VEGETATION HISTORY OF PLEASANT ISLAND, SOUTHEASTERN ALASKA, SINCE 13,000 YR BP

A 13,000-year history of late-quaternary vegetational and climatic cha nge has been derived from lacustrine sediments from Pleasant Island in the Glacier Bay region of southeastern Alaska. Early arrival of lodge pole pine and mountain hemlock, indicated by the presence of pollen an d conifer stomata, suggests expansion from refugia in the Alexander Ar chipelago. A short-term climatic reversal, possibly correlated with th e European Younger Dryas, is inferred from the expansion of tundra ele ments and deposition of inorganic sediments between 10,600 and 9900 C- 14 yr B.P. Two peat cores from the lake catchment verify Holocene vege tation changes and aid in the separation of biogenic from climatic for ces affecting vegetation history. Differences in pollen representation among the three cores illustrate the variation among pollen-collectin g substrates, as well as the spatial heterogeneity of peatland develop ment and its dependence on local hydrology. Initial peat accumulation and soil paludification, occasioned by increases in temperature and pr ecipitation in the early Holocene, allowed western and mountain hemloc k to replace sitka spruce 8500-8000 yr B. P. Open muskeg became widesp read about 7000 yr B. P. and allowed lodgepole pine to reinvade the re gion after a 2000-yr absence. The extensive replacement of fen element s by bog taxa at 3400 yr B. P. suggests increased paludification due t o changing hydrologic conditions; its correlation with the upland expa nsion of Tsuga heterophylla suggests the onset of a cooler/wetter Neog lacial climate in southeastern Alaska. (C) 1996 University of Washingt on.