Continental Boreal Regions

Maritime and Permafrost Regions

Figure 3.2 Ecosystem feedbacks in areas of high and low biodiversity in boreal regions. +, positive effcct of one property on another; -, negative effcct of one property on another o crant, rapidly growing hardwoods in the genera Be ml a and Populus. The tissues of these deciduous species have low resin and lignin contents, leading to high rates of decay and nutrient cycling and productivity {Flanagan and Van Cleve 1983; Pastor and Mladenoff 1992).

Conifers and hardwoods support different food webs. Those food webs, when coupled to the cycling of limiting nutrients such as nitrogen, may cause cyclic or chaotic alternation between these two functional groups (Wein and El-Bayoumi 1983; Pastor and Mladenoff 1992; Cohen and Pastor 1995). Hardwood tissues are poorly defended, and are the major food source for mammalian herbivores (Bryant and Kuropat 1980), which in turn are the major prey for predators such as lynx and wolf (Cards lupus). In contrast, the conifer food web is characterized more by insects and their avian predators. The intolerant hardwoods eventually succeed to the conifers, but this succession is hastened by intense browsing on the hardwoods, leading to slower nutrient cycles and lower populations of mammalian herbivores (Bryant and C'hapin ยก986; Pastor e( at. 1993a). However, conifer stands can revert to shade-intolerant hardwoods after outbreaks of insects such as spruce budworm or bark beetle, which kill the trees (Morris 1963; Blais 1968; Cole 1981). Bird populations in turn may control the timing and extent of insect outbreaks (Crawford et al. 1983; Holling 1992).

In contrast to this complex pattern of ecosystems of the more seasonal continental interiors, ecosystems of the wetter maritime areas and northern areas underlain by permafrost have simpler assemblages of species and less complex food webs (Figure 3.2). Here, the wet climate and nearly saturated soils preclude fires, except of very long frequencies of several centuries (Heinselman 1981: Payette 1992). This leads to several generations of dominance by conifers and low mammalian diversity. Furthermore, mosses and lichens dominate the forest floor; these in turn form a positive feedback with cold temperatures, further enhancing the low-nutrient availability and strengthening dominance by the slow-growing conifers (Bonan 1992).

These biogeographica) patterns of defense, food web assembly and nutrient cycling arc related to climate and fire history in the following two ways. Longitudinal variation in wildlife history is caused by geographic variation in climate (Johnson 1992; Payette 1992). Densities of boreal browsing mammals such as snowshoe hares are related to tire regimes because these species feed on woody plants adapted to the early stages of post-fire succession (Grange 1949, 1965; Fox 1978). Where fire is frequent, browsing mammals are abundant, and browsing imposes a strong selection for defense of the juvenile-stage of trees and shrubs, and significantly reduces deciduous tree requirement (Fox 1978; Mclnnes el al. 1992; Bryant et al. 1994). This pattern can easily be seen in the interaction between snowshoes hares and woody plants in subarctic North America. The return-time of fire is short (50 years} in the boreal forest of Alaska and western Canada, and much longer (500

years) in the moist boreal forest of maritime eastern North America (Payette 1992). The maximum density of snowshoe hares in Alaska ranges from 400 to 1200 hares per 100 ha, but in eastern Maine the maximum recorded densities are an order of magnitude lower (40 hares per 100 ha) (Keith 1990). Accordingly, juvenile-stage birch and quaking aspen from Alaska are more chemically defended against browsing by snowshoe hares in winter than congeners and conspecifics from the boreal forest of Maine (Bryant et al. 1994). Again, such biogeographic patterns of chemical defense have consequences not only for herbivore densities and diversities, but also for nutrient cycling rates because of the effects of secondary compounds, iignin and resins on the decay of plant litter (Bryant and Chapin 1986; Pastor and Naiman 1992; Pastor el al. 1993a).

As well as fire, low soil temperatures and their depression of decomposition and nutrient availability also influence the diversity of plant defenses and mammalian diversity. Limitation of growth by insufficient mineral nutrition alters the carbon/nutrient balance of individual plants, resulting in increased production of defensive chemicals that contain no nitrogen (Bryant and Chapin 1986). Consequently, boreal woody plants adapted to infertile soils are generally more strongly selected for chemical antiherbivore defense than are boreal woody plants adapted to more fertile soils (Bryant et al. 1983). This may explain why northern species and provenances are more defended than southern congeners and conspecifics (e.g. Niemelaa et al. 1989), with consequent depression of mammalian populations and diversity as well as nutrient cycling rates.

Beyond these functional patterns, the relationship between diversity and ecosystem properties is not well known. Regional species richness may be greater in continental interiors because of the heterogeneous environment, but the number of species that can be packed into a square meter may be greater in the wetter areas because of dominance by smaller statured life forms; in fact these areas often grade into tundra ecosystems. Although population cycles may enhance or even be driven by these feedbacks (Hansson 1979; Haukioja et al. 1983), the quantitative relationship between the cyclic nature of populations, nutrient cycling rates and species richness is also not fully known.

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