LANDSCAPE PATTERNS AND CLIMATIC CHANGE: 1200 YEARS OF VEGETATION CHANGE ON THE NORTHWEST WISCONSIN SAND PLAIN
Randy Calcote, Limnological Research Center, University of Minnesota
Sara Hotchkiss, Botany Department, University of Wisconsin, Madison
Elizabeth A. Lynch, Department of Biology, Luther College, Decorah, IA
The objective of this study is to determine the natural variability of the vegetation mosaic recorded in Public Land Survey (PLS) data of the 1850s. We used fossil pollen and charcoal from sediments of seven small deep lakes on the 450 km2 sand plain in northwestern Wisconsin to reconstruct vegetation and fire history over the past 1200 years. Our results indicate that vegetation and fire regimes have changed dramatically.
We used PLS data and pre-ragweed rise pollen assemblages to develop methods of interpreting the pre-European vegetation types from pollen assemblages. Cluster analysis identified five common vegetation types from PLS data within a 5km radius around 42 random points and 31 small, deep lakes. Pre-European pollen assemblages from the lakes were used to sample the range of pollen assemblages produced by each vegetation type. We then constructed a key to interpret fossil pollen percentages in terms of the five pre-European vegetation types using the analog method and ratios of pollen types. The key gives the correct vegetation interpretation of our pre-European pollen assemblages ~90% of the time.
At sites in the southern sand plain white pine increased in abundance about 700 years ago. Nonarboreal pollen decreased about the same time, suggesting that tree cover increased. The timing and degree of increase in tree pollen varies from site to site, but at several sites decreased charcoal influx precedes the increase of white pine and mesic trees by a century or more. This suggests that moister conditions decreased fire frequency, allowing succession to a more mesic forest. One site in the fire-prone central region had little change in vegetation during this period and the charcoal influx increased, presumably due to increased fuel production.
Paleoecological records from other Midwestern sites indicate a trend toward cooler/moister conditions 700-150 cal yr BP. Our results suggest that the vegetation of the sand plain changed dramatically in response to these relatively small regional climatic changes, but that local site conditions tempered the magnitude and timing of this response. Our results also suggest that the presettlement vegetation represented by the witness tree records may have been a result of several centuries of cool, moist conditions, and restoration goals need to take natural variability and future climate into account.
