Contrasting Patterns of Plant Distribution in Beringia

The Bering Land Bridge has been a major highway for Asian plants into North America, but also a barrier to some and a filter for others. Additionally, there are widely separated and highly local occurrences of Asiatic species in Alaska and adjacent Yukon that are thought by some to be relicts of lateglacial steppe-tundra. Molecular sequence data successfully clarify historical dispersal and vicariance events and will help resolve the role that Beringia played in facilitating and/ or inhibiting plant migrations. Furthermore, hypothesis testing through coalescence-based analyses of molecular sequence data provides a powerful means of investigating the region’s floristic history. Introduction Nearly 80 years have passed since the term Beringia was coined by E. Hultén (1937) to refer to the immense unglaciated areas of Alaska and the adjacent regions of northwestern Canada and northeastern Russia. A vast literature on Beringia is now available that addresses questions of persistence in the glacial refugium, exchanges of plants between Asia and America, diversification of plants in the refugium, and Beringia as a source of tundra plants for post-glacial expansion into deglaciated areas beyond its borders. Beringia as first proposed by Hultén has since been enlarged to encompass the entire region between the Lena River in northeast Russia, and the Mackenzie River in northwest Canada (Figure 1). At its closest point, North America is separated from Asia by only 50 miles (80 km), and portions of the strait are less than 100 feet (30 m) deep. Beringia was mostly exposed during the Tertiary, but the intercontinental land connection was flooded 4.8-5.5 million years ago (Marincovich and Gladenkov 2001). Throughout the Pleistocene, the sea level changed repeatedly, exposing and flooding the region accordingly (Hopkins 1959). During the Last Glacial Maximum, the Bering Land Bridge was extant between 60,000 years ago and 25,000 years ago, and a sea-levels fell of as much as 397 ft (121 m) between 20,000-18,000 years ago, causing the maximum extent of the dry land connection between Asia and America during the Last Glacial Maximum. Only limited exchange was possible after about 11,000 years ago (Hoffecker et al. 1993). It is quite impossible to understand fully the origin of Alaska’s flora without knowing a great deal about its Asian antecedents. During the Tertiary, easternmost Asia and northwestern North America were fully connected and essentially identical biotically. The Tertiary vegetation was dominated by mixed broad and needle-leafed trees. As climate cooled, forests receded from the region, but certain Tertiary floristic elements remained (Matthews and Ovenden 1990, Murray 1992). The major contribution of Asian tundra plants arrived in Alaska throughout the Quaternary, via the land bridge while it was exposed. Successive changes directly related to the waxing and waning of glaciers and consequent rising and falling of sea level caused the repeated appearance and disappearance of the Bering Land Bridge. These dynamics have together created conditions for plant dispersal and also diversification. Even when the land bridge was submerged, plant propagules, driven by wind, are presumed to have crossed from Asia to America in winter when the Bering Strait was ice covered (Savile 1972). Whereas one might suspect that when the land bridge was exposed it provided relatively unimpeded dispersal routes between Asian and America, the distribution of plants shows that for some species the land bridge was a filter or even a complete barrier to movement. There are examples of some species that barely reach the opposite shore (Populus balsamifera and Viburnum edule on Chukotka and Potentilla fragiformis and Ranunculus monophyllus in Alaska) (Figure 2, pattern IV). A complex biogeographic history with distinct spatial and temporal components has created a flora rich in the circumpolar element but also one of vicariant taxa, intercontinental disjunctions and, for this latitude, a high level of endemism. Several molecular studies support Hultén’s hypothesis that unglaciated Beringia was a Quaternary refugium for plants (Tremblay and Schoen 1999, Abbott et al. 2000, Thompson and Whitton 2006, Eidesen et al. 2007a,b). Beringia is therefore key to understanding post-glacial dynamics within and among species. Much of our understanding of Ice Age Beringia is based on the study of