The vegetation of the northwest Wisconsin sand plain is dominated by oak and pine communities maintained by periodic forest fires. Regional climate records suggest extreme, spatially extensive droughts in the western Great Lakes Region before ~3300, 1900-1600, and 1100 to 600 cal yr BP. The response of vegetation and fire regimes to these events is poorly understood and has management implications for responses to future climate change. We used pollen and charcoal in cores from three small, deep lakes located < 20 km from one another to test the hypothesis that forests on different textures of sandy soil would respond differently to climatic changes. Hell Hole (HH) and Lily (LY) lakes are in areas with soils of relatively fine sand; soils around Lone Star (LS) are coarser sand. We measured the degree of change of vegetation with ordinations and squared chord distance (SCD) between each fossil sample and the sample that corresponds to pre-European settlement vegetation and the rate of change using SCD between 50 yr increments. Charcoal accumulation rates (CHAR) and peak frequencies were analyzed using Charster.
Results/Conclusions
All sites have similar long-term patterns. Periods of high CHAR occur ~4000 BP. Oak pollen percentages and the magnitude of charcoal peaks decrease ~3000-2500 BP, consistent with regional cooler/wetter conditions. Oak pollen decreases further beginning ~1500 BP, as pine (primarily jack-red pine) increases. White pine pollen increases after ~600 BP at all sites and charcoal generally decreases, consistent with the regional interpretation of fewer or less severe droughts. Although this general pattern is the same at all sites the degree of change is smaller at the most xeric site (LS) where the SCD between fossil samples and the PLS sample is less than 0.15. The pollen assemblages at less xeric sites are very different from PLS before 3000 BP (SCD >0.3), with another period of high dissimilarity (SCD 0.12-0.2) from 2000-700 BP. Ordinations also show more vegetation change at the less xeric sites. We conclude that even small variations in soil texture play an important role in determining the stability of vegetation and fire regimes in response to climatic changes. Additional studies of the response of vegetation to climate change on contrasting soils will help with modeling ecosystem responses to future climatic changes.
