Boreal Ecosystem Development in the Northwestern Alaska Range since 11,000 yr B.P

tem processes, and topography in determining vegetation Analyses of pollen, plant macrofossils, macroscopic charcoal, composition, landscape patterns, and ecosystem function mollusks, magnetic susceptibility, and geochemical content of a (Van Cleve et al., 1983, 1986, 1991; Wien and MacLean, sediment core from Farewell Lake yield a 11,000-yr record of 1983; Bonan and Shugart, 1989; Shugart et al., 1993). Forest terrestrial and aquatic ecosystem changes in the northwestern footfires and soil processes affecting permafrost, forest floor hills of the Alaska Range. Between 11,000 and 8500 yr B.P., the organic layer, and soil temperature and moisture have been regional landscape was dominated by a Betula shrub tundra, in identified as the primary controls of forest dynamics. Ecosyswhich Populus-Salix communities were common. Abundant chartem modeling in interior Alaska suggests that these factors coal in sediments indicates that fires were common in the lake could greatly alter boreal forest responses to climatic forcing catchment during this period, and high mineral accumulation (Bonan et al., 1990). The roles of fire and soil-forming prorates, allogenic elemental content, and magnetic susceptibility suggest intense soil erosion. In addition, mollusks, pollen and macrocesses in the postglacial development of boreal forests have fossils of aquatic macrophytes, and biogenic silica provide evidence also attracted the attention of paleoecologists working in that the lake was substantially shallower and more productive Alaska and adjacent Canada (Ritchie, 1985, 1987; MacDon11,000–8500 yr B.P. than later. Low lake level and high aquatic ald, 1987; Hu et al., 1993; Anderson and Brubaker, 1994; productivity might have been caused by warm and dry summers Earle et al., in press). Ritchie (1985, 1987), in particular, associated with early postglacial insolation maximum in northern has stressed the need to examine charcoal and sediment geohigh latitudes. About 8000 yr B.P., Picea glauca arrived within the chemistry along with fossil pollen to elucidate the postglacial lake catchment, forming a forest tundra association until ca. 6000 history of fire and soil regimes in relation to vegetation and yr B.P. Alnus shrub thickets established in the region ca. 6500 yr climate changes. However, evidence for evaluating ecosysB.P., and Betula papyrifera arrived ca. 6000 yr B.P. Closed P. tem processes had been unavailable from this region until glauca forests developed ca. 6000 yr B.P. Picea mariana became important subsequently and replaced P. glauca as the dominant two recent studies (Hu et al., 1993; Earle et al., in press). tree species in the region ca. 4000 yr B.P. An increase in authigenic Hu et al. (1993) investigated pollen, macroscopic-charcoal, Fe/Mn ratios suggests that the development of waterlogged soils and geochemical content of a sediment core from Wien Lake accompanied this vegetation change. Fires increased in importance to infer the relation of soil processes and fire to the associated at this time and might have accelerated soil erosion. The establishvegetation changes during the past 12,000 years. Earle et ment of P. mariana forests probably reflected complex responses al. (in press) presented a 14,000-yr charcoal record from of forest ecosystems to the onset of cooler and wetter climate Sithylemenkat Lake and discussed potential importance of conditions during the late Holocene. q 1996 University of Washington. fires in major pollen zones. These studies provide evidence that changes in soil and fire regimes might have exerted INTRODUCTION proximate controls over vegetation responses to climatic Recent investigations of boreal forests emphasize the imchanges during postglacial time. portance of complex interactions between climate, ecosysIn addition to the scarcity of information on terrestrial ecosystem processes, postglacial lacustrine environments are 1 To whom correspondence should be addressed: Limnological Research poorly documented in Alaska and adjacent Canada (Ritchie Center, 220 Pillsbury Hall, University of Minnesota, Minneapolis, MN