montane réfugia and spread back later into interior Alaska; there is considerable debate about role of réfugia vs. several waves of invasion which is not yet resolved (Hopkins et ai. 1981; Brubaker et al. 1983; Ritchie 198?; Edwards et al. 1991).

A considerable variation in tissue chemistry along latitudinal and longitudinal gradients on each continent is related to these historical patterns of disjunct populations. Within a given continent, northern species arid provenances are generally less palatable and more defended than southern species and provenances (Niemola el al„ 1989; Swihart et al., 1994). Within eastern North America, there is a strong east-west gradient of chemical defenses. For example. Beringian birches are more heavily defended chemically (Bryant el a!. 1989) and are generally diploid (Dugle 1966). Eastern North

American populations are poorly defended (Bryant et al. 1989, 1994) and tetraploid (Dugle 1966). Furthermore, western North American populations of Abies balsamea arc chemically more defended than eastern populations (Hunt and von Rudolf 1974; Hunt 1993). These subspecies or population differences are expressed most strongly in the juvenile stage (Bryant et al. 1989, 1994).

3.2.2 Mammals

The mammalian fauna of the Holarctic boreal forest includes rodents (Rodentia), lagomorphs (Lagomorpha), ungulates (Artiodaetyla), carnivores (Carnivora) and insectivores {Insectivora). As with plants, most mammalian genera have circumboreal distributions, but contain few species. For example, only two hares {Lepus) are commonly found in boreal forests, but the genus Lepus is circumboreal, with the mountain hare occurring the in the Palearctic and the snowshoe hare occurring in the Nearctic.

The diversity of boreal mammalian fauna is intimately related to the history and diversity of tree species assemblages, as reviewed above. At the same time as the boreal forest flora was becoming assembled, there was a coincident radiation of several herbivores that depend on the mix of conifers and hardwoods for habitat. For example, the sole extant species of moose {Alces alces) radiated into eight subspecies 6500 7000 years ago, of which seven currently survive (Telfer 1984).

Even though mammalian species richness is low, there are marked biogeo-graphical gradients within the Holarctic boreal forest. It is generally accepted that species richness decreases with increasing latitude (Schall and Pianka 1978; Rosenzweig 1992). However, the notion of marked longitudinal gradients in species diversity is new. Danell et al. (1994) have recently found marked longitudinal variation in the species richness of mammalian herbivores in the Holarctic region, with the Beringian region particularly species-poor (Figure 3.1). Furthermore, the centers of the Eurasian continent and the North American continent are richcr in species than continental edges. The mid-continental peak in mammalian species diversity in the Palearctic is positively correlated with a high number of growing season degree days, the number of hardwood species, and the area of boreal forest. In the Nearctic, the mid-continent peak in mammalian species richness is correlated with length of growing season and the number of coniferous tree species. This indicates that on a continental scale species richness of boreal mammalian herbivores may be related to primary productivity (in boreal regions increased temperature is associated with increased primary productivity) and tree species richness. Furthermore, the fact that Beringia both supports the woody species most chemically defended against browsing by mammals and is also the region with the most species-poor mammalian herbivore fauna

Environmental Gradients and Selected Adaptive Traits Important to Ecosystem Functions jr 1

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