PHYSIOGRAPHIC CONTROLS ON FIRE REGIMES IN CENTRAL MINNESOTA OVER THE PAST 1000 YEARS

Forest history in the region around the Twin Cities of Minnesota offers an excellent opportunity to study interacting controls on landscape vegetation patterns. The “Big Woods,” which was an isolated stretch of dense forest surrounded by prairie, existed here at the time of European settlement and had replaced local prairies and oak savannas only 400 years ago. Climatic changes probably caused the transition from prairie to forest, but the transition also occurred on the time scale of forest responses to disturbance, such as fire. The bigwoods forest, therefore, may have developed from regional-scale biogeographic dynamics that result when climatic effects interact with disturbance (fire) regimes. Such dynamics need to be better studied given human effects on both climate and disturbance regimes at overlapping spatial and temporal scales. This study will test the postulated model that climate-fire interactions led to the regional mosaic of bigwoods forest, oak savanna, and prairie, by evaluating a key hypothesis that fires were less common in the bigwoods forest than elsewhere in central Minnesota because of firebreaks. To do so, we have counted charcoal fragments from lake sediment cores collected from a pair of lakes: one within the bigwoods forest, one just outside the forest. GIS analyses were used to choose the study lakes in order to best evaluate the role of physiographic fire breaks. A complementary study was conducted to determine whether or not climate changed in the Big Woods region about 400 yrs ago in a manner that enabled conversion of prairie and oak savanna into closed forest.