PARENT SESSION
Oral Session # 2: Paleoecology.
Presiding: B Lynch
Monday, August 4. 8:00 AM to 11:30 AM, SITCC Meeting Room 101.
Influence of late Holocene moisture variability on forest dynamics in the western Great Lakes region.
Booth, Robert*,1, Jackson, Stephen1, 1 University of Wyoming, Laramie, WY
ABSTRACT- High-resolution records of past moisture variability have recently been obtained from peatlands in the western Great Lakes region using testate amoebae as proxies for past surface-moisture conditions. We used these records together with pollen and plant macrofossil data to determine how late Holocene population dynamics of mesic tree species (Tsuga canadensis, Fagus grandifolia, Betula alleghaniensis) were influenced by climate variability in the region. In Upper Michigan, populations of mesic tree species underwent expansions during times of increasing moisture, particularly when these moisture increases were preceded by drought. For example, regional populations of B. alleghaniensis and local populations of T. canadensis expanded 4000 years BP during a rapid increase in moisture that was directly preceded by a high-magnitude, yet relatively short-duration (100-200 years) drought. High-resolution pollen and charcoal analysis spanning the drought interval indicates regional wildland fires and abundant Pteridium ferns, suggesting that the expansion of B. alleghaniensis and T. canadensis was caused by the combined effect of drought-induced disturbance and the subsequent rapid increase in moisture. Our results suggest a new explanation for the decline of F. grandifolia populations in southeastern Michigan and southern Ontario during the last 1000 years, which has been variously attributed to climatic cooling or anthropogenic disturbance. Our peatland proxy-climate record indicates that drought and an associated increase in fire frequency and/or intensity led to replacement of F. grandifolia by Pinus populations. Comparisons between records of paleoclimate and vegetation history show that major late Holocene vegetation changes in the western Great Lakes region were strongly influenced by moisture extremes, and demonstrate the power of independent paleoclimate records in evaluating records of vegetational change.
Key words: paleoclimate, paleoecology, vegetation dynamics, climate change