We used pollen and charcoal in lake sediments to ask how coarse vs fine textured soil and the abundance of fire breaks influenced the response of oak- and pine-dominated ecosystems to the climatic changes of the past 2000 years. We hypothesized that sites dominated by the most xeric soils would exhibit greater stability in community composition over time than would sites on less xeric soils. This hypothesis predicts that vegetation on the less xeric sites will have changed more when the climate became drier, but the most xeric sites would have had relatively stable jack pine vegetation because they would be resistant to invasion by species less tolerant of poor soils and drought. The resistance of xeric sites to invasion would be strengthened by stabilizing vegetation-fire feedbacks of jack pine. Our second hypothesis was that sites surrounded by fewer lakes and wetlands would exhibit greater stability in community composition than sites in areas with more lakes and wetlands. We predicted that more frequent fires would promote the maintenance of jack pine vegetation and its stabilizing vegetation-fire feedbacks on sites with fewer fire breaks. We tested the first hypothesis by comparing the vegetation and fire histories of sites that differ in soil drainage, but are matched in their abundance of fire breaks, and the second hypothesis by comparing sites that differ in their abundances of lakes and wetlands but have similar soil drainage characteristics.   
Results/Conclusions

Our initial results from six sites support both hypotheses. All six sites showed evidence of vegetation change between the Medieval Warm Period (ca. 1400-800 years ago) to the Little Ice Age (ca. 750-150 years ago), but fire regimes and the amount and nature of vegetation change differed among sites. Warner Lake, on excessively well-drained soils, and Lone Star Lake, on well-drained soils, had more stable community composition (evaluated using hierarchical cluster analysis, non-metric multidimensional scaling ordination, and pre-European-American pollen assemblages) than Hellhole and Lily Lakes, which are on less xeric soils but have similar abundance of fire breaks. Metzger and Elevenses Lakes, on excessively well-drained coarse sand with few fire breaks, had more stable vegetation than Warner Lake, which is also on excessively well drained soils but is surrounded by more fire breaks. Our results suggest that conditions that enhance drought and promote disturbance by fire support more stable vegetation composition over several thousand years on this pine-and oak-dominated landscape.