Alaska's Boreal Forest

Although forests have occupied interior Alaska for only 13,000 years, their floristic composition and patterns of distribution have remained relatively stable for the past 5,000 years. Here, at the present northern limit of forests, severe environmental conditions have prevented migration of new species from the south. The Bering Sea isolated Alaska from a taxonomically distinct flora in Eurasia. Mountains to the north and south of interior Alaska restricted the potential for latitudinal shifts in species in response to millennial-scale variations in climate. The stability of the Alaskan vegetation mosaic for the last 5,000 years contrasts with the substantial vegetation movements that have occurred in the eastern Canadian boreal forest (Bergeron et al. In press). Because of this long-term stability, the distribution of Alaska's boreal vegetation reflects a clear imprint of current environmental patterns on the landscape. This strong link between current environment and vegetation facilitates the use of state factors as a framework for describing floristic patterns. In this chapter we describe the major patterns of diversity and distribution in interior Alaska forests and discuss their potential future changes.

The composition of the boreal forest varies greatly throughout its circumpolar range in response to differences in both current environment and histories. The primary species, those that give the forest is distinctive appearance, include broad- leafed deciduous trees, needle-leafed evergreens, and needle- leafed deciduous trees. In Alaska, the predominant conifers are white and black spruce (Picea glauca and P. mariana, respectively); larch (Larix laricina) tends to be local, and pine is absent in interior Alaska but a prominent component of the forests to the east in Yukon and Northwest Territory. Important deciduous trees are two poplars (aspen [Populus tremuloides] and poplar [P. balsamifera]) and tree birch (Betula neoalaskana; formerly B. papyrifera). Important shrubs include alders and willows, but the common species in Alaska differ somewhat from their ecological equivalents in eastern North America. Nomenclature follows Furlow (1997).

Much of Alaska and the rest of Beringia were unglaciated, when most of the northern hemisphere was covered by Pleistocene ice sheets. This allowed the flora of Alaska to persist in the Beringian refugiam throughout the Ice Ages, whereas the flora and vegetation elsewhere in North America and Eurasia are entirely post-glacial. However, at the end of the last glacial maximum, the area occupied today by boreal forest was a treeless landscape that also lacked the common tall shrubs (e.g., alder) of today.

The boreal forest has fewer species than any other forested biome (Pastor and Mladenoff 1992). Consequently, many of its species have extremely broad distributions (Chapin and Danell 2001). Interior Alaska, for example, has only six tree species-three conifers and three broadleafed species. Typically only 1-2 of these tree species dominate a given stand. For comparison, the southern transition of the boreal forest to northern hardwoods in the Great Lakes region has 5- fold more tree species (42) and 10-fold more shrubs (300, compared to 24 in Alaska) (Maycock and Curtis 1960, Viereck et al. 1983, Viereck et al. 1992). The approximately 40 herbaceous species in interior Alaskan forests represents a much more depauperate flora than occurs at the southern margin with either northern hardwood forests or prairies. In contrast to vascular plants, interior Alaskan forests are much more diverse in mosses and lichens than are southern forests and share most of this diversity with arctic tundra to the north. Forests contain a relatively small proportion of the total plant diversity present in interior Alaska. For example, in the vicinity of Fairbanks, forests contain all trees (by definition), half of the shrub species, 10% of the herbaceous species, and an unknown proportion of lichens and mosses (Jorgensen et al. 1999). Forest edge and open habitats such as alpine tundra, bogs, fens, and dry steppe are typically more diverse than adjacent forests, just as in most forests of western North America, where there are typically very few tree and understory species. Even in forests, most herbaceous and non- vascular taxa are more diverse in forest gaps or relatively open stands. The reduction in herbaceous and non-vascular diversity beneath the shade of a relatively homogeneous canopy explains why forest diversity is less than in the arctic tundra to the north (Chapin and Danell 2001).

The diversity of boreal forests is a dynamic property that responds sensitively to disturbance. The diversity of vascular plants tends to decline through succession, whereas the diversity of nonvascular plants increases (Fig. 6.1) (Rees and Juday 2002). There is a 40% turnover in species from the colonists present 2-5 years after wildfire to those species present in a mature hardwood forest. After logging there is only a 30% turnover of species from early to late succession because many early successional fire specialists are excluded from recently logged sites (Rees and Juday 2002). Exotic species, which until recently have been largely excluded by the severe environment of Alaska, are appearing in some early successional sites.