VEGETATION HISTORY OF NORTHCENTRAL ALASKA - A MAPPED SUMMARY OF LATE-QUATERNARY POLLEN DATA

Fossil pollen data, as illustrated by isopoll and isochrone maps, docu ment the complex late Quaternary history of tundra and boreal forest d evelopment in northcentral Alaska. Major plant taxa behaved independen tly over time, resulting in substantial differences in the vegetation history of eastern and western regions. Major vegetation changes are i n general agreement with GCM simulations and confirm the importance of the continental ice sheet and insolation variations in determining la te Quaternary climatic trends. Herb-dominated tundra characterized the vegetation between 18 and 14 ka BP, with mesic graminoid tundra in lo wer elevations of western areas and more xeric, sparse tundra communit ies in the east and at higher elevations. Moist Betula tussock tundra rapidly replaced the western herb tundra ca. 14 ka BP. However, Betula shrubs expanded more slowly in the east, establishing relatively dry shrub tundra as the predominant regional vegetation by ca. 12 ka BP. R iver valleys and south-facing slopes supported Populus woodlands betwe en 11 and 9 ka BP, but shrub tundra continued to dominate most upland sites. Alnus shrubs first expanded in the southwestern Brooks Range be tween 10 and 9 ka BP, spreading rapidly throughout the entire region b etween 8 and 7 ka BP. Picea glauca populations also expanded between 1 0 and 9 ka BP, but from source areas in northwestern Canada. The P. gl auca forests were most abundant in riparian settings, but isolated sta nds probably also established in the shrub tundra. P. glauca reached t he central Brooks Range by ca. 8 ka BP, followed by an apparent popula tion decline between 8 and 7 ka BP. P. mariana became the dominant tre e species ca. 6 ka BP, when it invaded non-riparian P. glauca forests in eastern and central areas and moist shrub tussock tundra in the wes t. The modern distribution of communities in northcentral Alaska was a chieved between 6 and 4 ka BP.