SENSITIVITY OF LOCAL VEGETATION AND FIRE TO CLIMATIC CHANGES VARIES WITH SOIL DRAINAGE

R. Calcote, University of Minnesota
C. Drake, University of Minnesota
P. Woods, University of Minnesota
E.A. Lynch, Luther College
S. Hotchkiss, University of Wisconsin, Madison

The relative sensitivity of the vegetation and fire regimes of neighboring sites to climate change is poorly understood. Many factors that can influence a sites sensitivity to a given climate change, including soil texture, drainage, fire breaks, vegetation type, and herbivory. We tested the hypothesis that vegetation assemblages on sites with excessively well-drained soils are the most strongly water limited and are thus the most sensitive to climate change.

We analyzed pollen and charcoal data from sediment cores of four small (<10ha), deep (>8m) seepage lakes within 30km of each other on the northwestern Wisconsin Sand Plain in the upper St. Croix watershed to reconstruct the vegetation and fire regime changes over the last 2,000 years. These sites are all on sandy outwash soils, but span a gradient from excessively well drained (L. Round and Lone Star Lakes) to somewhat well drained (Hell Hole and Lily Lakes). The type and timing of changes at each site were compared with the general climate trends of the Medieval Warm Period (MWP, warmer and/or drier ~1100 to 700 years ago) and the shift to wetter and/or cooler conditions of the Little Ice Age (LIA) after ~700 years ago. Our hypothesis predicts that the sites with excessively well-drained soils will be the most responsive to increased moisture availability during the LIA.

The pollen data show that the vegetation at the sites on the driest soils changed the least at the onset of the LIA, while sites on the slightly wetter soils changed dramatically. All of our sites had an increase in the pollen percentages of white pine, the pine species least tolerant of drought and fire, and a decrease in the more drought- and fire-tolerant jack and/or red pine. Charcoal data suggest a decrease in fire frequency and/or severity after ~600-1,000 years ago, consistent with an increase in more mesic tree species. Changes in fire regimes are also stronger at wetter sites. These changes are consistent with the interpretation of increased moisture availability during the LIA, but are not consistent with the hypothesis that a strongly moisture-limited site will respond more strongly to an increase in moisture availability. We hypothesize that the wetter sites have enough moisture to sustain many tree species if a period of lower fire probability (increased moisture) allows natural forest succession. The driest sites in this study are dominated by jack pine, a fire-adapted species that is highly flammable. Jack pine and fire, therefore, produce a positive feedback loop that will maintain jack pine dominance over thousands of years at the driest sites.

Our results demonstrate the importance of local factors such as soil type and vegetation feedbacks on disturbance in determining the relative sensitivity of sites to regional climate changes.

Suggestions for Reading

S.C. Hotchkiss, R. Calcote, E.A. Lynch (2007) Response of vegetation and fire to Little Ice Age climate change: regional continuity and landscape heterogeneity. Landscape Ecol. 22:25–41.