A comparison of taiga flora in north-eastern Russia and Alaska/Yukon

Aim To understand the similarities and differences between the taiga floras of far north-eastern Asia and north-western North America in the light of their Tertiary and Quaternary histories. Does the taiga flora follow the tundra pattern (Asian-American commonality of species as a result of continuity through the Quaternary), the temperate forest pattern (distinct species because of late Tertiary disjunction), a combination of these two patterns, or some pattern unique to the taiga? Location The taiga regions of interior Alaska and the Yukon in North America (the 'Alaskan taiga'), and the Kolyma and eastern Indigirka River basins in Russia (the 'Kolyma taiga'). The study areas include both forested and unforested habitats below elevational treeline. The two regions have similar climate and topography and were linked via the Bering Land Bridge in the Tertiary and for several extended periods during Quaternary cold periods. Methods Systematic comparison of the vascular floras of the two regions from published sources; and review of palaeoecological literature for the region. Results Of the 796 species found in the study areas, 27% occur only in the Alaskan taiga, 35% occur only in the Kolyma taiga, and 38% occur in both the regions. The following subsets of species show a high proportion of species in common between the study areas (subsets are not mutually exclusive): plants that occur on the tundra and the taiga, non-flowering plants, abundant taiga understory plants, and wetland and aquatic plants. A lower proportion of shared plants was noted for warm, south-facing steppe communities. No tree species are common to both areas. Main conclusions The Bering Strait region in the Quaternary has acted as a biogeographical filter for taiga plants. Significant divergence between northeast Asia and northwest North America has developed among the more southerly ranging fraction of the flora (e.g. trees), while the more cosmopolitan and the most cold-adapted elements of the taiga flora are common to both areas. Many plants in the former group have been disjunct between Asia and North America for millions of years, while many plants in the latter group have probably maintained continuity between the study areas via the Bering Land Bridge through much of the late Tertiary and Quaternary periods. Repeated extirpation of the less cold-adapted species from both study areas during Pleistocene cold periods has probably enhanced floristic differences between the two regions.