THE ROLE OF VEGETATION CHANGE, DISTURBANCE, AND CLIMATE ON SEDIMENT RECORDS OF LAKE PRODUCTIVITY: SOUTHWEST ALASKA

Paleolimnological studies are combined with landcover vegetation analyses in selected lake ecosystems in the Southwest Alaska Network of the National Park Service in order to better understand the impacts of both rapid and recent (last century) and long-term (late Holocene) landscape changes on lake ecosystem dynamics. Evidence from previous studies suggests Alder Fixed Nitrogen (AFN) may exert an important influence on nitrogen cycling and productivity in sub arctic watersheds and lake systems. Paleo lake productivity is reconstructed from down core measurements of changes in organic carbon, nitrogen, phosphorus, and diatom accumulation as well as sedimentary ä 15N and ä 13C. Lake productivity records, ranging in age from 100 to 8000 years, have been reconstructed from at least three lakes in Southwest Alaska. Siliceous microfossils, particularly diatoms were analyzed to provide information on past and present changes in plankton abundance. Characterization of the history of vegetation abundance and spatial distribution is shown by combining pollen records retrieved from sediment cores with a suite of historical, high altitude photographs and landcover datasets. Recent vegetation change is illustrated by comparison of past and future photographic records and landcovers.

Results to date indicate vegetation change, namely an expansion and increased biomass of American Green Alder, is apparent over the last century as seen in the photographic record. Preliminary data suggest that recent increases in lake productivity are also apparent in the sedimentary records over the last 50 years. Disturbances driven by climate, glacial retreat, volcanism and other natural perturbations contribute to, and perhaps at times overprint, vegetation driven processes in lake sediment cores. The combined results allow for an integrated record of terrestrial and aquatic changes and evaluation of the effects of alder among aquatic ecosystems both spatially and temporally. Recent documentation that warming has accelerated over the last three decades may also allow comparisons between the climate driven `alder rise' of the early Holocene, and the alder expansion which we observe occurring today.