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CLIMATE, VEGETATION, AND FIRE HISTORY OF HELL HOLE LAKE: A PALEOECOLOGICAL APPROACH
Christa Drake, University of Minnesota
The interaction of upland vegetation, fire, and climate change in the St. Croix basin is preserved in the fossil record of lake sediments. We used fossil pollen and charcoal fragments to study the history of the northwestern Wisconsin sandplain. At the time of the Public Land Survey (PLS, ~1860) the area around Hell Hole Lake was dominated by a mixed forest of red, jack and white pine along with oak. The modern forest around this site is dominated by the same taxa, though with a greater abundance of oak (Radeloff et al. 1999).
A 2.5m sediment core was obtained from Hell Hole Lake in 2004. Subsamples were taken for pollen, AMS radiocarbon dating, and charcoal analysis. Four AMS radiocarbon dates provide a chronology for the past 4100 years. Charcoal samples were taken at contiguous 0.5cm intervals throughout the core. About 500 upland pollen grains were counted in each sample; all charcoal particles 20-120 microns were counted. We compared the changes in vegetation around Hell Hole Lake to the range of vegetation assemblages found on the sand plain at the time of the PLS by including 33 PLS age pollen samples (classified by PLS veg type; Hotchkiss et al. 1997) from across the sand plain and all pollen samples from Hell Hole Lake in a non-metric multidimensional scaling (NMS) ordination.
The pollen record shows that the area was dominated by Oak Savanna from 4180 to 2480 cal. yrs BP, with the highest oak pollen abundance of any time before European settlement. The charcoal abundance is consistent with frequent small fires that maintained an open savanna. After this period a more mixed pine-oak vegetation developed with perhaps a slight increase in frequency of intense fires. By 1600 years ago oak declined further and red/jack pine pollen increased. During this period the charcoal record has sporadic large peaks, consistent with a fairly dense canopy of pines maintained by occasional stand destroying fires. The most recent period before PLS had a large increase in white pine pollen and had low overall charcoal deposition. This shift is typical of other sites on the sand plain and has been attributed to the cooler/wetter climate of the Little Ice Age (Hotchkiss et al. 2007). The post-PLS period is clearly visible after ~80 years ago, with an increase in oak pollen at the surface. The last sample (surface mud) has high oak pollen percentages that suggest the modern vegetation is not similar to anything that has grown at the site since about 2500 years ago.
Hotchkiss S.C., R. Calcote, E.A. Lynch (2007) Response of vegetation and fire to Little Ice Age climate change: regional continuity and landscape heterogeneity. Landscape Ecology 22:25-41.
Radeloff, V.C., D.J. Mladenoff, H.E. He, M.S. Boyce (1999) Forest landscape change: the northwest Wisconsin Pine Barrens before European settlement and today. Can. J For Res 29:1649-1659.