This project uses pollen and charcoal in lake sediments to ask how soil texture and firebreaks influenced the response of oak- and pine-dominated ecosystems to climatic changes of the past 7000 years. Previous results demonstrate that during the last 2000 years the vegetation and fire regimes on a 450 square kilometer sand plain in northwestern Wisconsin were remarkably dynamic. Individual sites within the landscape responded differently to relatively small climatic changes. White pine increased during the Little Ice Age at most sites, apparently due to slight differences in soil texture and the abundance of lakes and other firebreaks. The current research extends the time period studied to include more extreme climatic changes, and tests hypotheses about the importance of soil types, firebreaks, and vegetation feedbacks in determining the response of local vegetation.
Predicting the response of local vegetation to climate change is an important goal of ecosystem modeling. Federal, state and county agencies are actively engaged in pine barrens habitat restoration in the area being studied, and the results of this work will be directly applicable to establishing realistic restoration goals and management strategies. Students from Luther College, the University of Minnesota, and the University of Wisconsin will participate in a collaborative learning environment in which skills in collection and interpretation of long-term records of environmental change are applied to questions about ecosystem management.
As climate changes, vegetation and disturbance regimes will also change, but little is known about how local site factors, such as soil quality or protection from wild fire, will affect the stability and resilience of ecosystems. Landscape paleoecology provides a way to address such questions by applying the tools of paleoecology to a network of strategically located sites to generate records of past ecological changes on the scale of interest to human land use management planning. The goal of this project was to collect and analyze records of past changes in vegetation and fire regimes from a network of sites on sandy soils in the western Great Lakes region in northwestern Wisconsin. Previous research demonstrated that during the past 2000 years the forest communities at some of these sites changed dramatically, while at other sites forest communities changed little (Hotchkiss et al. 2007). This project was designed to investigate the reasons behind these different responses to climatic changes. We used strategically located study sites to determine how landscape characteristics, specifically soil quality and the presence of fire breaks, contributed to the stability or resilience of the vegetation. We also extended the length of the paleoecological records at several sites to ask how vegetation and fire regimes changed during the hottest/driest period of the Holocene (8000-6000 years ago). Sediment cores were collected from four sites that had records exceeding 8,000 years and additional analyses were performed on cores previously collected. Accelerator Mass Spectrometry (AMS) radiocarbon dates were obtained for each core to determine sediment accumulation rates. Pollen and charcoal in sediments from 11 sites were analyzed to compare vegetation and fire histories across sites and over time. Using these records, we compared the stability and resilience of plant communities in sites with different soil quality and with different degrees of protection from fire spread. One important result from this project is that vegetation at the sites that were most protected from fire (many lakes and wetlands within 5 km) was less resilient when climate changed compared to vegetation at site exposed to fire. This result appears counter-intuitive, since fire can create such dramatic vegetation changes in a short period of time. However, in this sandy landscape feedbacks between jack pine (Pinus banksiana) and fire maintain jack pine dominated forests for millennia, leading to short term changes, but long term resilience (Lynch et al. 2014, Tweiten et al. in press). Results from this project also indicate that that both soil drainage (even within this narrow range of well-drained sandy soils) and fire breaks influence the responsiveness of local vegetation to climate change, with firebreaks having a stronger influence. Our results also support the hypothesis that the effect of wetlands as fire breaks changes as the regional climate changes. During dry periods, when water tables are low, sites that are well protected from fire by wetlands under modern conditions became more susceptible to fire, and the vegetation changes to a more resilient jack pine community with strong fire feedbacks. Sites protected from fire by surrounding lakes, on the other hand, remain protected from fire, and therefore continue to be more responsive to the range of climate changes that occurs during the dry period. This work has been conducted in collaboration with staff from the Wisconsin Department of Natural Resources (WDNR), who are developing management plans to protect federally endangered sand plain species (Karner Blue butterfly and Kirtland’s warbler) and to maintain sand plain habitats under predicted future climate changes. Most of our sites are in WDNR Conservation Opportunity Areas. This research was presented to the Society for Ecological Conservation (Hotchkiss et al. 2013) and the Chicago Botanical Garden (Hotchkiss et al. 2012) in invited symposium talks. Results will also be presented to The Nature Conservancy, in Madison, Wisconsin in the spring of 2015. The data from this project are being contributed to databases (North American Pollen Database, NEOTOMA, International Multiproxy Paleofire Database) in order to make them available to all researchers. These data are already being used by the PALEON project to improve understanding of the subcontinental vegetation history of eastern North America and to use the past to calibrate models of the response of vegetation to future climate change.
