Vegetation ecotone dynamics in Southwest Alaska during the Late Quaternary

To examine Late Quaternary vegetation change across the modern vegetation gradient from continuous boreal forest (central Alaska) to Betula shrub tundra (Bristol Bay region), pollen records from Idavain and Snipe Lakes are described and compared to those of four other sites in southwest Alaska. Major features of the vegetation history at Idavain Lake include herb-dominated tundra (ca. 14}12 ka BP), mixed herb/Betula shrub tundra (ca. 12}8 ka BP), and Alnus/Betula shrub tundra (8 ka BP to present). The Snipe Lake record reveals a brief period of herb tundra ('12 ka BP), Betula shrub tundra (ca. 12}8.5 ka BP), and Picea forest mixed with Alnus/Betula shrub tundra (ca. 8 ka BP to present). Comparisons with other pollen records indicate that southwest Alaska has been the location of major vegetation ecotones throughout the last 12 ka years. Northern areas have consistently been dominated by larger growth forms (shrubs or trees) than have southern areas. During the Betula period (12}8 ka BP), a dense Betula shrubland occupied central Alaska, changing to a mixed low-Betula shrub and herb tundra in the south. In the Alnus/Picea period (8 ka BP to present), Picea and Betula trees were more common to the north; Alnus and Betula shrubs more abundant to the south. Vegetation dynamics have been complex at individual sites and across the region. Each site shows both longand short-term shifts in major taxa, but the magnitude of these changes varies across the transect. In addition, some pollen changes appear to be synchronous among sites (within the constraints of existing chronologies), whereas others are strikingly time transgressive across the region. Similar vegetation dynamics at all sites are: (1) long-term decreases in herb taxa during the Betula period, (2) short-term oscillations between Betula shrubs and herbs during the Betula period, and (3) major increase in Alnus shrubs ca. 8 ka BP. Signi"cant di!erences among sites include: (1) major expansion of Populus trees in northern but not southern areas during the Betula period, (2) progressively later expansion of P. glauca at northern sites than at southern sites (ca. 9.5}4.5 ka BP), and (3) #uctuation of P. glauca populations in extreme northern areas during the early Alnus/Picea period. ( 2000 Published by Elsevier Science Ltd. All rights reserved.