at the ccosystcm level. We expect gain or loss of species to affect steady-state patterns of biogeochemistry within sites only if the species is extreme with respect to size, RGR and correlated traits of resource capture and tissue chemistry. This is most likely to occur with the invasion of a new functional group such as trees or nitrogen fixers.
Ecological theory suggests that species coexist at equilibrium only if they differ in the nature of growth-limiting resources (Tilman 1988). Although all plants use the same basic resources of light, C02, water and nutrients to grow, species differ in the range of conditions under which they most effectively acquire these resources (Chabot and Mooney 1985; Körner 1989), their efficiency in converting acquired resources into biomass (Vitousek 1982), and the effectiveness of resource retention (see above) (Chapin 1980). Specialization among species in a community in conditions for resource capture could increase ecosystem fluxes by expanding the total range of conditions over which resources are acquired by plants. We will consider this possibility separately for each resource.
Specialization into canopy and understory species generally has little direct effect on total ecosystem carbon gain because it does not affect the amount of light available to the community. In the presence of a closed canopy, most light is absorbed or reflected by the canopy, and carbon fixation by the understory (generally <2-10% of the total) will have substantial ecosystem effects only if the understory species differ from canopy species in ccosystcm impacts other than carbon fixation. For example, in the boreal forest, understory shrubs cycle many more nutrients than their biomass would suggest because they have less structural investment than trees (Yarie 1980; Chapin 1983) and can be important food for mammals (Bryant and
Chapin 1986), which in turn have multiple effects on ecosystem processes (see below).
In contrast to canopy height, specialization by rooting depth can enable deep-rooted species to tap resources that would otherwise be unavailable to the rest of the community. For example. Eriophorum vagina turn is the only tussock- tundra species with deep enough roots to access nutrients in ground water that flows laterally over the permafrost surface. By tapping nutrients at depth, its productivity increases 10-fold in sites with abundant groundwater flow, whereas the productivity of other species is unaffected by these deep resources (Chapin et at 1988). In the absence of this species, nutrient inputs and ecosystem productivity and nutrient cycling would be much reduced. Similarly, in temperate ecosystems where production is constrained by water availability, alien species such as Eucalyptus or Tamarix, which are more deeply rooted than native species, increase the amount of water absorbed and transpired, and, therefore ecosystem productivity.
Plant species can also specialize in absorbing different forms of nitrogen (N). For example, tundra health species meet much of their N requirement through amino acid absorption by their mycorrhizal symbionts (Read 1991) and have low nitrate reductase activity (suggesting iittle use of nitrate), whereas deciduous shrubs show high nitrate reductase activity (Shaver et al. 1996) and a low potential to absorb amino acids (Kielland 1990). VA mycorrhizal fungi, which are typical symbionts of herbaceous tundra species, cannot access organic N (Read 1991). Other tundra plant species absorb amino acids directly al rates sufficient to meel most of their N demand (Chapin et al. 1993; Kielland 1994). Similarly, in the alpine regions, species restricted to calcareous soils, where nitrate is the predominant form of inorganic N. grow better on nitrate than on ammonium, whereas species from acidic soils grow better on ammonium (Ingestad 1976). These physiological specializations enhance ecosystem fluxes in the field only when soil nitrogen is not utilized (or utilized at a different rate) in the absence of a particular uptake pathway. For example, all arctic species have some capability to absorb nitrogen as both amino acids or inorganic N (Kielland 1994). When grown on a single N source, one arctic sedge (Eriophorum vaginatum) adjusted its capacity to absorb nitrate, ammonium and amino acids so that it grew equally well on nitrate, ammonium or amino acids, for example by increasing nitrate reductase activity when nitrate was the only N source available (Chapin et al. 1993). Thus, arctic and alpine plants may exhibit sufficient flexibility in the form of N utilized that, if a competing species with a different preferred form of N were removed, it is unclear whether this would cause large changes in total N flux through ecosystems.
Arctic and alpine species may also differ in the form of P utilized. Eriophorum vaginatum has a root phosphatase activity sufficient to supply its annual P requirement entirely from organic sources (Kroehler and Linkins
1991). These root phosphatases remain active even after the roots die, and contribute substantially to soil phosphatase activity in the soil beneath the tussock. Other plant species have lower root phosphatase activity and presumably are more dependent on the supply of inorganic P.
Phenological specialization in the timing of plant activity could increase the time available for plants to acquire resources for their environment. For example, arctic mosses in sedge tundra gain a large proportion of their carbon early in the growing season before they become shaded by the vascular plant canopy (Oechel and Sveinbjornsson 1978). However, if nutrients strongly limit production, early-season carbon gain by one species may tie up nutrients that would otherwise have been used to support production later in the season by other species (Chapin and Shaver 1985). fn general, wc expect that phenological specialization will alter ecosystem-level fluxes of carbon and nutrients only if it allows access to new pools of limiting resources that would otherwise be untapped. In cases that have been evaluated critically, phenological differences in activity have surprisingly little effect on ecosystem-level fluxes. Evergreen trees acquire only about 7% of their annual carbon gain during spring and autumn when deciduous trees are leafless, and therefore do not greatly increase the annual carbon gain by extending the season for carbon capture (Schulze et al. 1977). It remains to be determined bow important phenological specialization is in determining carbon gain and biogeochemical cycling in arctic and alpine ecosystems.
Ecosystem differences in soil microbial potential to attack different substrates couid influence the speed with which nutrients are recycled, and therefore the productivity that could be supported by a given pool of nutrient resources. Although microbial groups differ strikingly in their enzymatic potentials to degrade common substrates (e.g. cellulosc, lignin, protein), and these substrates differ in abundance among ecosystems, the enzymatic potential to degrade these substrates remains surprisingly constant across a wide range of ecosystem types (Schimel 1995). In general, these cnzymatic potentials are ubiquitous, and microbial diversity with respect to these functions has no clear ecosystem consequences (Meyer 1993; Schimel 1995).
In summary, although specialization of resource capture is a theoretically attractive and potentially important mechanism by which species diversity might influence ecosystem processes, there is currently little information on the degree of physiological specialization under field conditions from which to draw conclusions.
Light availability to an ecosystem is determined by climate. It is the pool of available soil resources that determines the extent to which this light can be utilized to support plant production and nutrient cycling within a climatic zone (Field 1991). Some species alter ecosystem processes by changing the size of this soil resource base. For example, introduction of alien nitrogen (N)-fixers have greatly altered N availability and many properties of low-N ecosystems (Vitousek et al. 1987). Most arctic and alpine ecosystems increase their productivity in response to N addition in the short term. However, in the long term, regular N addition will eliminate any given community and lead to a new mix of taxa, usually less resistant to physical stresses (Shaver and Chapin 1980, 1986; Körner and Larcher 1988; Körner 1989). Although N-fixing plant species are well represented in the low-arctic and low-alpine floras (e.g. alpine legumes and bluegreen algae associated with arctic mosses), they are not abundant in the most widespread ecosystem types (e.g. sedge meadows), perhaps reflecting low phosphorus availability in acidic organic soils or less resistance to low temperatures. Loss of N-fixing species or gain of other more efficient N-fixers (e.g. the tall shrub Aimts crixpa in the arctic) could alter biogeochemical fluxes and productivity.
Like nutrient inputs, nutrient loss from arctic and alpine ecosystems may depend on the traits of a few species and therefore be sensitive to changes in the abundance of these groups. Half the annual N and P input to aquatic systems in the arctic occurs at snow melt (Whalen and Cornwell 1985), when the rooting zone of vascular plants is frozen and mosses serve as the major biologically active filter. Changes in the abundance of plants that are physiologically active during snow melt could alter the effectiveness of this filter. During the main part of the growing season, the roots of most species are physiologically active (Chapin and Bloom 1976), so that diversity among plants' potential to absorb nutrients may have little influence on nutrient loss from closed plant communities.
Soil microbial processes (e.g. nitrification and denitrification) govern gaseous N loss from ecosystems. Nitrification also determines the supply of nitrate, which is more susceptible to leaching loss than are other forms of soil N. Rates of these processes (and of other trace-gas fluxes such as methane) differ strikingly among ecosystems, and are carried out by a relatively small number of organisms. Consequently, diversity with respect to these processes could have profound ecosystem implications (Schimel 1995).
Animals can move nutrients laterally among ecosystems. Insects which grow and develop at low elevations and are carried upward in wind currents can be an important nutrient input to high-altitude ecosystems (Meyer and Thaler 1995). Migratory salmon, which acquire most of their resources for growth in the ocean, significantly alter nutrient inputs to streams and lakes where they spawn. However, in general, animal consumption and cycling of energy and nutrients is small relative to direct plant litter inputs to decom posers, so lhat the role of animals in material inputs to ecosystems will be important only when animals move from a relatively high-nutrient to an extremely low-nutrient environment, or where animals concentrate their activity in a small area. For example, sea birds and ground squirrels forage broadly but deposit much of their urine and fcccs in a restricted area near their nests or burrows. Lemmings in coastal tundra forage in many micro-habitats but spend most of their time (and release most of their nutrients) in polygon troughs where greater snow depth provides insulation. The resulting nutrient transport from polygon rims and centers to troughs may account for the high productivity of these troughs (Batzli et al. 1980). This heterogeneity in resource supply generated by animal activity may promote the coexistence of a greater number of species and enhance community diversity.
In summary, relatively few species account for most of the nutrient inputs and losses from ecosystems, so changes in the abundance or diversity of this small group of organisms could strongly alter the resource base that drives biogeochemical processes.
Climate determines the energy available to an ccosystem. However, plant size strongly influences the extent to which this energy is absorbed. Tall plants such as trees and tall shrubs, which are absent or uncommon in arctic tundra and alpine vegetation, reduce the albedo (reflectance) during periods of snow cover, thereby increasing annual energy inputs to eold-dominated ecosystcms. The invasion of these taller plants into the arctic or alpine belt would thus exaggerate any direct climatic warming and alter both nutrient and carbon flows because decomposition, nutrient supply and plant growth are so sensitive to temperature and the length of the growing season. Because the atmosphere is heated primarily by convective exchange with the ground surface, the amount of energy absorbed by an ecosystem rather than reflected back to space (as controlled in tundra by plant size) also determines the energy inputs to the atmosphere and therefore the regional climate. Simulation models suggest that if the boreal forest were suddenly converted to tundra, this would cause a large permanent climatic cooling that would be most pronounced at high latitudes, but would extend to the tropics (Bonan et al. 1992).
Plant size and RGR determine the avenue by which absorbed energy is dissipated. Where water is readily available, as in most arctic and alpine ecosystems, most energy is dissipated as évapotranspiration rather than as sensible heat. The greater coupling of tall plants to the atmosphere often does not increase transpiration because transpiring surfaces remain cool, whereas prostate "decoupled" plants warm up substantially, causing moisture gradients to steepen. These effects cancel each other, with little difference in évapotranspiration between adjacent forest, grassland and shrubland.
The overriding effect of plant size on microclimates in cold environments is best documented for short vegetation, which warms up substantially when exposed to sunlight (Körner and de Moraes 1979: Körner and Cochrane 1983). In fact, photosynthetic temperature optima reflect these bénéficiai thermal conditions by exhibiting few differences between alpine and lowland plants in short canopies (Körner and Larcher 1988). These effects are less pronounccd in lowland arctic tundra because of the lower solar angle. Accordingly, temperature optima for gas exchange in arctic plants are closer to prevailing ambient temperatures (Billings et al. 1971). Clearly changes in structural diversity which involve changes in plant stature could feed back to regional climate not only by altering albedo, but also by altering the relationship between latent and sensible heat flux.
Landscape heterogeneity provides an important component of ecological complexity in arctic and alpine ecosystems. Communities along topographie-gradients differ strikingly in ground-water chemistry and therefore in their effect on aquatic ecosystems (Kling 1995). For example, riparian shrub communities have strong nitrification potentials, so that ground water passing through these communities is enriched in nitrate, whereas ammonium dominates the soil solution of many other ecosystem types. Upland heath ecosystems are net sources of nitrogen to ground water, whereas lowland sedge-meadow communities are net sinks for nitrogen (Shaver el al. 1991).
Maintenance of ecosystem integrity over a complete cycle of common disturbance events (e.g. disruption of the soil surface by frost) is critical to the long-term persistence of ecosystem processes. Many arctic and alpine ecosystems are characterized by large areas of unoccupied space caused by disturbances associated with slope instability, frost action or animal activity (Batzli and Sobaski 1980; Körner 1995; Walker 1995). Here productivity and energy flow may be limited by the rate of seedling establishment or clonal spread; the role of early successional species in stabilizing soils is critical to the development of closed communities and closed biogeochemical cycles. For example, in the arctic, frost action often disrupts the vegetative cover, creating small-scale cyclic succession. Lichens and mosses, which require no firm attachment to substrate, are effective early colonizers that stabilize soil sufficiently for deep-rooted gramtnoids to establish (Gartner et al. 1986).
In the alpine regions, where vertical relief exaggerates soil instability caused by frost action, the role of plant diversity in ensuring ecosystem integrity is particularly important. A high diversity of rooting patterns is important in fulfilling various mechanical functions, consolidating the ground during succession, and creating mosaics of plant communities. Certain scree species must be present to consolidate the substrate and open it to other species, which in turn establish islands of humus where the quantity and quality of biomass is adequate to support herbivores. The elimination of deeply anchored pioneers would immediately destabilize the entire system. Thus, a few pioneer species of low abundance are critical to slope stability, accumulation of plant biomass and soil organic matter, and water retention. These factors, in turn, govern biogeochemical pools and fluxes, and the seasonality of water flow to rivers and hydroelectric power plants, i.e. they are critical to ecosystem and regional processes.
Loheleuria procumbens (Ericaceae) is an example of the risk of low diversity in maintaining ecosystem integrity. This prostrate, slow-growing evergreen dwarf shrub produces extremely acid soils and massive layers of raw humus. Its dense, cushion-like growth habit creates a favorable microclimate (Cernusca 1976), enabling the specics to dominate on windswept slopes. However, Loiseleuria is extremely sensitive to mechanical disturbance such as occurs on ski trails (Körner 1980) and to nitrogen addition (Körner 1984), both of which lead to rapid elimination of the species and loss of soil stability. A few pioneer species (e.g. Juncus trifidus) are able to invade bare Loiseleuria soils and provide transitory soil protection. The presence of these species is thus critical to the prevention of erosion. This is a case where the presence of a species in low abundance in a community can suddenly become of great functional importance.
Species diversity can be important in maintaining ecosystem integrity in response to rare events. For example, in the Andean paramos, variation in frost resistance is associated with different morphologies (Squeo et al. 1991). taller species are less resistant than smaller ones, which are also microclimat-ically better protected from frost. This diversity of frost tolerance ensures that the more slowly growing short-statured species survive even the most severe frost events, maintain some level of nutrient-cycling, and offer protection from frost for regrowth of the taller species. Similarly, rapidly growing grasses comprise less than 1% of plant biomass in undisturbed tussock tundra (Shaver and Chapin 1991), but expand and become extremely important in nutrient retention following pulses of nutrient addition (Shaver and Chapin 1986) or disturbance (Chapin and Shaver 1981; Shaver 1995).
Disturbance by animals plays a key role in determining the structure and diversity of arctic and alpine ecosystems. For example, moderate grazing of arctic salt marshes by geese maintains a high productivity (Jefferies and Bryant 1995), just as grazing by Pleistocene megafauna may have contrib uted to a productive steppe-grassland in Beringia 20 000 years ago (Zimov et a!. 1995). However, recent increases in grain availability in temperate wintering areas has augmented snow-goose populations beyond their summer carrying capacity, so that they are destroying widespread areas of summer salt-marsh. This illustrates how the impact of human activities outside the Arctic can alter the activity of key arctic organisms.
Browsing mammals are both a product and a cause of plant diversity in cold-dominated ecosystems (Jefferies and Bryant 1995). Fire and other disturbances create patches of early successional habitat that are essential to maintenance of populations of browsing mammals. These mammals selectively browse early successional vegetation, speeding the transition to dominance by late-successional species, which are more flammable, and increasing the probability of fire and return to early succession (Pastor 1995). In addition, browsing mammals maintain a mixed diet to minimize intake to any single plant secondary metabolite. One consequence of this mixed feeding strategy is that animals tend to eliminate rare species and avoid those species that are most common, thus reducing plant species diversity. Browsing mammals thus contribute to landscape diversity by speeding succession, but reduce species diversity within individual patches of vegetation. In contrast to browsers, grazers, which are particularly common in alpine regions, tend to increase plant species diversity by reducing competition from the dominant species and creating disturbed microsites for seedling establishment.
Species diversity can be a catalyst for community change as well as for maintenance of the existing community. In the Australian Snowy Mountains, mosaics of diverse shrublands support tree establishment above the current treeline (Egerton and Wilson 1993), whereas continuous carpets of tussock grasses prevent establishment of tree seedlings. A similar phenomenon was observed in New Zealand (Wardie 197); A.F. Mark, personal communication, 1994), where root competition by tussock grasses prevented establishment of Nothofagus above its current treeline.
Species diversity is functionally important because it provides insurance against large changes in ecosystem processes. Because each species shows a unique response to climate and resources, any change in climate or climatic extremes that is severe enough to cause the extinction of one species is unlikely to eliminate all members of a functional group. The more species there are in a functional group, the less likely it is that any extinction event or series of such events will have serious ecosystem consequences. For example, catastrophic events such as a snow-free winter could eliminate a large fraction of the snowbed species that depend on protection from snow
(Larcher 1980). If the community contains a few non-snowbed species or genotypes with higher frost resistance, the integrity of the ecosystem can be maintained. For these reasons, genetic and species diversity per se is important to the maintenance of ecosystem structure and function. Loss of species which have qualitatively unique effects on ccosystem processes (e.g. effects on inputs/outputs or disturbance) are especially likely to alter ecosystem processes.
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