Michael Tweiten

Background/Question/Methods Ecological theory, models, and limited empirical data predict that forest communities will vary in their responsiveness to climate change, depending on factors such as site quality, disturbance regimes, and competitive interactions. Paleoecological records from a dense network of sites in northwestern Wisconsin documenting changes in plant communities and fire regimes during the past 3000 years allowed us to test predictions about factors that stabilize vegetation during periods of centennial-scale climate change. In particular, we tested whether on poor quality sites jack pine (P. banksiana) communities would be more stable because of strong feedbacks between fire and jack-pine and lack of competition from other tree species. At sites on finer-grained sand we expected vegetation would change more because changes in climate (and possibly fire regimes) would alter the outcome of competitive interactions among species and provide the opportunity for a new c...

Background/Question/Methods To understand how the dynamics of variation in plant communities have responded to past climate changes paleoecologists must devise ways to translate changes in pollen assemblages to changes in plant communities. Fossil pollen data can be used to interpret vegetation in both typological and continuous ways. A typological interpretation of pollen records can be used to test models predicting changes in transition probabilities. We developed a method for vegetation interpretations of pollen assemblages for northern Wisconsin from a library of 77 modern and 65 pre-European analogs. Each potential analog was matched with vegetation data: LANDSAT classifications for the modern vegetation, and General Land Office (GLO) Survey data for the pre-European settlement period. Species relative abundance (modern) or relative basal area (pre-European) were summarized within 5km of each site. Cluster analysis was used to classify vegetation types. A decision-analysis fra...

Landscape-scale vulnerability assessment from multiple sources, including paleoecological site histories, can inform climate change adaptation. We used an array of lake sediment pollen and charcoal records to determine how soils and landscape factors influenced the variability of forest composition change over the past 2000 years. The forests in this study are located in northwestern Wisconsin on a sandy glacial outwash plain. Soils and local climate vary across the study area. We used the Natural Resource Conservation Service's Soil Survey Geographic soil database and published fire histories to characterize differences in soils and fire history around each lake site. Individual site histories differed in two metrics of past vegetation dynamics: the extent to which white pine (Pinus strobus) increased during the Little Ice Age (LIA) climate period and the volatility in the rate of change between samples at 50-120 yr intervals. Greater increases of white pine during the LIA occurred on sites with less sandy soils (R² = 0.45, P < 0.0163) and on sites with relatively warmer and drier local climate (R² = 0.55, P < 0.0056). Volatility in the rate of change between samples was positively associated with LIA fire frequency (R² = 0.41, P < 0.0256). Over multi-decadal to centennial timescales, forest compositional change and rate-of-change volatility were associated with higher fire frequency. Over longer (multi-centennial) time frames, forest composition change, especially increased white pine, shifted most in sites with more soil moisture. Our results show that responsiveness of forest composition to climate change was influenced by soils, local climate, and fire. The anticipated climatic changes in the next century will not produce the same community dynamics on the same soil types as in the past, but understanding past dynamics and relationships can help us assess how novel factors and combinations of factors in the future may influence various site types. Our results support climate change adaptation efforts to monitor and conserve the landscape's full range of geophysical features.