equatorial location, there is no seasonal but rather a continual growing season with cold periods. The daily temperature is minimal because of the insulating effects of the water, which gains and loses heat very slowly. Maximum daytime temperatures of only 30[degrees]C are typical, but the heat is oppressive due to the high humidity. Diurnal temperature fluctuations are on the order of only 5[degrees]C.

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This consistent temperature warms the atmosphere, therefore creating a moist climate with no extreme temperature fluctuation and no cold season provides a stable and favorable environment for plant growth. The vegetation is dominated by tall (50 to 60 m), broadleaf, evergreen trees that branch near the crown to form a solid layer, or canopy, of leaves. Because of the density of the trees in the forest, light availability is the primary limited factor to plant growth below the canopy. Competition for light has resulted in tall, fast-growing trees that always have leaves on them. Certainly, leaves do become old, diseased, or shaded so that they die and fall to the forest floor, but this happens throughout the year rather in accordance with seasonal variations that affect most deciduous trees.

The tree canopy shades the forest floor so that smaller trees and herbaceous plants cannot survive; therefore, the floor is open, dark, and damp. Where the canopy is broken and light penetrates, dense undergrowth of plants results. This may occur at the banks of a river, where an old and diseased tree falls, or in areas where the frost has been cleared.

The warm, moist conditions of the tropics are ideal for the bacterial and fungal processes that decompose dead plants and animals and return their rich organic nutrients to the soil. Because of the density of living plants needing these nutrients, deep topsoil rich in these decomposition products is not able to accumulate. The trees therefore have shallow root systems that quickly take water and nutrients out of the soil to be used for new growth. Support roots aid the stabilization of the shallow-rooted trees.

In addition to having nutrients confined to shallow topsoil, many tropical soils are lateritic--they have certain metals in combination with large amounts of clay that pack very tightly if not kept loose with organic materials. Annual plowing for cultivation and crop harvesting removes the source of organic matter. Without plants to take up the larger amounts of water, rainfall causes leaching of existing nutrients out of the topsoil. The lateritic soils become compact and harden, so that after only a few years of cultivation such areas are as hard as concrete and further cultivation or revegetation is impossible. Therefore, clearing vast tropical forests for cultivation does not solve world food shortages, and it is an ecological disaster, resulting in permanent loss of these areas. The loss of this vegetation affects not only ecological balance but broad climate patterns as well. In addition, the extinction of species eliminates their unique genetic potential.

The tropical rain forests are the oldest vegetative biome because their equatorial position has shielded them from the effects of past periods of glaciation. This great age (some 200 million years long), combined with climate stability and abundance of all resources, has produced phenomenal diversity in plants and animals native to these areas. Although tropical rain forests are relatively unexplored and poorly understood biologically, there are still more species of plants and animals described from this biome than in all the other biomes combined (see Figure 3-3). It would be sad indeed to see this unparalleled natural resource destroyed. Currently, these forests are being destroyed faster than they can be studied and understood. This situation has resulted in a race against time to classify and catalogue tropical plants before they become extinct. Soon a herbarium (plant museum) may be the only place to find samples of the diverse tropical vegetation.

Tropical diversity is exemplified in plant groups that have developed unusual methods of survival. Epiphytes are plants that grow with roots attached to another plant by a holdfast. These plants do not harm their benefactors in any way; they are not parasitic, but rather coexist with them. Bromeliads and orchids include many common epiphytes. Long lianas (hanging vines) use the trunks and branches of the tall forest trees to climb into the canopy, where they produce leaves for photosynthesis. Since these vines are rooted, they obtain water and nutrients from the soil. They often grow from one tree to the next, trailing long looping sections between trees. These would be ideal for Tarzan to swing from tree to tree without having to touch the ground. Many native primates do in fact travel throughout the forest in such a manner. Entire communities of animals inhabit the forest canopy; exotic and beautiful plants, birds, insects, and other organisms are all part of this spectacular biome. Although the given species change from one tropical rain forest to the next, the general picture is very similar.

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Nearly half the forested areas of the earth are tropical rain forest. The Amazon River Basin of South America, the Congo Basin in Africa, and certain areas of Southeast Asia are the three largest areas; but, tropical forests also exist in Australia, Central America, New Guinea, the Philippines, Malaysia, the East Indies, and many of the Pacific Islands. Since many of these forests are found in countries with overpopulation and food shortage, they are in danger of being cleared and cultivated.

Occasionally rain forests occur in regions where one would not expect to find them, such as the Olympic Peninsula in the state of Washington. Located on the Pacific in a cool temperate region and warmed by the Japan Current, the land mass receives constant precipitation. Many of the species found there are of genuine rain forest origin.

Savannas such as the one in Figure 3-4 are usually found between tropical rain forests and deserts. Their proximity to either of these two areas greatly affects the annual rainfall. The "normal" range often falls between 80 and 160 cm per year. Because of their latitudinal distance from the equator, savannas have seasonal temperature fluctuations even though they have no true cold period. Precipitation is also scattered; long dry periods, which are often very hot, are followed by heavy warm-season thunderstorms. There is little rain during the cool season. This lack of rainfall for a prolonged period apparently excludes many species that otherwise might occur there and gives the savanna its characteristic vegetative composition.

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Savannas are primarily grassland with scattered deciduous trees. Since there is no true cold season, these trees generally lose their leaves during the long dry season each year, leafing out again when the rains come, and generally flowering while leafless. There are few annual plants in the savannas because of the density of the perennial grasses. A number of perennial herbs do thrive here, emerging from underground heat-resistant bulbs after the rains begin. The trees have smaller leaves than do those in the tropical forests. Because it is necessary to reduce water loss in the dry season and because light is not a limiting factor, the increased surface areas characteristic of tropical forest species are not found. In savanna areas that border the drier desert regions, the trees are smaller, denser, and often thorny. These areas are called thorn forest.

Some of the most extensive savannas are in the central and eastern African veldt (pronounced "felt"), as seen in Figure 3-5. A large variety of animal life, including many species of antelope, zebras, giraffes, elephants, and their predators depend on the grass species found there. Other savannas are found in Brazil, India, Southeast Asia, northern Australia, and North America.

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The maintenance of this grassland component requires periodic burning. Burns clear the dead dry grass so that the new lush grass can grow when the rains come. Fire also keeps the trees thinned and scattered by removing young seedlings. The mature trees often have a thick bark, and since they are leafless during the dry season, minor trunk scorching is usually the only damage done. If young tree seedlings were allowed to grow to a size that would protect them from these grass fires, their density would form a canopy that would shade out the undergrowth of grasses and perennial herbs. For the proper balance of grasslands, animal forage, and scattered trees for shade and nesting sites, fires are essential. Lightning is a natural source of fire in the savannas as well as in other biomes.

For centuries, inhabitants have set fire to grasslands, recognizing its role in the balance of plant and animal populations. In some parts of the world, including North America, controlled burns are used to bring about the same kind of ecological manipulation that occurs in nature. If kept under control, these burns are not a misuse of fire. However, this practice has come under scrutiny because of the devastating forest fires that started as controlled burns. These fires have accidentally destroyed millions of acres in the forests of North America. The study of fire ecology is an important part of overall management in several biomes.

Most of the desert areas of the world are found in a belt from 20[degrees] to 30[degrees] north and south latitude with rain-shadow deserts at other latitudes (see Figure 3-6). A few deserts have no vegetation and lots of shifting sand dunes, such as parts of the Sahara, but most have scattered low-growing vegetation.

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Deserts receive 25 m or less average annual precipitation. The driest deserts, including the Sahara, have less than 2 cm average rainfall, and all desert areas can have extended droughts with no rainfall for several years. In the Atacama Desert in northern Chile, for example, the total rainfall over a period of 17 years was 0.05 cm, with only three showers in that 17-year span that were heavy enough to measure. It is important, therefore, to note that the rainfall figures mentioned previously are average over many years, with considerable fluctuation possible. When the rains do come, they can be heavy enough to produce flash floods or can be light showers.

Because of the dryness, diurnal temperature fluctuations are great. It is not unusual to have a 25[degrees]C or greater drop in temperature at night, since there is little moisture to hold the heat produced during the day. Once the sun is down, the heat source is gone. Conversely, it warms up rapidly after sunrise. It is common for vacationers, camping in the North America desert in the summer months, to bring no blankets or sleeping bags. A thin sheet provides little warmth, and these campers usually spend a sleepless night in the car!

Desert temperatures also follow the seasons, with occasionally harsh winters, freezing temperatures, and snow. Elevation, proximity to the coast, and latitude also combine to produce some "warm" deserts, which seldom have freezing temperatures. Such is the case of the Sonoran Desert of North America, the only North American desert having the giant saguaro cactus, a plant that cannot tolerate the freezing temperatures found in the Chihuahuan, Mojave, and Great Basin deserts.

Desert vegetation is amazingly diverse and uniquely beautiful, falling into three major categories based on the physical adaptation that allow their survival in times of drought. Succulents are plants that store water in thickened leaves or stems and protect that supply with thorns and spines. Most of the cactus family are stem succulents with either jointed pads, as in the prickly pears and chollas, or barrels with a single thickened stem. The spines are modified leaves that protect the cacti from herbivores in search of moisture. Other plants, called leaf succulents, store water in modified leaves. The century plant (Agave), Spanish dagger (Yucca), and the low-growing Sedum and Portulaca are common examples, although many different plant groups have succulent members adapted to dry zones, see Figure 3-7.

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Low-growing, small-leafed shrubs with spines or sharp branches survive in the desert by conserving water in their woody stems and reducing water loss through reduced leaf surface area. In addition to or instead of spines, these shrubs often have foul-tasting compounds in their leaves to discourage herbivores from browsing. Creosote bush (Larrea) and tar bush (Flourensia) are such examples. Both spines and thorns protect cat claw (Acacia) and mesquite (Prosopis).

A third group of desert plants survives in a totally different manner. Instead of storing water, they wait until an adequate supply is available. They exist as heat- and drought-resistant seeds that will not germinate unless a heavy rain washes off a self-produced chemical inhibitor to germinate. The rain also provides ample soil water for these plants to germinate, grow to maturity, flower, reproduce, and set seeds before the supply of water is gone. This all happens in a short period of time--possibly a week or two, at most a few weeks, but certainly in one season. These desert annuals are sometimes called ephemeral because of their short life cycle. Their seed may have to wait as long as 20 or 30 years before conditions are right for germination and completion of their life cycle. Consider the fortitude of a seed able to survive so long in desert soils where surface temperatures are regularly over 60[degrees]C in the summer. If a year is wet, the flowering desert is as beautiful as any area.

The largest and driest desert in the world is the Sahara, followed in size by the Australian desert. Figure 3-6 shows where many of the other major deserts are located, including the four North American deserts, the Gobi of Mongolia, deserts in India, the Middle East, other parts of Africa, and in South America. Approximately onethird of the earth's surface is arid or semi-arid. Deserts are recent in origin, some no older than 12,000 to 15,000 years and possibly none older than 5 to 6 million years. The relative youth of deserts is even more striking when compared with the age of the tropics, which have existed for some 200 million years.

Deserts have expanded due to worldwide drying trends since the last glaciation period, which ended some 15,000 years ago. Human activities are accelerating this process of desertification (conversion to deserts) in many areas through overgrazing, cultivation of marginal areas, and general removal of plant life and water for our own uses. It is estimated that the Sahara is advancing to the southwest at approximately 17 km per year. With wise use and controlled developmental studies being conducted by government agencies, universities, and private groups in essentially every country containing desert lands, some of the most interesting and promising research is being done in the area of desert agriculture. There have been attempts to develop strains of already existing crop plants that require less water, can withstand higher temperatures, and can tolerate a great level of soil salinity.

Some studies are also aimed at finding commercial uses for plants that naturally occur in desert regions and are therefore already well suited to desert climates. It is probable that this latter approach has the greatest potential, and plants such as saltbush (Atriplex), prickly pear Cactus (Opuntia), and ironweed (Kochia), show great promise as forage plants. Guayule (Parthenium argentatum) for rubber, jojoba (Simmondsia chinensis) for oil, and several plant species for fuel biomass also have significant potential for commercial use.

Below ground desert dwellings using solar energy have been designed for experimental habitation, as have large, plastic dome greenhouses for single-family food production. Industries not requiring water can use desert land, which is plentiful and inexpensive for building new factories. Almost one-third of the earth's surface may some day be used more extensively than even before.

Grasslands

This biome is dominated by areas of perennial grasses (see Figure 3-8). Predominately in temperate latitudes, grasslands receive from 30 to 150 cm annual precipitation and have distinct seasons. Temperatures are often above 40[degrees]C in summer and far below freezing in winter, with occasional extremes. The annual precipitation is usually distributed throughout the year with occasional summer peaks. On the more mesic (wetter) end of the range, grasslands grade into savannas or temperate deciduous frost, whereas on the xeric (dry) end, they grade toward deserts.

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Because of the matted turf of fibrous grass roots, there are very few annual plants in the grasslands. The herbaceous plants found there are mostly perennial with underground storage structures such as tubers, bulbs, and rhizomes. Occasional prairie fires help maintain the integrity of the grasslands (much as in savannas), but in the dried grasslands the invasion by desert species is common.

Overgrazing of such areas has increased their desertification, allowing mesquite, cacti, and weedy annuals to become well established in the thinned turf. There are few native types of grassland (prairies) intact in the United States, most of these areas having been overgrazed by domestic animals or cleared for cultivation, as seen in Figure 3-9. Other major grasslands are found in Eastern Europe and parts of Russia, Central Asia, Argentina, and New Zealand.

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The dust bowl of the 1930s in the Oklahoma and Texas panhandles was caused by an extreme drought that drastically reduced productivity, and the few water wells were not adequate for irrigation. Without the cultivated plants to hold down the soil against the ever-present spring winds, the topsoil was literally blown away, as Figure 3-10 shows.

This biome is found in all the major continental areas of the Northern Hemisphere but is almost absent from the southern hemisphere (see Figure 3-11). The average precipitation of 75 to 225 cm per year is usually scattered throughout the year. Warm summers and cool to cold winters are typical. These areas rarely have droughts and only limited periods of snow and subfreezing weather. The trees drop their leaves each fall, hence the name temperate deciduous forest (see Figure 3-12a).

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[FIGURE 3-11 OMITTED]

Because the forest leaf canopy is not intact year-round, the ecology of these regions is unique. Low growing, understory vegetation dominates in the early spring before the trees completely leaf out to form the shade-producing canopy. Once the canopy is formed, the forest trees are the dominant vegetation, followed again in the fall by a second-story understory assemblage that develops in response to the available light resulting from the leaf drop (see Figure 3-12b). Thus three-district vegetational assemblage exists during the growing season, resulting in a complex ecological situation.

[FIGURE 3-12a OMITTED]

[FIGURE 3-12b OMITTED]

Amazingly, however, similar types of trees are found in temperate deciduous forests throughout the world. Many of these trees are hardwoods, highly valued in the furniture-making industry (Chapter 8).

In drier portions of the biome, where the winters are cool and moist but the summers are hot and dry, a unique vegetational association called chaparral exists. Characterized by smaller, often thorny or roughly branched evergreen trees and shrubs and deciduous trees, the chaparral has a short spring growing season interrupted by the heat and drought of the summer. Often referred to as a Mediterranean climate because of the winter rainfall and extensive chaparral areas along the shores of the Mediterranean Sea, such localized areas are also found in southern California, southern Africa, coastal Chile, and coastal western and southwestern Australia. Although they are isolated from one another and contain different species of plants, their climatic conditions give these areas a similar appearance.

Coniferous Forest

Coniferous trees are the cone-bearing members of the gymnosperms. Conifers, valuable for their use in the lumber industry, are evergreen, except for larch, bald cypress, and tamarack. Their needle-shaped leaves have a thick covering (cuticle), which helps prevent water loss. Many conifers have shallow soils found commonly in mountainous areas (see Figure 3-13). Mountains in Europe and Asia, as well as the Rocky Mountains and Appalachian Mountains of North America, have coniferous forests with similar climates and rainfall patterns.

The ability to thrive in thin, rocky or sandy soil that often contains little moisture explains the significant strands of coniferous forests (see Figure 3-14) in the southeastern United States and in the western coastal areas of California, where giant redwoods (Sequoia) grow.

The far northern coniferous forest, found almost exclusively in the Northern Hemisphere north of the 50[degrees] latitude, is referred to as a taiga (snow forests). The average precipitation ranges from about 35 to 100 cm per year, most of it falling in the summer. Winters are very long and cold and have a persistent snow cover. Although the winter air is dry, the ground remains moist because of the low evaporation rate resulting from the cold temperatures. These forests grade into either grasslands or temperate deciduous forest to the south, depending on precipitation levels.

At their northern limits, the coniferous forest gradually gives way to the tundra, as shown in Figure 3-15. Found predominately in the Northern Hemisphere north of the Arctic Circle, the tundra comprises approximately one-fifth of the earth's total land surface. With less than 25 cm of annual precipitation and strong, dry winds, the subzero temperatures and long periods of winter darkness create an exceptionally harsh environment for plant growth. The ground is frozen solid for most of the year, thawing only to a depth of about 1 m during the short summer. This soil permafrost causes plant root systems to be relatively shallow yet extensive. In the moist sedge-dominated communities, underground plant parts may be as much as 10 times greater than above-ground biomass.

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Tundra vegetation is typically composed of scattered, low-growing woody perennials that are well adapted to the drying winds and extreme cold. Certainly, tundra plants must survive long periods of moisture unavailability, and their morphological adaptations commonly include thick waxy cuticles and dense leaf pubescence. Additionally, their prostate to shrubby woody trunks are often covered by a protective coat of lichens or moss that helps prevent desiccation as well as provide protection from the cold. Although no tundra vegetation grows taller than 1 m, and the harsh climate has limited the number of well-adapted species, the successful plants are abundant. Essentially every native plant community is dominated by only two or three species, but there are huge expanses of such areas, as shown in Figure 3-16a.

The mean daily temperature is above freezing for only about one month of the year, providing tundra plants with a very short growing season. During this period, soil moisture is available because the ground thaws above the permafrost, and photosynthesis is possible for all 24 hours of summer daylight. Growth is generally minimal, however, because the replenishment of stored food reserves in the roots and woody stems is the primary plant function during this short period of favorable conditions. Some plants add only a few new leaves to each twig before the cold temperatures again become restrictive.

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Sexual reproduction is an even more tenuous function than vegetative growth, since the plants only have 4 to 6 weeks to complete the entire cycle of flower development, pollination, fertilization, and fruit and seed maturation. Many species have developed dormant buds at the end of the previous summer, thus, when the snows melt; the mature flowers emerge within a few days (see Figure 3-16b). Many of these flowers are modified to concentrate the heat of the sun, thus increasing the maturation process by speeding up metabolic activity. The arctic poppy has a white cup-shaped flower that tracks the sun, focusing the sun's rays on its reproductive parts. In full sunlight, the temperature inside the flower can be as much as 28[degrees]C higher than the air temperature around it. Insects attracted to this warmth affect pollination, and seeds are mature about 3 weeks after the flower opens. Not all the arctic plants have such sophisticated flowering adaptations: some depend on vegetative reproduction, only managing to set mature seeds in unusually long and mild growing seasons, which may not occur for 50 years or more.

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Once mature, seeds of arctic plants must be able to remain dormant for long periods. It is a rare year that provides a growing season sufficient in length to allow for seed germination and development to mature protective woody growth. The deep-freeze conditions of the arctic enable seeds to retain their viability for an unusually long time, however. As an extreme example, a seed of an arctic lupine (genus Lupines, family Laminaceae), found in a Yukon deposit dated at 10,000 years old, germinated after dry storage for 12 years at room temperature. Long dormancy enables seeds to remain viable until the conditions are optimum for success.

Some mountainous areas have climatic conditions and vegetation similar to those of the arctic tundra. These mountain tundra zones are at elevations above tree line in essentially the entire world's larger mountain ranges. The farther from the poles these ranges occur, the higher the elevation before tundra conditions can be found.

At extreme elevations and polar latitudes, no vascular plants occur. The icecaps of both polar regions support a few algae and fungus species but no higher or vascular plants. The tundra is therefore the farthest limit of plant growth.

Aquatic Biomes

The aquatic biome is comprised of marine (saltwater) and freshwater.

Marine

Covering nearly 75% of the earth's surface with an average depth of approximately 5 km, the marine biome is phenomenally large (see Figure 3-17). The plant life there exists under unique circumstances. The most important is the light penetrates to an average effective depth of only a few meters. Below this shallow zone of adequate light, only the short wavelength blue and green portions of the spectrum penetrate effectively, and even then it is essentially dark below 660 to 750 m. Thus the vast majority of plant life is limited to the lighted surface, with a few organisms capable of using shortwave light for photosynthesis at greater depths. Bacterial, fungal, and animal forms inhabit the oceans even at their greatest depth, but as on land, the base of their food chain is still plants.

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Water itself is obviously not in short supply; however, only organisms that can grow in salt water exist in the oceans and seas. These organisms are, for the most part, single-celled algae having no need for the complex vascular system, supportive tissues, and reproduction organs of most terrestrial plants. Only in the shallow waters along the coastal shelves do more complex algal types exist. Simple transport systems, anchoring devices, and other multicellular modifications have been developed by these organisms in response to the constant wave action along the shores.

The climate for marine plants is to a great extent a function of the ocean currents. Plant distribution and temperature are especially current dependent. The most important factor in producing ocean currents is patterns of air circulation. In combination with temperature, which affects water densities, and the deflection of currents off the continental landmasses, these predictable wind patterns create massive water movements around the world.

Ocean currents affect not only the distributions of plants and animals in the oceans, but some climate patterns on land as well, as shown in Figure 3-18.

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The Gulf Stream and Japan Current, for instance, move water warmed in the tropical latitude northward across the Atlantic and Pacific oceans, respectively, to northern latitudes. The British Isles and Alaska are affected by these currents, which change not only the climate, but also the terrestrial vegetation along the coasts and for some distance inland. The coniferous forests of North America, therefore, do not extend as close to the coasts in the west as they do in the east because of the Japan Current. These currents also affect the entire assemblage of marine organisms along these coasts. For example, seals can be seen off the coast of southern California when the cold water moves from northern latitudes south with the California Current. Another ocean phenomenon that occurs is the El Nino--Spanish for "the Christ child"--which refers to an interval of especially warm ocean temperatures that intermittently appear around Christmas in the equatorial Pacific. The phenomenon is associated with weather changes around the globe, including in the Pacific Northwest, where it causes the winters to be especially mild. The effect occurs with a frequency that varies from two years up to a decade. Unusually cold ocean temperatures in the equatorial Pacific characterize la Nina--Spanish for "little girl." This other phenomenon occurs after an El Nino and has the opposite effect on the weather, which in turn makes winters colder with more precipitation. These are but a few examples of how ocean currents modify climate and vegetation worldwide in both aquatic and terrestrial biomes.

Marine organisms are sometimes classified as pelagic or free floating and benthic, or bottom dwelling. The free-floating organisms are primarily phytoplankton (single-celled plants) and zooplankton (single-celled animals). Phytoplankton is composed of single-celled algae, primarily diatoms. The open waters nurture millions of these organisms, plus eggs and larval forms of fish and invertebrates, to provide the early stages of the marine food chain. The larger pelagic animals are thus provided with a reliable and abundant food source. Animals of the benthic zone are usually sedentary or slow-moving clams, starfish, snails, worms, and sea anemones, sponges, and the larger fish. Bacteria and fungi also inhabit this zone, thriving on organic debris that settles from the pelagic zone.

Not all ocean zones are considered productive; some regions are almost devoid of essential nutrients, and therefore few phytoplanktons can survive. Since there is no food source for the zooplankton and larger marine animals, such regions have been referred to as the ocean deserts. The commercial fishing industry is well aware of these unproductive zones.

One interesting and important part of the marine biome is the coral reef. Reefs have been formed only in warm, well-lighted waters of the world (see Figure 3-19). The largest one is the Great Barrier Reef off the coast of northeastern Australia. Extending for a distance of about 200 km, it, like other coral reefs, is composed of colonial coelenterates and encrusting algae. These organisms secrete calcium and become quite hard. Primary production (photosynthesis) is provided by symbiotic algae living among the coral. Coral exists in a variety of colors, and the reflection of light rays in tropical waters shows off a spectacular display of natural beauty.

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Freshwater

Essentially all terrestrial organisms require a supply of fresh water. Plants obtain their moisture from the soil. Some animals are capable of securing all the water they need directly from eating plant tissue. The vast majority of animal life, however, needs additional freshwater supplies, and they find it in lakes, ponds, rivers, and streams (see Figure 3-20). Of the basic natural resources, humans require oxygen, food, fuel, fresh water, and raw materials for shelter, clothing, medicine and industry. Loss of any one could theoretically limit future human growth. The one that first becomes inadequately available, however, is the limiting factor. Since only slightly more than 2% of the world's total land surface area is covered by standing or running fresh water, this resource is a prime candidate as the limiting factor for human population.

[FIGURE 3-20 OMITTED]

The ecology of freshwater areas is complex. From the banks, where many vascular plants such as trees, shrubs, and herbaceous plants grow, to shallow water, where some specially adapted vascular plants such as cypress trees can exist, to progressively deeper water, where nonvascular plants, primarily algae, thrive, each slight change produces a new zone of plant species. Whether the water is still, as in ponds and lakes, or flowing, as in streams and rivers, it plays an essential ecological role.

Scientists who study freshwater biology divide this biome into standing water and running water. In a sense, it is difficult to classify running water as an ecosystem because the water and nutrients are not recycling within given boundaries. Standing water lakes may be large or small, and the life zones are classified as the littoral zone, at the edge of the lake and quite productive; the limnetic zone, a region of open water where phytoplankton are abundant in the upper layers, and the profundal zone, the region below the limnetic zone where there is no plant life. Principal occupants of this third zone are the scavenging fish, fungi, and bacteria.

The climatic zone of lakes is similar in composition to the open water in the ocean (although the species may be different), but the littoral zone of the lake is unique. The shoreline may contain bottom-rooting aquatic angiosperms (flowering plants) such as cattails and rushes; water lilies and other rooted plants may extend further out for a considerable distance.

Running water arises from melting ice or snow, from artisan water below the soil surface, or as an outlet from lakes. The flux or quantity of water transported per unit time determines to a large extent the kinds of organisms that prevail. Slow-moving streams may be rich in phytoplankton, whereas rapids and fast-moving water may have very few. Rapid water movements also preclude the attachment of angiosperms to the bottom of the river or stream. Under such conditions most productivity is confined to the quiet shallow areas, where algae and mosses can attach to rocks.

Even latitude, which affects temperature, has a major effect on biological productivity, especially in thermal stratification (which causes turning over) of lakes and ponds. In winter, as the air temperature drops, so then does the water temperature, gradually cooling from the surface downward. Since water is an excellent insulator, gaining and losing heat slowly, several days of subfreezing air temperature may be required before the surface water finally begins to form ice at 0[degrees]C. During this gradual cooling toward the freezing point, the top layer of water reaches 4[degrees]C, which is the temperature at which water is most dense (the water molecules are actually packed most closely together). Thus, at 4[degrees]C the top layer is more dense than water below, which is not in direct contact with the air and does not cool as rapidly at the lower levels. The denser upper layer of water sinks to the bottom; since this top layer is more saturated with oxygen than are lower levels, as it sinks it oxygenates the pond or lake and stirs up organic matter. This results in a period of growth for freshwater organisms.

When ice forms, which is less dense than liquid water, it floats on the surface. This ice layer will slowly thicken if the air temperature remains cold enough, and the ice layer helps insulate the water below from the air temperature. Thus, the entire body of water does not become an ice block. In the spring, as the surface ice warms from 0[degrees]C to 4[degrees]C, the lake turns over again, with results comparable to those at the onset of cold weather. This may happen repeatedly until all surface water is warmed above 4[degrees]C.

[FIGURE 3-21 OMITTED]

Water is truly a unique liquid. If, as with other liquids, the freezing point produced a solid state denser than the liquid, ice would sink to the bottom. The pond or lake would be filled with ice, which would thaw slowly from the top down in the summer and probably never completely melt. This would eventually result in a solid block of ice in winter and only the upper portions even being in the liquid state for part of the year. A vast majority of our deep freshwater bodies then would not support aquatic life, nor would it be available for human's needs.

Because standing bodies of water have living organisms that reproduce, grow, and ultimately die, these bodies of water slowly fill with organic debris. During periods of eutrophic (nutrient-rich) conditions, bursts of growth and reproduction occur. As in the oceans, a majority of plant activity is at the surface where light is available. This often produces algal blooms, which cover the surface and initiate oxygen-poor conditions below. Excess algal growth increases the animal populations that feed on them. The ultimate effect is a population crash in the pond or lake. This process of eutrophication is often artificially accelerated by human activities such as the dumping of raw sewage; the runoff of agriculture fertilizers and hog waste provides rich nutrient supplies for the plant life to grow.

Other human activities that not only disturb the ecological balances but also render the water unsafe for use include various forms of pollution. Chemicals, trash, and sewage are among the most serious water pollutants. Whatever the source, any negative use of the limited supplies of fresh water, could have very serious consequences. We need to understand as best we can freshwater ecology and use this knowledge to wisely manage this important resource (see Figure 3-21).

Summary

Biomes are worldwide groups of similar ecosystems. The terrestrial and aquatic biomes are summarized next.

1. Tropical rain forests: Equatorial, large evergreen trees with a dense leaf canopy, little diurnal or annual temperature fluctuation, 200 to 500 cm of rainfall annually, shallow topsoil that is often lateritic, and many species of plants and animals.

2. Savannas: At subtropical latitudes, there is some seasonality, 80 to 160 cm annual precipitation, perennial grasslands with scattered deciduous trees, maintained by periodic burning; the driest savanna areas are called thorn forest.

3. Deserts: Primarily found in a latitude belt between 20[degrees] and 30[degrees] in both northern and southern hemispheres, only 25 cm or less annual precipitation, extreme daily temperature fluctuations, and annual season; low growing, scattered vegetation is either small leafed shrubs, succulents, or herbaceous annuals.

4. Grasslands: Receiving 30 to 150 cm of annual precipitation, distinct seasonality; constant coverage of perennial grasses is normal but cultivation and overgrazing are permanently changing much of the world's native grasslands; in the more xeric grasslands, desertification is a common direction of change.

5. Temperate deciduous forest: Found primarily in the Northern Hemisphere, 75 to 225 cm of annual precipitation is seasonal; deciduous trees (hardwoods) dominate the vegetation in the fall, an autumn and spring understory of shrubbery and herbaceous plants becomes evident; the xeric end of the precipitation range produces what is termed chaparral or Mediterranean vegetation.

6. Coniferous forest: Known as taiga in the northern latitudes and high elevation mountain areas; predominately Northern Hemisphere; annual precipitation of 35 to 100 cm limits vegetation to coniferous gymnosperms (softwoods), which are evergreen; at more southern latitudes this biome is produced by sandy soils.

7. Tundra: Mostly north of the arctic circle, about one-tenth of the earth's surface area; less than 25 cm of precipitation annually; tundra plants are scattered and low-growing perennials; permafrost soil conditions exist year-round and the growing season is very short; mountain tundra occurs above the tree line in higher elevation mountainous areas; vegetation and climatic conditions are very similar to arctic tundra but at more southern latitudes.

8. Aquatic biomes: The aquatic biomes include marine and freshwater systems; the marine biome includes nearly 75% of the earth's total surface area and, although the oceans average 3 km in depth, most plant life is limited to the top few meters; most plant life is single celled, with the shallow water along the shores the only locality for the more complex kelps and other multicellular algae; ocean currents such as the Gulf Stream and Japan Current affect terrestrial vegetation by modifying the temperatures of the area and thus the climate; the freshwater biome includes flowing water and standing water ecosystem types; vegetation in both is more complex with more species than in the marine biome.

Something to Think About

1. What is a biome?

2. Identify the vegetation in a tropical rain forest.

3. Describe the vegetation in a savanna.

4. List the vegetation in a desert.

5. Identify the vegetation in a grassland.

6. Describe the vegetation in a temperate deciduous forest.

7. Identify the vegetation in the coniferous forest.

8. Discuss the vegetation in the tundra.

9. Name the vegetation in the aquatic biome.

10. Where are the following biomes found: tropical rain forest, savanna, desert, grassland, temperate deciduous forest, coniferous forest, tundra, and aquatic biome?

Suggested Readings

Dale, V. H., C. M. Crisafulli, and F. J. Sawanson. 2005. Ecological responses to the 1980 eruptions of Mount St. Helens. New York: Springer-Verlag.

Dashini, K. M. M., and C. L. Foy. 1999. Principles and practices in plant ecology: Allelochemical interaction. New York: CRC Press.

Kalman, B., and J. Langille. 1998. What are food chains and webs? New York: Crabtree Publishing.

Sprent, J. 1987. The ecology of the nitrogen cycle. Cambridge: Cambridge University Press.

Wigley, T. M. L., and D. S. Schimel. 2000. The carbon cycle. Cambridge: Cambridge University Press.

Internet

Internet sites represent a vast resource of information. The URLs for Web sites can change. Using one of the search engines on the Internet, such as Google, Yahoo!, Ask.com, or MSN Live Search, find more information by searching for these words or phrases: biomes, grassland, tundra, tropical rainforest, temperate deciduous forest, coniferous forest, savanna, desert, aquatic, carbon cycle, and nitrogen cycle.
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Title Annotation:PART 1: Plants and Nature
Publication:Fundamentals of Plant Science
Date:Jan 1, 2009
Words:7664
Previous Article:Chapter 2: Plants and ecology.
Next Article:Chapter 4: Basic design I.
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