Seed and seedling ecology of pinon and juniper species in the pygmy woodlands of western North America.
Pinon-juniper woodland is one of the most widespread vegetation types in the western United States, covering at least 17 million hectares (West et al., 1975; West, 1988). The current distribution of the woodland is primarily the result of climate change that occurred during the Holocene. Since the end of the Wisconsin ice age 11,500 years ago, some species moved upslope as much as 1000 to 1500 m and northward as much as 6 [degrees] latitude. For example, Pinus monophylla and P. edulis have undergone parallel northward expansions of some 640 km from ice age refugia in southern Arizona and New Mexico and northern Mexico. Pinus monophylla has migrated through the Great Basin as far as southern Idaho, while P. edulis has migrated over most of the Colorado Plateau and southern Rockies (Betancourt, 1987; Van Devender, 1987; Wells, 1987; Thompson, 1990). Although climatic changes have obviously influenced tree species migration and current distributions, recent expansions appear to be occurring at an accelerated rate and to be influenced by human activities. Before Anglo-American settlement of the West, fires burned through much of the woodlands as frequently as every 50-100 years (Wright & Bailey, 1982). This limited local expansion of the trees and resulted in a mosaic of early seral grasslands, mid-seral shrublands, and late seral woodlands (West & Van Pelt, 1987). However, overgrazing by livestock during the past 150+ years has depleted the highly palatable but grazing-intolerant grass and forb species (Caldwell et al., 1981), resulting in a decrease in fine fuels and, consequently, a decrease in fire frequency. Active fire-prevention policies and a reduction in the number of fires set by Native Americans has contributed to additional decreases in fire frequency (Miller et al., 1994). The lack of competition from grasses and forbs in combination with the decrease in fire frequency has resulted in an increase in the relatively unpalatable and fire-intolerant woody species (West & Van Pelt, 1987; Miller et al., 1994; Pieper, 1994; Young, 1994). Pinon and juniper woodlands are expanding into adjacent grass- and shrub-dominated communities throughout their range (Cottam & Stewart, 1940; Johnsen, 1962; Arnold et al., 1964; Blackburn & Tueller, 1970; Tausch et al, 1981; West, 1984; Miller & Rose, 1995; but see Lanner, 1977, 1993) and tree density is increasing within existing stands in many areas. Between about 1870 and 1980, woodlands in the Great Basin increased by about 2.5 times in both distribution and density (Tausch et al., 1981). In southwestern Utah alone, pinon and juniper increased fivefold in area and six- to twentyfold in density between 1864 and 1940 (Cottam & Stewart, 1940).
Understanding prehistoric migration and historic expansions and the effects of human activities on pinon-juniper woodlands requires knowledge of the seed and seedling ecology of the tree species. Although seed production, seed dispersal mechanisms, and seedling establishment processes are key determinants of both the past migration and recent expansion of the woodlands, little research has focused in this area. Here we review the available literature on the reproductive ecology, seed dispersal, post-dispersal seed mortality, seed dormancy and seed germination requirements, and seedling establishment of pinon and juniper species within the woodlands. We begin with a brief description of the species within the woodlands and of its geographical extent. Then, to provide the framework for our discussion, we present generalized seed and seedling fate diagrams for pinons and junipers [ILLUSTRATION FOR FIGURES 1 & 2 OMITTED]. In the sections that follow, we detail the various fates of pinon and juniper seeds and seedlings from seed development to seedling establishment. Finally, we examine the implications of this information for the prehistoric and historic expansions of the woodlands and for their current management, and make recommendations for future research.
III. Woodland Description
One of the defining features of the woodlands is that they are semi-arid, receiving 25-50 cm total annual precipitation. The seasonality and effectiveness of precipitation as well as the growing-season temperature differ considerably across the broadly distributed woodlands and, consequently, the species of Juniperus and Pinus vary regionally. Junipers are more widely distributed both geographically and elevationally and extend into drier and colder habitats than pinons, but in the true woodland usually one juniper species and one pinon species comprise the tree synusia. The woodland extends from the east slope of the Sierra Nevada eastward throughout the mountains of the Great Basin in Nevada, Utah, and southern Idaho, and the foothills of Wyoming, onto the flanks of the Rocky Mountains in Colorado (as well as mesas of the Colorado Plateau and interior valleys), then southward into Arizona, New Mexico, western Texas, and northern Mexico (West et al., 1975; West, 1988). The primary tree species include two pinon pines, P. edulis Engelm. and P. monophylla Torr. & Frem., and seven junipers, d. monosperma (Engelm.) Sarg., d. occidentalis var. occidentalis Hook., d. occidentalis var. australis Vasek, d. osteosperma (Torr.) Little, d. scopulorum Sarg., J. deppeana Steudel., and d. coahuilensis (Mart.) Gaus. (Malusa, 1992; Adams, 1993; Kral, 1993). Dense stands of d. occidentalis var. occidentalis without pinons extend farther north into northeastern California, eastern Oregon, and southwestern Idaho. Additional tree species are found on the Mohave border in southern California, including P. californiarum Bailey subsp. californiarum, P. californiarum subsp. fallax (Little) Bailey, and d. californica Carr., and on the Edwards Plateau in central Texas and Trans-Pecos-Chihuahuan Desert border in western Texas and southeastern New Mexico, including P. cembroides var. bicolor Little, P. cembroides var. remota (Little) Bailey & Hawks, J. ashei J. Buch., and d. pinchotii Sudworth. Our focus here is on the species that occur within the true pinon-juniper woodlands, although references for other species are included where informative.
IV. Overview of Seed and Seedling Fates
The seed and seedling fate diagrams outline the potential pathways and fates of seeds from seed or fruit development to seedling establishment for pinons [ILLUSTRATION FOR FIGURE 1 OMITTED] and junipers [ILLUSTRATION FOR FIGURE 2 OMITTED]. pinon and juniper species within these semi-arid woodlands exhibit interesting similarities and differences in their seed fates.
In pinon pines, it takes slightly more than two years to produce mature "seeds in the trees." In the different juniper species, seed maturation takes from one to two years. Many of the seeds that are produced by both pinons and junipers are unfilled due to environmental and developmental constraints; others are eaten by insects or destroyed by animals before they are fully developed. Once the seeds are mature, they are harvested or eaten by a variety of animals, some that act solely as seed predators and others that are seed dispersers.
One of the major differences in the seed fates of pinons and junipers is the dispersal pathways of mature seeds. In pinons the seeds are adapted for dispersal by birds, corvids such as jays and nutcrackers, which harvest the seeds from the trees. While some of the seeds are eaten by the birds, many are scatter-hoarded in shallow caches in the soil, and a portion of these are recovered and eaten at a later date. Although not well researched, some of the recovered seeds are probably moved to new locations, where they may be recovered again and then either eaten or moved to yet another location (e.g., Vander Wall, 1995; Vander Wall & Joyner, 1998). Some cached seeds remain unrecovered and die due to abiotic or biotic factors, but others survive to later germinate. In contrast to pinon seeds, juniper seeds are adapted for dispersal by frugivorous birds and mammals which either eat the seeds while they are on the trees or after they fall to the ground. If the fruits are ingested by frugivorous mammals, the pericarps are digested by the animals and the seeds are defecated onto the ground. Depending on the species of juniper and animal disperser, some of the seeds are killed. Many survive ingestion only to be eaten later by other animals, or die of abiotic or biotic causes. Surviving seeds may remain on the soil surface or be abiotically buried where they may suffer additional mortality or germinate.
A dispersal pathway that occurs in both pinons and junipers but that has only recently been documented involves scatter-hoarding by rodents such as chipmunks, mice, and kangaroo rats (Vander Wall, 1997; Chambers et al., 1998). The fates of the cached seeds are similar to those of pinon pine seeds cached by birds. Many of the cached seeds are later recovered and eaten, but others are probably recached (e.g., Vander Wall & Joyner, 1998; Vander Wall, 1995). While some of the cached seeds that are not recovered by rodents die of abiotic or biotic causes, others survive to germinate. If either pinon or juniper seeds are harvested by rodents, a portion may be placed in large underground larders, where the majority are either eaten by the rodents or die because of unsuitable conditions for germination and establishment (Vander Wall, 1990). If juniper seeds are harvested by rodents, it is likely that they are husked before they are cached.
Regardless of the dispersal agent, if the seed has been dispersed to a microhabitat with environmental conditions suitable for germination and growth and maturation, seedling establishment can occur. However, it may still be susceptible to rodent predation, insects, or pathogens during this phase. If the seed has been dispersed to a microhabitat unsuitable for growth and survival, mortality by biotic or abiotic causes is inevitable.
V. Seed Production
In pinon pines, portions of three growing seasons are required to produce mature seeds [ILLUSTRATION FOR FIGURE 1 OMITTED]. The exact timing of the various developmental events varies among pinon species and with the elevation or latitude of the pine stand, but the processes are similar. Pinons, like all pines, are monoecious (Mirov, 1967; Vidakovic, 1991). Male strobili (cones) are located in tight clusters of 10-50, usually on the lower branches of trees. Female strobili are scattered singly or in small clusters of 2-3, usually at the base of terminal shoots on the upper portion of the tree (Little, 1938, 1941). Primordia of Pinus edulis strobili are produced in winter buds that form between August and October. The following spring, strobili resume their development with the male strobili initiating development about two weeks earlier than the female strobili. By mid-June the male strobili are mature and shed their pollen, and the female strobili are open and have become receptive. Pollination is usually complete by the end of June. The male strobili die and fall from the tree shortly thereafter, but the female strobili continue to grow until late August or early September. At this time they are one-sixth to one-fourth the length of mature cones. Growth resumes in the spring as cones turn bright green and swell. In early July, as cones approach mature size, fertilization of the ovules takes place and seed development continues. At this time, seed coats are soft, white, and spongy. The seed contents consist of a small diploid embryo embedded in female gametophyte, which is haploid and derived entirely from female tissue. based on the dry mass of developing seeds (Vander Wall, 1988), by early August the female gametophyte is rich in water but still lacks storage products. Over the next 6 weeks, carbohydrates, proteins, and lipids are translocated into fertilized seeds (Botkin & Shires, 1948; Vander Wall, 1988) and stored in the female gametophyte. The seed coats gradually become woody and pigmented by late August. Seeds are ripe by early September. Water and other materials stop flowing into the cones at this time, and the cones gradually dry out and begin opening by mid-September.
In pinon, many of the seeds that are produced by the trees are unfilled or are eaten by insects before they mature [ILLUSTRATION FOR FIGURE 1 OMITTED]. Seeds can develop that are fully formed and have a normal woody seed coat but that are virtually empty, the undeveloped female gametophyte having collapsed during the summer of the second year of development. In most cases this is the result of self-pollination and subsequent fertilization (Lanner, 1980). Seed fill can be highly variable both between and within species. In Pinus edulis, nearly one-half of the seeds in mature cones are empty (Vander Wall & Balda, 1977; Ligon, 1978), and for P. monophylla in the Pine Nut Range of western Nevada, 18% and 21% of the seeds were empty during two successive years, respectively (Vander Wall, unpubl. data).
A variety of insect species feed on the developing cones and seeds of pinon pines. The two most important genera are Conophthorus and Dioryctria (Keen, 1958; Christensen & Whitham, 1991, 1993). Conophthorus species are cone beetles of the family Scolytidae. The larvae eat through the cone scales and seeds causing the second-year cones to wither and turn brown. In some years, up to 90% of the cone crop can be destroyed by beetle larvae (Keen, 1958). The larvae of Dioryctria albovitella, juvenile moths known as "pine coneworms," attack terminal shoots, potentially reducing current and future cone production. Larvae tunnel through cones, eating cone tissue and seeds. Individual Pinus edulis trees vary genetically in their ability to avoid being attacked by Dioryctria albovitella (Whitham & Mopper, 1985; Christensen & Whitham, 1991, 1993; Gehring & Whitham, 1991; Moppet et al., 1991). Christensen and Whitham (1991) found that slightly more than one-half of the P. edulis trees at their Arizona study site lost at least 50% of the cones to Dioryctria. Fourteen percent of trees lost their entire crop of cones. At the population level, cone mortality ranged from 30% to 100%. Some trees lack effective chemical defenses against insect herbivores and, consequently, are repeatedly attacked by coneworms each year. These trees have an abnormal shrub-like form and reduced ectomycorrhizal associations; they rarely produce healthy cones and they function, in effect, like male trees (Gehring & Whitham, 1991; Mopper et al., 1991; Del Vecchio et al., 1993).
After the various forms of predispersal seed losses are accounted for, mean filled seeds per cone ranges from about 4 in Pinus edulis (Vander Wall & Balda, 1977; Ligon, 1978) to about 16 in P. monophylla (Vander Wall, 1997). During a year of heavy seed production, large trees can produce more than 1000 cones, although the average tree produces fewer. A typical mean seed crop during a year of heavy cone production ranges from 2000 to 8000 filled seeds per tree, although values for individual trees vary tremendously. Seed production in a stand of P. monophylla in the Pine Nut Range of western Nevada was 1873 filled seeds per tree in a year of moderate cone production and 5936 filled seeds per tree in a year of fairly heavy cone production (Vander Wall, 1997). More extensive data on annual and individual variation in seed production are needed.
In junipers, it may take one (e.g., Juniperus monosperma), two (e.g., J. osteosperma), or even three (e.g., J. communis) growing seasons following pollination to produce mature "fruits in the trees" ([ILLUSTRATION FOR FIGURE 2 OMITTED]; Fechner, 1976; Adams, 1993). In contrast to pinon pines, junipers are generally dioecious, although J. occidentalis and J. californica may be either dioecious or monoecious and J. osteosperma is generally monoecious (Johnsen & Alexander, 1974; Tueller & Clark, 1975; Adams, 1993). However, even typically dioecious species such as J. monosperma occasionally produce both male and female strobili on a branch (Johnsen, 1962). In d. occidentalis, gender may be environmentally labile since open stands contain many female trees while dense stands are dominated by males (Eddleman, 1984; Miller & Rose, 1995), but variable gender expression in some juniper species appears to be at least partly under genetic control (Jordano, 1991). Male strobili (cones) are terminal on branchlets and have 3-7 pairs or trios of sporophylls, each containing 2-6 pollen sacs. Female strobili, borne at the tips and along the length of short branchlets, have 3-8 pointed scales each containing 0-2 ovules. During development the fleshy scales of female cones fuse, forming indehiscent strobili often referred to as "berries." Unlike pines, there is no evidence that male and female cones are spatially segregated within a crown in monoecious species. Reproductive phenology varies with species and climate, but it appears that, for most junipers, male strobili mature in late summer to fall and release pollen in spring as female strobili are emerging from beneath leaf scales (Tueller & Clark, 1975; Fechner, 1976; Schupp, unpubl. data). Little information is available on juniper seed development, but in the one-year-maturing eastern red cedar (J. virginiana), the female gametophyte begins development in the spring a few weeks after pollination and fertilization occurs by late June or July. In the three-year-maturing common juniper (J. communis), the female gametophyte begins development in the spring of the year following pollination (Ottley, 1909). Thus, depending on the phenology of the species, junipers appear to have a delay between pollination and fertilization of several months to a year or more.
In some juniper species, ripe fruits can remain on the tree for two or even three years. As fruits ripen, usually from September to October, they turn from green to blue, blue-black, brown, or reddish brown, although a waxy coating gives most species a bluish look (Johnsen & Alexander, 1974; Adams, 1993). The fruit may be fleshy and resinous, as in Juniperus occidentalis, or dry and woody, as in J. osteosperma. Through time, the cones slowly dry out and, in some species, shrivel (Johnsen, 1962) Seed number is variable both within and among species, ranging from those species generally producing only a single seed per cone (e.g., J. osteosperma and J. monosperma) to those producing four to five or more seeds per cone (e.g., J. deppeana) (Adams, 1993). Although seed bearing begins at 10-20 years of age (Johnsen & Alexander, 1974), significant fruit production starts at 50-70 years and continues for centuries (Tueller & Clark, 1975; Eddleman, 1984; Noble, 1990; Miller & Rose, 1995).
As with pinon pines, there is extensive loss of potential seed production between pollination and seed maturation with many fruits failing to fill or being eaten by insects or other predators [ILLUSTRATION FOR FIGURE 2 OMITTED]. Junipers ripen many fruits with fully developed seed coats but without an embryo or endosperm (Johnsen & Alexander, 1974; Noble, 1990; Fuentes & Schupp, 1998), although the proportion of unfilled seeds is highly variable among and within species. For example, the 1993 cone crop in a Juniperus osteosperma population in Tintic Valley, western Utah, varied among individuals from 0% to 17.3% filled seeds with a mean of 5.6% filled (Fuentes & Schupp, 1998). In the same year, a population about 50 km away at U.S. Army Dugway Proving Grounds had less than 1% of its seeds filled, yet produced roughly 33% filled seeds three years later (Fuentes & Schupp, 1998; Schupp, unpubl. data). There are several apparent causes of empty seeds. Because seed and fruit development is initiated by pollination, not fertilization, any genetic or environmental factors yielding fertilization failure or embryo abortion or death can result in a fully developed yet empty seed (Fechner, 1976; Farmer, 1997). Empty seeds in other conifers have often been attributed to embryo death due to both selfing (Allen, 1942; Sarvas, 1962; Lanner, 1980) or extreme climatic events (Dogra, 1967). Although causes are poorly understood in juniper, it is thought that the proportion of filled seeds varies with the age, structure, density, and community composition of a stand, with physiography, and with weather during pollination and/or seed development (Noble, 1990).
Further losses to invertebrate and vertebrate predation on developing seeds can have a significant impact on seed production. Although the extent of infestation is unreported, caterpillars of the moths Periploca atrata and Ithome spp. feed on juniper seeds, consuming the entire embryo and endosperm (Keen, 1958; Powell, 1963; Furniss & Carolin, 1977), and the cecidomyiid midge Walshomyia juniperina galls fruit of Juniperus osteosperma (Furniss & Carolin, 1977). The juniper-berry mites Trisetacus quadrisetus and Eriophyes ramosus have been known to destroy the entire fruit crop of some trees (Morgan & Hedlin, 1960; Furniss & Carolin, 1977). The chalcidoid wasp Eurytoma juniperina destroyed about 25% of the seed crop of Juniperus occidentalis near Hilt, California, in 1913 (Keen, 1958). In the most quantitative study to date, Fernandes and Whitham (1989) found that the larvae of an unidentified beetle (Anobiidae) heavily attacked J. monosperma near Sedona, Arizona, and that insect attack increased the likelihood of fruit abscission. Considering only fruits with filled seeds, larvae were present in 65.7% of the abscised fruits and 26.9% of the fruits still on trees. Fruit abscission may be beneficial to juniper, since beetle mortality was 4.3 times greater in abscised fruits than in fruits retained on the tree. We know of no published information on predation by vertebrates on developing seeds, but a white-tailed antelope squirrel (Ammospermophilous leucurus) was observed destroying nearly the entire developing crop of a single J. osteosperma tree over a several-month period while ignoring nearby trees (Schupp, unpubl. data). Much remains to be learned concerning the impact of seed predation on seed availability and population dynamics of juniper.
Despite large losses from abortion and predation, substantial fruit production can still occur. Salomonson and Balda (1977) estimated that winter feeding territories of Townsend's solitaires (Myadestes townsendi) in northern Arizona contained about 27.2 million Juniperus monosperma cones per hectare in a good year and 1 million cones per hectare in the following poor year. In central Texas, a single J. ashei can ripen 7744 to 226,944 cones in a good year, depending on crown area (Chavez-Ramirez & Slack, 1994). It is unknown, however, how many of these fruits are filled and potentially viable.
VI. Seed Dispersal and Seed Predation
A. ADAPTATIONS OF CONES AND SEEDS FOR DISPERSAL
Pinon pine cones and seeds are constructed to promote seed harvest and dispersal by birds of the family Corvidae ([ILLUSTRATION FOR FIGURE 1 OMITTED]; Vander Wall & Balda, 1977). The ovate cones range from small (3-6 cm long in Pinus edulis) to medium-sized (4-9 cm long in P. monophylla). The cones are of rather flimsy construction, compared to cones of other pines found in the region, due to a reduced lignin content of the cone scales. They lack a strong central axis and the tips of scales (umbos) lack spines. Except for the pitch (oleoresin) they exude, cones are poorly defended against most vertebrate foragers. Some corvids, like Clark's nutcrackers (Nucifraga columbiana) and pinon jays (Gymnnorhinus cyanocephalus), have relatively long, pointed bills and can pry open unripened cones and extract seeds (Vander Wall & Balda, 1981).
Relative to other pines, the seeds of pinon pines are highly conspicuous within the cones. The sessile cones are arranged on branches singly and in clusters of two or three. At maturity the cone scales open widely, but the seeds do not fall readily from the cones, as do the seeds of many wind-dispersed conifers. The wingless seeds are produced in deep pockets of the cone scales and are held in these pockets by thin flanges. Also, cones do not hang downward, like the cones of many wind-dispersed pines, but are oriented in all directions (i.e., sideways and upward). The flanges and cone orientation impede the shedding of seeds, and the widely spreading scales and short cone axis result in the seeds being visible to animals perched in the canopy of the tree or flying. Filled seeds are often darkly colored, contrasting with the pale axial surface of the cone scales and increasing visibility. Although there is considerable intra- and interspecific variation, seed coats of filled seeds are usually dark brown whereas empty seeds are dull yellow-brown. In contrast, seeds of many pines with wind-dispersed seeds are mottled, which may camouflage them as they lie on the ground. pinon jays and nutcrackers are known to use the coloration of Pinus edulis and P. monophylla seeds to distinguish between filled and empty seeds (Ligon & Martin, 1974; Vander Wall & Balda, 1977; Johnson et al., 1987; Vander Wall, 1988).
Pinon seeds are highly nutritious. The dry mass of the edible portion of Pinus edulis seeds weighs 125-225 mg and that of P. monophylla seeds 200-300 mg. Pinus edulis seeds are very high in fats (63%), whereas P. monophylla's have less fat (26%) but are rich in carbohydrates (60%) (Botkin & Shires, 1948, values adjusted to dry weights). Protein ranges from 11 to 14%. Despite differences in size, caloric values of the two species are similar because of the differences in composition. A 175 mg P. edulis seed supplies 5.39 kJ of energy, while a 250 mg P. monophylla seed supplies 5.63 kJ (calculated from Botkin & Shires, 1948).
Seeds of Pinus edulis and P. monophylla have thin coats relative to many other large-seeded pines (e.g., P. sabiniana), and even small birds such as mountain chickadees (Parus gambeli) and red-breasted nuthatches (Sitta canadensis) are able to open the seeds. Texas pinon (Pinus remota) is known for its thin-shelled seeds, whereas Mexican pinon (P. cembroides) has relatively thick-shelled seeds (Lanner, 1981). The seed coat thickness of pinons appears to exclude or reduce the efficiency of weak - billed foragers while maintaining the attractiveness of the seeds to strong-billed corvids. Although the cones and seeds of pinons do not appear to be designed to attract rodents, they nevertheless do so (Vander Wall, 1997), and several species of rodents appear to be effective agents of P. monophylla seed dispersal.
Alternative means of seed dispersal do not seem to be available to pinons. Unlike seeds of most wind-dispersed pines, pinon pine seeds are relatively large, nearly round in cross section, and without wings. Thus, wind dispersal is ineffective in moving seeds away from source trees. Lanner (1972) estimated that a wind of 160 km/h would be needed to move a pinon seed 10 m if it fell from a height of 10 m. Most seeds that fall from tree crowns land on the ground below the canopy of the parent tree. When especially large crops of pinon seeds are produced, corvids, other birds, and rodents continue to harvest seeds from the ground throughout the autumn and winter.
All of these traits result in seeds that are highly attractive and readily harvested by food-storing birds and rodents, as well as other animals. Corvids (nutcrackers and jays), which are thought to be the most important group of seed harvesters, are all scatter-hoarders (Vander Wall & Balda, 1977, 1981; Ligon, 1978). Rodents, the second most important group of seed harvesters, also scatter-hoard many seeds (Vander Wall, 1997). A fraction of cached seeds are not recovered, and these seeds germinate in the spring and establish trees. Pinon pines appear to present their seeds to animals, seed predators and seed dispersers alike, because the greater the number of seeds that are harvested by animals, the greater the number of seeds that are likely to get buried in sites where they will germinate and establish seedlings.
With their round and more or less fleshy cones, all 60+ species of junipers in the world are clearly adapted for dispersal by frugivorous vertebrates rather than by wind [ILLUSTRATION FOR FIGURE 2 OMITTED]. The "flesh" (pericarp) of a Juniperus monosperma cone has an energy content of 1.32 k J, making it a reasonably rich energy source for dispersers (Salomonson, 1978; Balda, 1987). Such a "fruit" is unusual in North American Coniferophyta. Fleshy animal-dispersed cones are present in all five species of North American Taxaceae and all 13 species of Juniperus (Cupressaceae), but are absent in the remaining Cupressaceae (8 genera, 17 species) and in the North American Pinaceae (6 genera, 64 species) (Givnish, 1980; Adams, 1993). Salomonson (1978) suggests a number of traits of cones that may be related to dispersal by birds. The fruits can be very abundant, they are conspicuously colored blue or reddish (which is generally considered attractive to birds), they are easily accessible on the outer layers of foliage, they are energy rich, and they are available over extended periods of time. The thick, hard seed coat may also be related to endozoochorous dispersal, allowing seeds to pass undamaged through the guts of birds and mammals. Interspecific variation in structure of the cone, although perhaps not an adaptation per se, may influence the disperser assemblage, with smaller more fleshy and resinous cones being dispersed more by birds and larger fibrous or woody cones being dispersed more by mammals such as lagomorphs.
B. SEED DISPERSAL AGENTS
Pinon pines are dispersed by corvids (jays and nutcrackers) and rodents (chipmunks, mice, and kangaroo rats) that harvest seeds from the trees and scatter-hoard them in shallow caches in the soil [ILLUSTRATION FOR FIGURE 1 OMITTED]. The available information suggests that, among birds, the most important dispersers are scrub jays (Aphelocoma coerulescens), Steller's jays (Cyanocitta stelleri), pinyon jays, and Clark's nutcrackers (Vander Wall & Balda, 1981). Pinon jays, which travel in flocks of 50-300 birds, cache an impressive quantity of seeds (Balda & Bateman, 1971; Ligon, 1978; Marzluff & Balda, 1992). They range widely throughout their large home ranges locating productive pinon seed crops and carrying seeds back to the vicinity of the traditional breeding grounds (Balda & Bateman, 1971; Ligon, 1978). Seed dispersal distances are typically between several meters and 5 km but can be as much as 10 km (Vander Wall & Balda, 1981). Because scrub jays occupy relatively small territories, seed dispersal distances probably seldom exceed 1 km (Vander Wall & Balda, 1981). Scrub jays live in pairs and are more secretive than pinon jays, but it is possible that on a regional scale their population densities exceed those of the more conspicuous pinon jays. Scrub jays may also cache more seeds than pinon jays, but their effects on pinon seed dispersal have received little study. Steller's jays typically live in coniferous forests above the pinon-juniper woodland, but descend mountain slopes to gather pinon seeds when abundant. Most of these are carried upslope out of the pinon-juniper woodland and cached in the soil. These birds probably account for many of the scattered pinons that can be found in lower portions of the ponderosa pine (Pinus ponderosa) forests and other transition-zone coniferous forests. Dispersal distances range up to about 3 km. Clark's nutcrackers are conspicuous and bold harvesters of pinon seeds that transport large loads of seeds long distances, but their effects on seed dispersal appears to vary greatly depending on the situation (Vander Wall & Balda, 1977; Christensen et al., 1991). They cache some seeds in pinon-juniper woodlands but routinely carry seeds 5-10 km to mid- and high-elevation cache sites. In some cases, seeds are carried to high-elevation sites where pinon pines cannot survive or to mid-elevation, warm, south-facing slopes where pinon can survive but where reproductive potential may be low. The value of pinon dispersal by Clark's nutcrackers may lie in the fact that some seeds are transported long distances to other plant communities, thereby facilitating pinon colonization of new areas.
The jays typically place one seed in each cache site, whereas nutcrackers cache 1-10 seeds with a mean of about 4 seeds per cache (Vander Wall & Balda, 1981). Cache depth is influenced by bill length and the hardness of the substrate but is typically 2-4 cm. While many of the cached seeds are recovered and eaten by the birds, others are not recovered and thus have the potential to germinate and produce seedlings. Although not well documented, some of the recovered seeds are probably recached in new locations, where they may be recovered once again and then either eaten or moved to yet another location (e.g., Vander Wall, 1995; Vander Wall & Joyner, 1998).
The number of seeds stored has not been determined for most corvids, but scatter-hoarding by Clark's nutcrackers has been estimated at 22,000-33,000 Pinus edulis seeds (Vander Wall & Balda, 1977) or 17,900 P. monophylla seeds (Vander Wall, 1988) per individual in good seed crop years. Ligon (1978) estimated that a flock of 250 pinon jays could cache about 4.5 million P. edulis seeds over a period of 5 months. Seeds are carried long distances, often to areas where potential cache pilferers are less abundant (Ligon, 1978; Vander Wall, 1988). Microhabitat distribution of caches has not been quantified for corvids caching pinon seeds, but almost any site in mineral soil or plant litter appears to make an acceptable caches site; brushy areas and dense grass seem to be avoided.
Some corvids are highly dependent on the seeds they store (Vander Wall & Balda, 1981). Pinyon jays have been known to breed in the autumn, an unusual time for temperate-zone birds to breed, in years when a bumper crop of cones is being produced. Gonadal development has been linked directly to the development of green cones on pinon trees (Ligon, 1974). Clark's nutcrackers not only survive the winter on cached pine seeds, but feed seeds to nestlings and to the young after they fledge (Guintoli & Mewaldt, 1978; Vander Wall & Hutchins, 1983). When cone crops fail, nutcrackers leave their home ranges in search of developing cones. The timing of these movements (beginning in early August, long before food shortage is manifest) suggests that they are triggered by the lack of green cones developing on trees (Vander Wall et al., 1981).
While most mammals are predators of pinon seeds, some rodents may serve as important dispersers [ILLUSTRATION FOR FIGURE 1 OMITTED]. Rodent dispersal of pinon pine seeds has been little studied, and rodents have generally been considered to act as seed predators with little or no positive effects as seed dispersers. However, recent research has shown that several species of rodents - including deer mice (Peromyscus maniculatus), pinyon mice (Peromyscus truei), Great Basin pocket mice (Perognathus parvus), and Panamint kangaroo rats (Dipodomys panamintinus) - scatter-hoard large quantities of Pinus monophylla seeds (Vander Wall, 1997). All of these rodents cached P. monophylla seeds in captivity. Field studies indicated that some, if not all, of these rodents gather seeds from below productive trees and scatter-hoard them up to 39 m away and between 5 and 30 mm deep. Rodents placed 36% of caches under shrubs, 39% in the open, and 25% at the edge of shrub canopies. Although not specifically documented in this study, it is possible that the caches may be relocated several times by the rodent individuals that initially cached the seeds or by other individuals that later discover them (e.g., Vander Wall, 1995; Vander Wall & Joyner, in press). At other localities, chipmunks such as cliff chipmunks (Tamias dorsalis) and Panamint chipmunks (T. panamintinus) are almost certainly important dispersers of pinon seeds. Most of these species, unlike the corvids, forage for seeds on the ground. Except in years of large cone crops, most cones do not have an opportunity to open before nutcrackers and pinyon jays remove the seeds. Christensen and Whitham (1991), for example, found that only 7.6% of Pinus edulis cones in one year opened naturally and could potentially drop seeds to the ground. Vander Wall (1997) estimated that about 10% of P. monophylla seeds fell to the ground in a year of moderate seed production and that 55% of seeds fell during a year of heavy seed production. Of course, some rodents can also gnaw into cones to remove seeds and extract seeds from open cones in the tree canopy.
Dispersal of junipers occurs both through frugivory, or the ingestion of fruit and defecation of mostly unharmed seeds by birds and frugivorous mammals, and through scatter-hoarding by rodents [ILLUSTRATION FOR FIGURE 2 OMITTED]. Burkhardt and Tisdale (1976) argued that most Juniperus occidentalis dispersal in southwestern Idaho is by gravity and trampling disturbance by large mammals, but most have considered birds to be the primary agents of juniper dispersal (e.g., Phillips, 1910; Balda, 1987). There is growing evidence, however, that dispersal systems are more complex than are generally thought and that birds are not the primary dispersal agents of all juniper species.
At least 12 species of birds feed on the fruits and potentially disperse the seeds of Juniperus occidentalis (Maser & Gashwiler, 1978), 13 species are known to disperse J. ashei (Chavez-Ramirez & Slack, 1994), and 52 species have been observed feeding on J. virginiana (Van Dersal, 1938). Of the wide diversity of birds involved, the most important are probably species of the highly frugivorous Turdinae (Muscicapidae) subfamily, such as bluebirds (Sialia mexicana and S. currucoides), Townsend's solitaire (Myadestes townsendi), and the American robin (Turdus migratorius), or members of the Bombycillidae family, such as the waxwings (Bombycilla garrulus and B. cedrorum) (Gabrielson & Jewett, 1940; Salomonson, 1978; Holthuijzen & Sharik, 1985a; Chavez-Ramirez & Slack, 1994). Members of the Turdinae are also important dispersers of European junipers (Jordano, 1993).
The earliest quantitative studies on juniper seed dispersal of which we are aware were for Juniperus monosperma in northern Arizona and New Mexico (Salomonson & Balda, 1977; Salomonson, 1978). The vast majority of apparently bird-dispersed seeds in the Sandia Mountains of New Mexico were found beneath juniper canopies, with very few found more than 2 m away from a trunk. While this suggests that few seeds were dispersed away from parent plants, it is likely that many seeds were carried away and deposited beneath another juniper, which provided about 81% of the available perches in the woodland. Birds generally disperse seeds preferentially to structurally complex microhabitats where they perch while digesting fruit and voiding seeds (McDonnell & Stiles, 1983; Robinson & Handel, 1993; McClanahan & Wolfe, 1993; Schupp, 1993; Chavez-Ramirez & Slack, 1994). Near Flagstaff, Arizona, Townsend's solitaires established juniper feeding territories of 0.7 ha in a winter of abundant cone production and of 3.85 ha in a winter with few cones (Salomonson & Balda, 1977). These territories, which were defended intra- and interspecifically, were larger than necessary for a winter's fruit supply, so substantial numbers of cones remained undispersed after the solitaires departed in the spring. The overall consequences of such behavior for seed dispersal are unknown, but defense of a fruit source may reduce the number of seeds dispersed, alter the seed shadow by reducing the number of species feeding on fruit, affect seed survival and germination by delaying the timing of dispersal, and promote occasional longer-distance dispersal as intruders are chased away after consuming a few fruits (Schupp, 1993).
Although Juniperus virginiana is an eastern species, studies of its dispersal (Holthuijzen & Sharik, 1985a, 1985b; Holthuijzen et al., 1987) provide insight into dispersal of similar western species such as J. scopulorum and perhaps J. occidentalis. In J. virginiana, more than 65% of the seed crop was dispersed by birds away from parent crowns, mostly to distances greater than 12 m. Nonetheless, the highest density of seedfall was beneath juniper canopies, with the seed shadow declining rapidly with distance from the canopy edge. As expected, density of voided seeds was dependent not only on distance from a seed source but also on structural complexity of the environment with disproportionate seedfall beneath bird perches such as fence rows.
The most thorough study to date on juniper dispersal was done by Chavez-Ramirez and Slack (1994), on avian dispersal of Juniperus ashei on Edward's Plateau of west Texas. Although they recorded 11 bird species feeding on fruits, 93% of all fruit-feeding observations were of American robins and cedar waxwings, both of which were dependent on juniper fruit during winter. Individual robins spent more time in a tree and ingested more fruits per visit, but individual waxwings visited more frequently and thus dispersed more seeds per day (683 vs. 555). Because they were also more abundant, the waxwing population dispersed far more seeds per day (38,800 vs. 5,800). Post-foraging movement, and thus the distribution of fallen seeds, was non-random for both species. A waxwing flock tended to repeatedly use the same perch, typically the tallest group of trees near productive junipers, while robin flocks flew farther and used a much greater variety of perches, including the ground. As a result of differing post-foraging behavior, densities of fallen seeds were 22,250 seeds/[m.sup.2] beneath known waxwing perches, 30 seeds/[m.sup.2] beneath known robin perches, and 5 seeds/[m.sup.2] in open sites away from potential perches. Seedling emergence was density dependent, and a greater proportion of seedlings occurred beneath robin perches than beneath waxwing perches. Additionally, for each type of perch the ratio of seedlings to seeds decreased with increasing seed density. Nonetheless, substantial recruitment occurred beneath known waxwing perches, because of the large numbers of seeds deposited.
Little is known about the importance of frugivorous mammals in juniper seed dispersal. Phillips (1910) and others have considered frugivorous mammals to be of little importance in dispersal, but this view has little quantitative support. Mammals consuming juniper fruit and dispersing seeds endozoochorously include woodrats (Neotoma spp.), Virginia opossum (Didelphis virginiana), Nuttall's cottontail (Sylvilagus nuttallii), desert cottontail (S. audubonii), black-tailed jackrabbit (Lepus californicus), coyote (Canis latrans), red fox (Vulpes vulpes), gray fox (Urocyon cinereoargenteus), black bear (Ursus americanus), ringtail (Bassariscus astutus), racoon (Procyon lotor), mule deer (Odocoileus hemionus), white-tailed deer (O. virginianus), and assorted livestock (Miller, 1921; Van Dersal, 1938; Parker, 1945; Smith, 1948; Martin et al., 1951; Johnsen, 1962; Maser & Gashwiler, 1978; Salomonson, 1978; Chavez-Ramirez & Slack, 1993; Willson, 1993; Schupp et al., 1997a, 1997b). All of these species pass at least some seeds that are intact and germinable (Miller, 1921; Johnsen, 1962; Schupp et al., 1997a).
In general, frugivorous mammals may be quantitatively more important for some species of junipers than for others, and the relative importance of different species of mammals probably varies among juniper species. For example, of the mammals dispersing Juniperus occidentalis, only coyotes are quantitatively important (Schupp et al., 1997a). In other junipers, however, other mammals may be important. Juniperus ashei is dispersed extensively by cottontail rabbits, jackrabbits, and carnivores (Smith, 1948; Chavez-Ramirez & Slack, 1993), and J. osteosperma is endozoochorously dispersed almost exclusively by cottontail rabbits and jackrabbits (Schupp et al., 1996, 1997b). As a first generalization, we suggest that coyotes and, perhaps in some species, foxes are the major mammalian endozoochorous dispersers of juniper species with fleshier, moister fruit, while rabbits and jackrabbits may be most important for species with woodier, drier fruit.
The importance of scatter-hoarding for juniper seed dispersal has probably been greatly underestimated. Clumps of juniper seedlings have been observed emerging from rodent caches in the spring (Vander Wall, 1990; Schupp, unpubl. data), and in west-central Utah, a minimum of 16-33% of all natural Juniperus occidentalis recruits [less than or equal to]2 m tall emerged from clumps, presumably resulting from rodent caches (Schupp, unpubl. data). To quantify the role of rodents in the dispersal of J. osteosperma, Vander Wall (unpubl. data) placed 500 labeled seeds under four different source trees in the Pine Nut Range, Nevada, and monitored the fates of the seeds. Slightly less than one-half of the seeds were taken (41%), and of those, 27% were cached. Although a relatively low percentage of seeds were cached, the study was conducted in mid-summer, when other, possibly more desirable shrub and forb seeds were available. A low preference for juniper seeds may result in low recovery of cached juniper seeds and, thus, a high potential for seed germination and establishment. Also, in contrast to pinon, many juniper seeds are available under the trees on a year-round basis, potentially resulting in more caching activity than this single study indicates.
Although most species of junipers are probably dispersed by both birds and mammals, it is unlikely that there is a generalized dispersal system that applies equally well to all species of junipers. The most common view has been that all juniper species are dispersed by a large suite of species of which birds are clearly most important and mammals are of incidental importance (e.g., Phillips, 1910; Martin et al., 1951; Maser & Gashwiler, 1978; Balda, 1987). In reality, this view may simply represent one end of a continuum for seed dispersal from predominantly bird-dispersed to predominantly mammal-dispersed species. Species such as Juniperus occidentalis, with small fleshy fruits that do not remain long on the tree, are probably dispersed predominantly by birds (Gabrielson & Jewett, 1940; Maser & Gashwiler, 1978; Schupp et al., 1997a). Species such as J. ashei are dispersed extensively by birds (Chavez-Ramirez & Slack, 1994), lagomorphs (Smith, 1948), and carnivores (Chavez-Ramirez & Slack, 1993), and J. osteosperma appears to be dispersed primarily by lagomorphs (Schupp et al., 1996, 1997b) and seed-caching rodents (Vander Wall, unpubl. data). The role of seedcaching rodents in juniper dispersal is probably more important than previously believed, but may be highly variable among species. Such variation in dispersal systems will have important ecological and evolutionary consequences that should be explicitly addressed.
C. SEED PREDATORS
Seed dispersers often kill or consume a portion of a seed crop while dispersing seeds, but many vertebrates act solely as seed predators [ILLUSTRATION FOR FIGURES 1 AND 2 OMITTED]. In pinon pines, hairy woodpeckers (Picoides villosus) and common flickers (Colaptes auratus) drill into cones to feed on developing seeds. Porcupines (Erethizon dorsatum) eat the whole cones when they are still green and soft. Mountain chickadees, plain titmice (Parus inornatus), and red-breasted nuthatches remove seeds from open cones and either eat them or store them for future use, but they are ineffective seed dispersers because they store most seeds in bark crevices where seedling establishment is impossible. Several species of woodrats (Neotoma) and ground squirrels (Spermophilus) larder-hoard large quantities of pinon seeds in their nests, where seedlings have no chance of establishing. And a variety of animals, including mule deer, black bear, quail, turkeys, and humans, consume seeds that have fallen to the ground. In summary, a host of animals turn to pinon seeds to supplement their diets when the seeds are available.
In juniper, some species feeding on the fruits are seed predators rather than dispersers. Salomonson (1978) noted that evening grosbeak (Hesperiphona vespertina) and Cassini's finch (Carpodacus cassinii) were predators on Juniperus monosperma seeds in the Sandia Mountains, New Mexico. Near Blacksburg, Virginia, purple finches (Carpodacus purpureus) destroyed 0.9% of the J. virginiana cone crop in a year with small cone crops and 3.1% in a year with large cone crops (Holthuijzen & Sharik, 1985; Holthuijzen et al., 1987). Although Salomonson (1978) suggested they were neither seed dispersers nor seed predators, plain titmice (Parus inornatus) feed extensively on J. osteosperma seeds in west-central Utah (Fuentes & Schupp, 1998). After plucking a fruit from a branchlet, they move to a larger branch, remove the pulp, hammer the seed coat open with the bill, and extract and eat the embryo and endosperm. Interestingly, birds fed more heavily on trees having higher proportions of filled seeds, suggesting the possibility of a selective advantage to retaining and maturing fruits containing empty seeds. This is in contrast to the apparent selective advantage of abscising insect-attacked fruits, since abscission can increase predator mortality and thus potentially reduce future damage (Fernandes & Whitham, 1989).
The roles of rodents in juniper population dynamics are poorly understood, but many act exclusively as seed predators. Species believed to be primarily seed predators include the white-tailed antelope squirrel Ammospermophilous leucurus, the ground squirrels Spermophilus lateralis and S. variegatus, the cricetid mouse Peromyscus difficilis, and the woodrats Neotoma albigula, N. stephensi, and N. mexicana (Martin et al., 1951; Frischknecht, 1975; Maser & Gashwiler, 1978; Salomonson, 1978; Zeveloff, 1988). The complete list of species feeding on juniper seeds is surely much longer. Fruits are harvested both directly from the trees and from the ground. Neither the use of pulp versus seed nor the fates of harvested seeds has been studied in any species of juniper, although large piles of split seed coats are evidence that very many are consumed and destroyed and that rodents are important juniper seed predators.
D. DIFFERENCES IN DISPERSER EFFECTIVENESS
The concept of disperser effectiveness provides a useful framework for evaluating the link between dispersers and woodland tree dynamics. Disperser effectiveness has been defined as the contribution a disperser makes to the future reproduction of a plant population (Schupp, 1993). In terms of the seed and seedling fate diagrams [ILLUSTRATION FOR FIGURES 1 AND 2 OMITTED] effective dispersers are those that result in the establishment and survival of seedlings. Effectiveness has both a "quantitative component," the number of seeds dispersed, and a "qualitative component," the likelihood that a dispersed seed will survive to produce a new adult in the population. Here we use the available information to assess the effectiveness of the various dispersers described above for the recruitment of pinons and junipers.
The relative effectiveness of seed-hoarding birds and rodents for the dispersal of pinons varies among disperser taxa and pinon species. Although poorly quantified, avian dispersers potentially scatter-hoard many more pinon seeds than do rodent dispersers and, thus, are quantitatively highly effective. Birds such as nutcrackers and pinon jays begin harvesting seeds from closed cones before most scatter-hoarding rodents have access to the seeds. When seed crops are small to moderate in size, the birds may deplete the supply of seeds before rodents have much opportunity to forage. Only during large seed crop years, when many of the seeds fall to the ground, do scatter-hoarding rodents have the opportunity to harvest a significant portion of the seed crop. Qualitatively, rodents may be highly effective dispersers during large seed crop years. While jays and nutcrackers cache a large proportion of seeds in open environments, rodents appear to cache a high proportion of seeds under shrubs (Vander Wall, 1997). Because pinons have a fairly strong nurse plant requirement (Drivas & Everett, 1988; Callaway et al., 1996; see below), establishment is frequently higher under shrubs than in open environments. An important aspect of bird dispersal is that corvids have the potential to disperse seeds much farther than rodents, thus increasing the chance of founding new populations.
For endozoochorous dispersal of juniper, the relative effectiveness of birds and mammals likely varies with species of juniper, species of birds or mammals involved, and site conditions. For most juniper species under most site conditions, birds may be more effective dispersers than frugivorous mammals. In species such as Juniperus occidentalis, birds appear to disperse the majority of seeds and are thus quantitatively important (Maser & Gashwiler, 1978). They may also frequently be qualitatively important. Birds tend to regurgitate or defecate relatively few seeds in a group, while important mammals such as carnivores frequently deposit hundreds of seeds in a single defecation (Howe, 1980; Levey, 1986; Chavez-Ramirez & Slack, 1993; Schupp, 1993; Schupp et al., 1997a). Consequently, seeds dispersed by birds may be less likely than those dispersed by mammals to die from density-dependent seed predation or competition. This is not a universal expectation, since cottontail rabbits, jackrabbits, and deer generally deposit few seeds together, and repeated use of a single perch by birds can lead to high densities. of seeds. Also, while birds tend to deposit seeds beneath woody vegetation used as perches, mammals generally deposit seeds in the open, where there are fewer obstacles for walking (Bustamante et al., 1992; Chavez-Ramirez & Slack, 1993, 1994; Schupp, 1993, 1995; Schupp & Fuentes, 1995; Schupp et al., 1996, 1997a, 1997b). Because the dependence of juniper on nurse plants appears to be variable (Chambers et al., 1998; see below), the consequences of different dispersers likely vary with environmental conditions. Although gut passage time for birds is frequently less than one hour, gut passage for most mammals can take from a day to a week or more (Levey, 1986; Janzen, 1982; Schupp, 1993; Murray et al., 1994), resulting in longer-distance dispersal. A limitation of both bird and mammal dispersal is that seeds are deposited on the soil surface and are dependent on other mechanisms of burial.
The consequences of seed-hoarding rodents for juniper dispersal and recruitment are not well understood. Preliminary data indicate that rodents cache relatively few juniper seeds (Vander Wall, unpubl. data), so they may not be quantitatively very important. Because rodents bury seeds in the soil where conditions are favorable for hydration and germination, they may be qualitatively very effective. Also, since juniper seeds do not appear to be highly desirable, once cached they may survive unrecovered.
The overall effectiveness of different juniper dispersal agents probably varies geographically among species. In those areas where juniper establishment and survival benefit from nurse plants, such as extensive areas of the Great Basin, frugivorous birds and scatter-hoarding rodents are probably most effective because of their tendency to deposit seeds under shrubs or trees. In grasslands with few shrubs and with sufficient summer precipitation for seedling establishment, such as areas of the Southwest and Edward's Plateau of Texas, frugivorous mammals may be highly effective dispersers of juniper despite depositing seeds in clumps. Under such conditions, extensive establishment may occur in the open without benefit of nurse plants. Both Miller (1921) and Smith (1948) noted early on that dispersal by frugivorous mammals led to widespread recruitment, or invasion, of Juniperus monosperma throughout the southwestern grasslands, while frugivorous birds were responsible for patchier recruitment around potential perches such as shrubs, trees, and fence lines.
Even within a class of dispersal agent, interspecific differences in behavior can lead to very different effects on juniper recruitment. This is clearly demonstrated by Chavez-Ramirez and Slack's (1994) study of Juniperus ashei dispersal. Cedar waxwings were quantitatively far more important dispersers than robins because there were many more waxwing individuals, each of which visited trees more frequently. However, based on patterns of seedling emergence, it appears that robins were qualitatively superior. While waxwings deposited most seeds in very high-density patches beneath specific perches where they apparently suffered extensive mortality and/or reduced germination, robins scattered seeds more widely where they were more likely to successfully establish. Although it is not clear which species is ultimately the most effective disperser of J. ashei, it is clear that dispersal systems are more complex than is generally appreciated and that it is difficult to evaluate the effectiveness of dispersal without thorough, well-designed studies.
E. EFFECTS OF SEED CROP SIZE ON SEED DISPERSAL AND PREDATION
In pinon pines, the proportion of the seed crop eaten by animals (vertebrate and invertebrate) is thought to vary inversely with cone crop size, and this relationship is thought to be responsible for the regionwide synchrony in cone production and the marked annual variation in cone crop size (Vander Wall & Balda, 1977; Ligon, 1978; Smith & Balda, 1978). pinon trees begin cone production at 10-20 years of age, but early cone crops are typically small. Heavy seed production does not begin until age 100, and continues for about 200 years (Lanner, 1981). Most trees within a region are synchronized, with Pinus edulis producing large cone crops (mast crops) every 5-7 years (Jeffers, 1994) and P. monophylla producing large crops every 2-3 years (Tueller & Clark, 1975). In the intervening years, the size of cone crops ranges from moderate to nil. Different regions (i.e., populations separated by several hundred kilometers) are often not synchronized, so one region may produce a bumper crop of cones while another region produces none. Synchronization of cone crops presumably is caused by a populationwide response to weather patterns at a critical stage in cone development, although the exact environmental variables are unknown.
Small cone crops prevent the buildup of populations of harmful insects that can destroy large portions of the seed crop (Forcella, 1980). Insect populations may peak after bumper seed crops, but the harmful insects do not have sufficient time to respond to the abundance of cones and most cones escape insect herbivores. The following year, when the cone crop is small and insect populations high, most insect pests die without having the opportunity to breed. The reproductive success of pinyon jays also fluctuates with the size of pinon seed crops (Ligon, 1978). Corvids and rodents respond to seed abundance by caching most seeds (e.g., Christensen et al., 1991; Christensen & Whitham, 1991), often many more than they could possible consume. Although the abundance of food may trigger breeding and increased reproductive success (Ligon, 1974, 1978), a large proportion of the seeds escape predation because the birds cannot respond numerically fast enough to cause significant seed losses. In short, the bust/boom cycles of pinon pine seed production create conditions under which seed predators cannot track the food supply, but are periodically satiated so that on average a higher proportion of seeds survive to germinate. Although the proportion of seed caches recovered by corvids and rodents has never been accurately estimated, it seems safe to assume that most seeds are eventually recovered either by the cachets or by other seed predators and consumed during the winter and early spring. Because a sufficiently large number of seeds survive to germinate, animal caching serves as an effective means of seed dispersal.
When cone crops are small to moderate in size, corvids and rodents compete for seeds (Christensen & Whitham, 1993). Herbivorous insects can alter this competitive balance. When Pinus edulis trees experience heavy insect infestations, the proportion of the seed crop taken by corvids and rodents declines. Corvids tend to avoid trees with numerous cones infested by insects and, when everything else is equal, prefer trees with large displays of healthy cones. Rodents appear to be less sensitive to the presence of infested cones. Consequently, the proportion of seeds taken by rodents, relative to that taken by birds, increases on trees heavily infested by insects. It is still unclear whether this shift is important to the trees because rodents may be as effective as corvids in dispersing seeds.
For junipers, the frequency of large seed crops is variable among species, populations, and individuals. Heavy seed production may occur annually in Juniperus occidentalis (Deal, 1990), but most species tend to fruit irregularly, skipping 2-5 years between large cone crops (Johnsen & Alexander, 1974; Noble, 1990). Although this may be viewed as a form of masting at the population level, regionwide synchrony is apparently not as great as it is in pinon pines. Nearby populations may be out of synchrony, and even within a given population some individuals produce large crops frequently while others do so only rarely (Schupp, unpubl. data). Consequently, most populations have some individuals ripening fruit most years (Johnsen & Alexander, 1974), and even in years of generally low seed production, some individuals may produce very large crops (Schupp, unpubl. data). Consequences of variation in cone production at the population and individual levels for seed dispersal and predation are unknown.
VII. Post-Dispersal Seed Mortality
In addition to being eaten by animals, the causes of post-dispersal seed mortality in pinon and juniper, as in other species, include insects, pathogens, physiological death, and failed germination ([ILLUSTRATION FOR FIGURES 1 AND 2 OMITTED]; Chambers & MacMahon, 1994). Few data are available on post-dispersal mortality in pinon and juniper, and most of what is known is derived from laboratory or greenhouse experiments. In general, pinon seeds are short-lived, losing viability rapidly after one year of storage (Meewig & Bassett, 1983). Seed viability of newly harvested seeds of Pinus monophylla ranged from 56% to 90% for different populations harvested on the west side of the Great Basin (Gilleard, 1985). Six months after harvest, viability of these seeds was observed to decrease an additional 16% (Gilleard, 1985). Although it has been reported that P. edulis seeds can be stored for 5-10 years at -18 [degrees] to -7 [degrees] C and 5-10% moisture without losing viability (Jeffers, 1994), this has not been adequately substantiated. Several species of fungi occur on pinon seeds that can decrease both seed viability and germination percentage (Meagher, 1943; Kintigh, 1949; Gottfried & Heidmann, 1986). Fungi found on fresh seeds of P. edulis include Penicillium, Alternaria, Cledosporiumn, Bispora, Gliscladium, and Rhizopus (Gottfried & Heidmann, 1986); those on P. monophylla included Penicillium, Botrytis, Aspergillus, Rhizopus, Alternaria, Beauvearia, and Trichoderma (Gilleard, 1985). Laboratory germination of P. monophylla tends to be inversely related to the amount of fungal infection and can be increased by the application of fungicides (i.e., Captan and Dithane) (Gilleard, 1985).
In contrast to pinon, junipers are characterized by long-lived seeds. Tests of stored juniper seeds showed that 45-year-old Juniperus osteosperma still had 17% germination, 21 -year-old J. monosperma had 54% germination, and 9-year-old J. deppeana had 16% germination (Johnsen, 1959). The lower germination percentage for the youngest seeds may reflect differences in the seed lots, as seed germination in junipers varies among both years and ecotypes (Van Haverbeke & Comer, 1985; Young et al., 1988).
VIII. Seed Germination
In addition to being short-lived, pinon seeds also have little innate dormancy. Thus, it is likely that pinon seeds are capable of germinating only during the first or rarely during the second year after being scatter-hoarded by birds or mammals [ILLUSTRATION FOR FIGURE 1 OMITTED]. For Pinus monophylla seeds artificially cached in several different microhabitats in two separate years and protected from animal foragers, seedlings emerged only the first year after seeding (Chambers, unpubl. data). Also, seedlings that germinated on the soil surface always died, indicating that seeds must be buried in order for establishment to occur.
Laboratory data indicate that optimum germination temperature varies among pinon species and that the more southerly distributed species have higher temperature optima for germination. Pinus cembroides had the highest germination percentage at 21.1 [degrees] C (Kintigh, 1949) or 25 [degrees] C (Floyd, 1981), P. edulis at 20 [degrees] C (Floyd, 1981), and P. monophylla at 13.6 [degrees] C (Gilleard, 1985). For P. monophylla, the highest overall germination occurred with an alternating temperature regime of 2-15 [degrees] C. It is likely that temperature optima also vary within species, but no data are available on ecotypic differences. Although cold/moist stratification apparently has no affect on total germination of pinon seeds, it can increase the rate of germination. For P. edulis seeds, a 60-day cold/moist stratification period resulted in more rapid germination than a 30-day period (Gottfried & Heidmann, 1986). Also, cold/moist stratification has been observed to increase the rate of germination of P. monophylla seeds (Chambers, unpubl. data).
In junipers, germination of the long-lived seeds can be delayed and emergence can occur one to several years after defecation by birds or mammals or scatter-hoarding by rodents [ILLUSTRATION FOR FIGURE 2 OMITTED]. In a field germination experiment of Juniperus osteosperma in west-central Utah, seedlings emerged in each of the last three years and emergence is expected to continue (Schupp & Gomez, unpubl. data). Delayed germination and emergence may be due to impermeable seed coats, immature embryos, embryo dormancy, or the presence of inhibitors (Pack, 1921b; Gerbracht, 1937; Barton, 1951; Djavanshir & Fechner, 1976). The seed covering consists of a pericarp or an outer fleshy layer of pectic substances, and a seed coat or thick lignified stony layer and thin inner membranous and suberized layer that can interfere with moisture uptake (Johnsen & Alexander, 1974). Seeds of J. scopulorum and J. virginiana apparently have restricted moisture uptake because the hilum is obstructed by vascular tissues at the fruit base (Johnsen & Alexander, 1974).
Most of the studies on juniper seed germination have been conducted to increase nursery production or seeding success, not to understand the ecology of the species in the wild. Efforts to increase juniper seed germination usually begin by depulping, or removing the outer fleshy layer (pericarp), to increase the permeability of the seed coats (Afanasiev & Cress, 1942; Van Haverbeke & Barnhart, 1978). Removal of the pericarp alone can increase germination of viable seeds (Johnsen, 1962). Other treatments to increase seed coat permeability and to leach possible germination inhibitors have included soaking seeds in sodium-lye (Webster & Ratliffe, 1942), alcohol or boiling water (Chadwick, 1946; Rietveld, 1989), concentrated sulfuric acid (Barton, 1951; Rietveld, 1989), citric acid (Cotruto, 1963; Van Haverbeke & Comer, 1985), and hydrogen peroxide (Trappe, 1961; Riffle & Springfield, 1968; Fisher et al., 1987; Rietveld, 1989); and freezing seeds in ice (Jelley, 1937). These treatments have met with varying success.
A frequently used and successful treatment for breaking seed dormancy of juniper seeds, particularly physiological dormancy, is stratification of fully imbibed seeds at low temperatures, 0.5-5 [degrees] C for up to 6 months (Johnsen, 1962; Riffle & Springfield, 1968; Fisher et al., 1987; Young et al., 1988). Germination rates and levels are often increased further by a combination of warm/moist and cold/moist stratification. For Juniperus scopulorum, the highest germination percentage reported was obtained by warm/moist stratification (20 [degrees] C night/30 [degrees] C day) for 45-90 days, followed by cool/moist stratification (5 [degrees] C) for 30-120 days (Johnson & Alexander, 1974). Germination of eastern red cedar (J. virginiana) seeds was enhanced by a combination of warm/moist stratification (24 [degrees] C for 6 weeks) followed by cool/moist stratification (5 [degrees] C for 10 weeks) (Van Haverbeke & Comer, 1985). Stratification in aqueous solutions with near saturation of the solution with oxygen increased germination of both J. monosperma and J. osteosperma seeds (Young et al., 1988). Addition of 0.289 m mol/L gibberellic acid (G[A.sub.3]) further increased seed germination.
In field situations, passage of juniper seeds through the digestive track of an animal appears to have varying effects on germination. Seeds of Juniperus monosperma from the droppings of bird, fox or coyote, packrat, jackrabbit, and sheep exhibited higher rates of germination than uningested seeds (Johnsen, 1962). However, the passage of J. monosperma seeds through the guts of Townsend's solitaires reduced germination (Salomonson, 1978). Because individual plants differ in seed germination percentages and individual animals differ in treatment in the gut, Murray (1988) warned that these types of data must be interpreted with caution. To really understand the importance of endozoochory on seed germination, it is necessary to use multiple animals, each fed seeds from multiple parents.
IX. Seedling Establishment
Seedling establishment probabilities of both pinon and juniper depend on seeds being dispersed to suitable microhabitats [ILLUSTRATION FOR FIGURES 1 AND 2 OMITTED]. As pinon and juniper stands mature, they often develop a characteristic heterogeneity that includes under-tree canopy, under-shrub canopy, and interspace (i.e., areas without woody plant cover) microhabitats. Because of the large rooting volumes of shrubs and especially of juniper and pinon in these semi-arid ecosystems (Everett & Sharrow, 1985), interspace microhabitats are often sparsely vegetated. On different mountain ranges in Nevada and Utah, average total vegetation cover of juniper and pinon woodlands ranged from only about 25% to 50% (Tueller et al., 1979), indicating that cover of bare soil, litter, gravel, and rock was 50-75%. As pinon and juniper expand into shrub environments, the percentage of under-shrub microhabitats declines (Eddleman, 1987; Miller & Rose, 1995), as does the herbaceous plant cover in the interspaces between trees (Tausch et al., 1981).
Seedlings of both pinon and juniper often occur under shrubs or other trees, indicating that the microhabitats created by nurse plants provide conditions important for establishment (Johnsen, 1962; Jameson, 1965; Burkhardt & Tisdale, 1976; Everett et al., 1986b; Eddleman, 1987; Callaway et al., 1996). Seeding or transplanting experiments have shown that seedling survival is higher under at least some species of nurse plants than in interspace environments for Juniperus osteosperma and Pinus monophylla (Callaway et al., 1996; Chambers et al., 1998). In fully stocked pinon-juniper woodlands, higher numbers of seedlings typically occur under the trees than under sagebrush or other potential nurse plants. Seedlings were more abundant in under-tree microhabitats than in interspaces for P. monophylla in Nevada (87%) (Everett et al., 1986b), J. monosperma in Arizona, (84%)(Johnsen, 1962), and J. occidentalis in Oregon (86%) (Miller & Rose, 1995). In contrast, in sagebrush communities being invaded by juniper, higher numbers of seedlings often occur under sagebrush than under trees. In Artemisia tridentata subsp. vaseyanal J. occidentalis communities in Oregon and Idaho with 34% tree cover, 52.3% of the juniper seedlings were located under sagebrush, while 31.3% were found under trees (Eddleman, 1987). In communities with 6% tree cover, 58% of the seedlings were under shrubs and 29% were under junipers (Miller & Rose, 1995), and in communities with 3% tree cover, 65% of the seedlings established under sagebrush and 17% established under trees (Burkhardt & Tisdale, 1976).
A more detailed examination reveals large differences in the nurse plant requirement both among and within species. pinon seedlings rarely establish in interspaces or open environments (Everett et al., 1986b; Drivas & Everett, 1988; Callaway et al., 1996). This is well illustrated by a total lack of first year survival of Pinus monophylla seedlings in interspace microhabitats in the Pine Nut Range, Nevada (Chambers, unpubl. data). In contrast, juniper seedlings appear to be capable of establishing in these microhabitats if the proper conditions exist. In the Great Basin and more northern areas of the woodlands, establishment occurs in interspaces but at lower frequencies than under trees or shrubs. First year survival of Juniperus osteosperma seedlings in the Pine Nut Range, Nevada, was less in interspace microhabitats than in under tree habitats, but was as high or higher than in under sagebrush habitats (Chambers et al., 1998). In Tintic Valley, Utah, emergence and survival of J. osteosperma differed among shrub, tree, and interspace habitats and among years, but seedlings established in interspace habitats in all years (Chambers et al., 1998). In expanding J. occidentalis communities, 18-47% of established seedlings occurred in interspaces (Burkhardt & Tisdale, 1976; Miller & Rose, 1995). For J. osteosperma on stabilized Lake Bonneville sand dunes in Utah, most of the few natural seedlings occurred in open interspaces, the most abundant microhabitat, and recruitment was statistically independent of microhabitat (Schupp, unpubl. data). Similarly, in southwestern grass- and shrublands, establishment of juniper seedlings in open environments occurs routinely (Miller, 1921; Johnsen, 1962; Salomonson, 1978).
Differences between pinon and juniper in the requirement for a nurse plant may be related to differences in their physiological characteristics. Juniper species have a greater drought tolerance and a higher capacity to obtain water resources from interspace microhabitats and shallow soils (Nowak et al., 1998). This may enable seedlings to establish in unshaded interspaces with higher soil temperatures.
In those ecosystems with little summer precipitation, such as much of the Great Basin, under-tree and -shrub microhabitats appear to provide the most favorable environments for seedling establishment of both pinon and juniper. In general, the microhabitats under both shrubs and trees have lower irradiance, soil temperatures, and effective precipitation, but higher humidity and delayed summer dry-down relative to open interspaces or grass communities (Johnsen, 1962; Vetaas, 1992; Stark, 1994). Also, soils beneath shrubs and trees often have lower bulk densities, higher concentrations of limiting resources (e.g., N and P), higher organic matter content, higher infiltration and soil water-holding capacities (Doescher et al., 1984; Everett et al. 1986a; DeBano & Klopatek, 1987; Doescher et al., 1987; Klopatek, 1987; McDaniel & Graham, 1992; Gutierrez et al., 1993), larger populations of microorganisms (Bolton et al., 1993), and higher rates of nutrient cycling (Charley & West, 1977; Bolton et al., 1990; Virginia et al., 1982; Evans & Ehleringer, 1994). Several of these characteristics may facilitate pinon and juniper establishment, but the most important appear to be shade and temperature modification. Seedling establishment of many conifer species is enhanced by shading, and shade has been demonstrated to increase survival of Pinus edulis seedlings (Meagher, 1943; Fowells, 1965). In Artemisia tridentata var. vaseyana/J. occidentalis communities in Oregon, shading resulted in summer soil surface temperatures that were 45-57% lower under nurse plants than on bare soil (Burkhardt & Tisdale, 1976).
Although nurse plants can facilitate tree seedling establishment, they also compete for light, water, and nutrients, with the outcome of the interaction being a balance between competition and facilitation (Bertness & Callaway, 1994). pinon and juniper seedlings exhibit higher survival in artificial shade (Phillips & Mulford, 1912; Meagher, 1943), but seedlings in full sun grow faster than those beneath shrubs (Burkhardt & Tisdale, 1976; Harrington, 1987; Miller & Rose, 1995; Callaway et al., 1996). Seedlings and juveniles may have physiological characteristics that result in more favorable water relations than those exhibited by adult trees or sagebrush nurse plants. Seedlings of Juniperus occidentalis have tighter stomatal control over water use than adult trees and are more responsive to the changing environment (Miller et al., 1992). Also, Pinus monophylla seedlings associated with sagebrush exhibit reduced stomatal conductance, while sagebrush water use continues to increase during summer and reaches levels up to five times greater (per unit leaf area) than associated pinons (Drivas & Everett, 1988). Even so, seedling mortality increases during droughts and it is likely that the competitive interaction with the nurse plant exacerbates this mortality. As noted above, the importance of nurse plant facilitation for pinon pines clearly outweighs the competitive effects in all situations. Also, facilitation tends to favor juniper in regions with little summer precipitation, such as the Great Basin and more northern woodlands. However, higher establishment of juniper in open or interspace environments in the southwestern portion of the woodlands indicates that the balance shifts with differences in precipitation patterns. Summer monsoonal rains in the southwestern grass- and shrublands often occur during the hottest months of the year. This may reduce the beneficial microenvironmental effects of nurse plants without reducing the competitive effects. Thus, the net effects of shrubs and trees on juniper seedling establishment in the Southwest are less facultative and may even switch to predominantly competitive.
Causes of seedling mortality unrelated to facultative or competitive interactions include animals, insects, and pathogens. Animal predation on seedlings has been observed to significantly decrease survival rates in Juniperus scopulorum (Fisher et al., 1990) and Pinus monophylla (Callaway et al., 1996). Seedlings appear most susceptible to animal predation during the first two years of establishment. Potential predators in pinon-juniper woodlands include domestic livestock such as cattle, sheep, horses, goats, and wildlife such as elk, mule deer, jackrabbits, cottontails, chipmunks, woodrats, kangaroo rats, and mice. Effects of insects and pathogens on seedling survival of the tree species have received little study. Parasitism of P. edulis and J. monosperma seedlings by different nematode species can result in reduced growth (Riffle, 1972) and, undoubtedly, many of the insects and pathogens that infect adult trees influence seedling growth and survival.
A. REGIONAL MIGRATION AND LOCAL EXPANSION
1. Prehistoric and Historic Migration of the Woodlands
The geographic distribution of the pinon and juniper species within the woodlands have changed dramatically during the Holocene, or last 11,500 years. During the Pleistocene ([greater than]11,500 B.P.), species distributions were shifted far to the south of their current ranges and were limited to climatically protected or lower-elevation areas. In response to ameliorating climate during the Holocene, the tree species within the woodlands moved upwards in elevation as much as 1000-1500 m and migrated northward as much as 1000 km to occupy their current ranges. At the end of the Wisconsin ice age, Pinus edulis and P. monophylla were found in what is now desert scrub habitat at 550-1525 m in southeastern California, southern Nevada, western and southern Arizona, southern New Mexico, and the states of Baja California, Sonora, Chihuahua, and Coahuila in Mexico (Lanner, 1981; Van Devender, 1986; Betancourt, 1987; Wells, 1987; Thompson, 1990). Currently, the northern limits of P. edulis are in northeastern Utah and northwestern Colorado, while those of P. monophylla extend to northern Nevada and southern Idaho. The movement of junipers during the same time period is somewhat more complex because of the number of species involved and the larger distributions. During the Pleistocene ([greater than]11,500 B.P.), Juniperus osteosperma was widely scattered in the lower-elevation and climatically protected areas of the western, south-central, and southern Great Basin (Thompson, 1990; Wigand et al., 1995). Juniperus occidentalis was in the lower foothills around the Sacramento and San Joaquin Valleys on lower-elevation areas of higher mountain ranges between the present Mojave Desert and the southern Sierra Nevada Mountains (Cole, 1983). The northernmost populations of J. scopulorum were apparently located in refugia in southeastern Oregon, southern Idaho, southeastern Wyoming, and northern Colorado, with the populations in the center portion of the range being displaced far out into the Great Plains (Adams, 1983). As the climate warmed in the Holocene, J. osteosperma moved northward and expanded outward from more northerly refugia into higher-elevation areas of the Great Basin in the west (Nowak et al., 1994) and Wyoming in the east (Betancourt, 1987). At the same time, J. occidentalis begin moving into higher-elevation areas of the Sierra Nevada Mountains and also north and east into northeastern California and southern Oregon (Mehringer & Wigand, 1987). Juniperus scopulorum migrated northward to its current northern limit in southern and central British Columbia, southern Alberta, and northern Montana (Adams, 1983). While patterns for J. monosperma appear to parallel those for P. edulis (Betancourt, 1987), the record is not as clear as it is for many of the other species.
This expansion of the different species within the woodland during the Holocene was not continuous or unidirectional. During the warmest parts of the Holocene (mid-Holocene, 8,000-5,500 B.P.; Post Neoglacial Drought, 2,500-1,300 B.P.) the woodlands distributions decreased and their northward migration was slowed or stopped. During periods more favorable to plant growth (early Holocene, 11,500-8,000 B.P.; Neoglacial, 4,500-2,500 B.P.; Little Ice Age, 550-150 B.P.), expansion of the woodland was the most rapid. During dry periods the areas dominated by woodlands were smaller and the woodlands were more open, whereas during favorable climatic periods, the woodlands were more broadly distributed. The current spatial distribution of the woodlands was reached by, or shortly after, 1,000 B.P. Concurrent with the end of the Little Ice Age and settlement of the West by Anglo-Americans around 200 years ago, the woodlands began to expand within their current range. This ongoing expansion has resulted in possibly the largest total area dominated by woodlands and the highest tree density within the woodlands during the Holocene. While past expansions occurred with increasing precipitation and invasion into previously xeric sites, recent expansions have been into more mesic shrub and grasslands and have occurred during a period of increasing aridity (Mehringer & Wigand, 1990; Miller & Wigand, 1994). This has led many to believe that recent changes in woodland dynamics are closely linked to human activities and changes in land use patterns (Miller & Wigand, 1994; see below).
2. Short- and Long-distance Dispersal Events
Recent theoretical and empirical work indicate that short-distance dispersal, local population growth, and long-distance dispersal are all important when considering the migration rates of species (Clark et al., 1998). Although rapid migration is driven proximately by relatively rare long-distance dispersal events, the likelihood of such long-distance movement increases with seed abundance at the existing front. The greater the local population growth, the greater the seed production, and the more seeds produced, the higher the probability that some will move long distances. Whether concerned about local migration (invasion) or long-distance migration (population founding), it is crucial to consider both the local component of seed dispersal that is responsible for rapid local population growth and the long-distance component of seed dispersal that is responsible for the "leaps."
As is the case for other plant species (see Clark et al., 1998), different classes of dispersers appear to be most responsible for local (short-distance) vs. long-distance dispersal in pinons and junipers. In pinon pines, relatively short-range dispersal that results in recruitment within existing stands is accomplished largely by rodents, scrub jays, Mexican jays, and pinyon jays that cache seeds in the pinon-juniper woodland, often within a few hundred meters of the source tree (Vander Wall & Balda, 1981; Vander Wall, 1997). Unexploited caches produce plants, causing population growth, local range expansion, and replacement of senescent individuals. Relatively long-range dispersal, by which seeds cross inhospitable habitats to colonize new areas, is achieved by nutcrackers, pinyon jays, and Steller's jays - birds that often carry seeds long distances to new habitats. These birds can easily cross arid valleys, rivers, and mountain ridges, and other areas inhospitable to pinon pines. Seed dispersal out of the pinon-juniper woodland introduces the species to habitats that may or may not be suitable for pinon pine establishment. If suitable, the cached seeds can establish a founding colony of pines from which a new stand might eventually arise. Vander Wall and Balda (1977) found Pinus edulis growing at middle elevations on steep south-facing slopes in the San Francisco Peaks of northern Arizona, more than 500 m above the typical upper distributional limit of pinon pine in this region. Lanner (1972) reported scattered P. monophylla growing along ridges in northern Utah, 16-37 km from the nearest stand of trees. Many of these trees were of mature size but seemed to produce few viable seeds. Despite their low seed production, these trees are important in that they represent the potential of pinon pines to colonize widely scattered patches of suitable habitats at great distances from source areas. Scatter-hoarding corvids are likely responsible for transporting seeds to these distant sites. If these new habitats are favorable for establishment and if the rodents and corvids necessary for local dispersal exist at these locations, founding populations can establish and a relatively rapid build up of local populations can occur.
The two extreme spatial scales of dispersal do not appear as clear-cut for juniper as for pinons, but similar processes surely occur. Depending on the juniper species, local dispersal leading to recruitment and population increases within existing stands is likely due largely to territorial or resident frugivorous birds and lagomorphs and scatter-hoarding rodents. Most birds have short gut-retention time and fly only short distance to a perch for processing fruit before returning to the same tree or moving to a nearby fruit source (Hoppes, 1987; Schupp, 1993). Thus, most dispersal is likely within or near the woodlands. Although rodents and lagomorphs are capable of moving seeds on the order of hundreds of meters, they cache or deposit most seeds within or near the woodland. Thus, they are unlikely agents for long-distance dispersal across inhospitable terrain.
Long-distance dispersal of junipers is likely effected by birds and large frugivorous mammals. Migrating or wandering flocks of robins, bluebirds, and waxwings potentially disperse seeds longer distances (Gabrielson & Jewett, 1940). Large frugivorous mammals are also likely candidates for long-distance dispersal, because they have long gut-retention times and can travel extensively across diverse terrain. For example, foxes and bears may move [greater than] 10 km in a day (Storm & Montgomery, 1975: Willson, 1993) and similar travel distances are expected for coyotes. Although the effectiveness of dispersal may be relatively low for such mammals, the occasional, successful dispersal event may be responsible for regional migrations.
3. The Role of Ecotones
Ecotones are important because they form the interface between the woodlands and adjacent communities and can provide valuable information on the mechanisms of tree expansion. They can occur as transitional zones between the woodlands and adjacent grasslands and shrublands, or as relatively abrupt boundaries between the woodlands and areas that have been disturbed by natural causes such as fire, or anthropogenic causes such as chaining or other tree removal. Ecotones provide areas for seedling establishment with little competition from adult trees and, depending on the availability of suitable establishment microhabitats, may be areas of high tree expansion. Because ecotones are utilized by animal dispersers that occur primarily in one community or the other as well as by those that utilize both communities, they may be particularly active zones of seed dispersal. Frugivorous mammals are often very active on the ecotone (Mason, 1981) and frequently forage hundreds of meters out of the woodland into shrub- and grasslands. Lagomorphs are known to deposit seeds in grasslands up to 1.6 km from the nearest woodland (Smith, 1948; Frischknecht, 1975; Schupp et al., 1997b). However, in a study of an ecotone between a Juniperus osteosperma woodland and a disturbed area dominated by the weedy, annual grass Bromus tectorum, lagomorphs deposited most seeds within or directly adjacent to the woodland, with juniper seed density declining rapidly with distance from the woodland (Schupp et al., 1997b).
The use of bird species to ecotonal areas appears to be species specific. In general, important dispersers of pinon, such as the corvids, are dependent on the woodlands but utilize the ecotonal areas (Mason, 1981; Vander Wall, pers. obs.). Because important avian dispersers of junipers such as the Turdinae and waxwings are more likely to use other trees and larger snags as perches, they may be more important for seed movement within the woodlands or following disturbances that leave a portion of the trees in place. Miller (1921) suggested that Juniperus osteosperma was successfully invading grasslands in northern Arizona while J. monosperma was recruiting almost exclusively within the existing woodland because the former was dispersed widely by mammals, including sheep, while the latter was dispersed almost exclusively by birds using juniper trees as perches.
The dispersal of pinon and juniper seeds by scatter-hoarding rodents across these ecotones has not been examined. However, studies of the changes in rodent species densities following fire or human disturbance indicate that the potential is high for seed movement from the woodlands into recently disturbed areas. Following tree removal (burning or chaining), rodent species known to disperse pinon and juniper seeds generally exhibit similar or higher species richness and densities on treated plots and in ecotonal areas (Mason, 1981; Sedgewick & Ryder, 1987; Severson, 1986: Albert et al., 1994; Kruse, 1994). Even species such as pinon mice that are dependent on the presence of trees (Severson, 1986; Sedgewick & Ryder, 1987), and Great Basin pocket mice and chipmunks that prefer some tree or shrub cover, utilize ecotonal areas once a vegetational cover has established (Mason, 1981). Species that prefer more open environments including deer mice, western harvest mice (Reithrodontomys megalotis), and Ord's kangaroo rats (Dipodomys ordi) occur in both the woodlands and ecotonal areas, and tend to increase in density following tree removal (Mason, 1981; Severson, 1986). Some broadly adapted species, such as deer mice, frequently exhibit higher densities in ecotonal and tree removal areas than in the woodlands. Undoubtedly, these species play a significant role in moving seeds at least short distances across ecotones.
B. NATURAL AND ANTHROPOGENIC DISTURBANCE
1. Fire, Livestock Grazing, and Human Intervention
The distribution and dynamics of the woodland are controlled not only by climate change, but also by disturbance regimes and the effects of humans on those disturbance regimes. Fire has been and continues to be the major natural disturbance influencing the local expansion of pinon and juniper woodlands. Before Anglo-Americans settled the West, fires used to occur as frequently as every 50-100 years throughout much of the woodland (Wright & Bailey, 1982) and as often as 20-30 years for more productive valley bottoms and grasslands (Burkhardt & Tisdale, 1976). Most fires kill all of the young ([less than or equal to]50 years old) and most of the adult pinon and juniper as well as the non-root-sprouting shrubs in the understory (Johnsen, 1962; Wright et al., 1979; Fisher et al., 1987; Miller et al., 1994). However, most grasses and forbs survive and assume dominance of the site after the fire. Following fires in that portion of the woodlands with shrub understory, such as most of the Great Basin, the successional sequence includes a grass- and forb-dominated stage, a shrub-dominated stage and, finally, a tree-dominated stage (West & Van Pelt, 1987). In that part of the woodlands where grasses dominate the understory, such as the southwestern grasslands, junipers establish directly into the grasslands (Salomonson, 1978).
During this century, fire frequencies have been reduced throughout the West due to the indirect effects of livestock overgrazing and the direct effects of removing Native Americans from the ecosystem and implementing active fire prevention programs. Overgrazing by livestock within the woodland belt has resulted in the depletion of the highly palatable but grazing-intolerant grass and forb species (Caldwell et al., 1981), and in the increase of the relatively unpalatable woody species (West & Van Pelt, 1987; Miller et al., 1994; Pieper, 1994; Young, 1994). Because overgrazing reduces the fine fuel buildup attributable to grass and forb species (Miller et al., 1994), it has resulted in a reduction in natural fire frequency. In the absence of fires, the shrub- and tree-dominated seral stages are increasing at the expense of the grass-dominated stages.
Because of the concern over the recent expansion of the woodlands, a variety of management techniques other than fire have been developed over the past 40 to 50 years to reduce tree densities. Early treatments included cabling, chaining, and, later, bulldozing or tree pushing with the primary objective of increasing forage for livestock production (Dalen & Snyder, 1987). More recently there has been increasing emphasis on wildlife habitat enhancement and fuelwood production. Fuelwood harvest techniques use standard silvicultural practices designed to ensure natural regeneration (Dalen & Snyder, 1987). Wildlife habitat enhancement projects and other tree removal projects rely on many of the original treatment methods, especially chaining. These treatments are less frequent and encompass much smaller land areas than the fires that occurred prior to western settlement. They often result in lower mortality of both adult trees and seedlings than fire (Stevens, 1987) and, thus, a more rapid return to tree dominance. Also, they often include seeding of introduced grasses and forbs in those cases where the understory grass and forb cover has been depleted by livestock grazing or tree competition. These introduced species are often highly competitive and in many of the areas where they have been seeded are effectively replacing the natives (Richards et al., 1998).
2. Species Life History Attributes and Disturbance Characteristics
In pinon juniper woodlands, as in other ecosystems, both the life history attributes of the species and the characteristics of the disturbance determine the establishment probabilities of seeds and seedlings (White, 1979; Pickett & White, 1985). Because of differences in the life histories of pinon and juniper, they exhibit different responses to disturbance. Disturbances that remove both trees and understory shrubs in pinon-juniper woodlands have a relatively greater effect on the establishment of seeds and seedlings of pinon than juniper. Pinons have short-lived seeds that persist only one or rarely two growing seasons following disturbance, whereas juniper have long-lived seeds capable of persisting until conditions occur that are favorable for establishment (Johnsen, 1959; Gilleard, 1985). Seeds and seedlings of pinon that survive the disturbance have a minimal chance of establishment due to the requirement for a shrub or tree nurse plant, while those of various juniper species have a higher probability of establishment due to the ability to establish in open environments. Also, pinon seedlings appear to be less tolerant of competition from grasses and other herbaceous vegetation than the seedlings of juniper species such as Juniperus occidentalis (Burkhardt & Tisdale, 1976; Eddleman, 1987; Miller & Rose, 1995). Consequently, initial establishment of juniper seedlings is frequently higher than that of pinon seedlings following disturbance in the Pinus monophylla/J. osteosperma woodland (Everett & Ward, 1984; Tausch & West, 1988) and probably other woodland types.
The fates of seeds and seedlings will depend on the type of disturbance. Most tree seedlings are killed by fire. Although the fates of the residual tree seed bank following disturbance has received little study, it can be assumed that seeds of both pinon and juniper, especially those that are cached, have a reasonable probability of surviving fires depending on the microhabitat in which they are located. Under-shrub and under-tree microhabitats often experience higher soil temperatures than interspace areas, especially if the entire shrub or tree and the duff burns (Wells et al., 1979), and would be expected to exhibit the highest seed mortality during fires. Seeds that are cached undoubtedly have higher survival during fires than seeds on the soil surface, especially if they are located in interspace environments. Rodents cache Pinus monophylla seeds at a depth of 3-29 mm (Vander Wall, 1997) and Juniperus osteosperma seeds at a depth of 8-31 mm, and birds cache P. monophylla seeds at about 10-30 mm (Vander Wall, unpubl. data).
Mechanical disturbances can have a much different effect on both seed and seedling fates than fires. Tree seedlings frequently survive mechanical disturbances, such as chaining, and the "seedling bank" under mature trees can be effectively released following removal of the overstory. Seeds that have already arrived on the soil surface or been dispersed are left in place, although some redistribution and even burial may occur due to the activity of the equipment. The establishment probabilities of both seeds and seedlings will depend on the characteristics of the microhabitats remaining after treatment. If downed trees and understory shrubs are left in place, more shaded microhabitats and nurse plants for seedling establishment will be available. If the trees are piled and burned and understory shrubs are removed, fewer desirable microhabitats for seedling establishment will remain.
The timing of fires or tree removal in relationship to the timing of seed dispersal influences the soil seed bank of the tree species following the disturbance. Fires or tree removal treatments that occur during June, July, or August before seed maturation and dispersal will have a greater effect on the current year's seed crop of pinon and both the current and residual seed crop of juniper than fires that occur in September or October after significant seed dispersal has occurred. Spring treatments should only affect the residual seed crop of juniper. Depending on the timing of fires or removal treatments relative to seedling emergence, a significant portion of the current year's seedling crop may be killed. For example, seeds of Pinus monophylla cached in the fall could potentially survive a late fall or early spring burn and yet be killed as newly emerging seedlings in a fire the following summer.
Although not well documented, other disturbance characteristics may also influence seed and seedling establishment probabilities. More severe disturbances such as hotter fires or mechanical treatments that completely remove trees and shrubs undoubtedly decrease rates of reestablishment, whereas less severe disturbances that leave some trees and shrubs in place result in higher rates of establishment (Stevens, 1987). Larger disturbances and those that minimize the amount of edge decrease dispersal into the disturbed area from the woodlands and thus seedling establishment, while smaller disturbances with maximum edge should result in higher seed dispersal and seedling establishment.
XI. Areas for Future Research
Several areas relating to the seed dispersal and seedling establishment of pinon and juniper require additional research if we are to understand the dynamics of the woodlands: (1) the factors influencing seed production in both pinon and juniper; (2) types and behaviors of rodent dispersers of pinon and of all dispersers of juniper; (3) effectiveness of different types of animal dispersers for seedling establishment; (4) environmental requirements for seedling establishment; and (5) the differences in seed dispersal and seedling establishment among species and regions. Ecotones that serve as the transitions between pinon- and juniper-dominated areas and adjacent communities as well as ecotones between disturbed and undisturbed ecosystems appear to be particularly promising for increasing our understanding not only of dispersal and establishment processes but also of woodland dynamics.
We thank R. J. Tausch for summarizing the paleoecology of the juniper species within the Great Basin. Financial support for E. W. Schupp included The Ecology Center, Utah State University, the Utah Agricultural Experiment Station (Project UTA908), and the Utah State University Vice President for Research Office.
XIII. Literature Cited
Adams, R. P. 1983. Infraspecific terpenoid variation in Juniperus scopulorum: evidence for Pleistocene refugia and recolonization in Western North America. Taxon 32: 30-46.
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|Author:||Cambers, Jeanne C.; Vander Wall, Stepen B.; Schupp, Eugene W.|
|Publication:||The Botanical Review|
|Date:||Jan 1, 1999|
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