Responses of forests on sandy soils to post-European land-use changes

Background/Question/Methods

We compared pollen assemblages immediately before European settlement with surface pollen to determine if there were regional differences in vegetation dynamics following logging, post-logging fires and other land use changes. We compared sites across northern Wisconsin with soils derived from sandy outwash.  Soil texture is variable in different areas of the northwestern Wisconsin sand plain (NWS), ranging from excessively well-drained to moderately well drained, with vegetation ranging from jack pine barrens to mixed pine forests. We compared vegetation history on the range of soil textures of NWS to Northern Highland (NH) sites ~140 km to the east. Pre-European pollen assemblages were identified by the rapid decrease in white pine and rise in Ambrosia pollen percentages associated with European settlement. A non-metric multidimensional scaling (NMS) ordination of 247 surface, pre-European, and fossil samples from the last 600 yrs was used to visualize trajectories of vegetation change over time. Twenty four common pollen types were included in the ordination but Ambrosia (ragweed) was excluded in order to de-emphasize the disturbance effects of European land use.

Results/Conclusions

Preliminary results show broad overlap in the ordination scores of pre-European pollen assemblages from the 20 NH sites and the 31 NWS sites. Pollen assemblages changed dramatically after European settlement in most regions. Sites on less coarse sand in the southern region of NWS had a large (10-30%) increase in the percentage of oak pollen. Sites on similar soils in the northern region of NWS generally had red pine-birch vegetation in PLS and switched to higher birch percentages, and sometimes also oak. Sites in the central NWS where soils are excessively well drained had the least change in pollen assemblages and the least consistency in the direction of change. Although presettlement pollen assemblages from two of the northern highland sites were similar to oak-pine sites in NWS, a third fell within the red pine-birch ordination space. Birch pollen percentages increased at all three NH sites (8-10%) and the post-settlement samples shifted toward the birch region of the ordination space. Our results demonstrate that vegetation and pollen assemblages have changed dramatically since 1850 CE and that the trajectories of change were different between regions and soil textures, even within the range of outwash soils.