Print PageForest composition and tree species range limits are predicted to shift in response to climate change, although current empirical knowledge of the interrelated roles of climatic, environmental, and biotic drivers shaping forest change is minimal. Initial forest shifts should be detectable along ecotonal boundaries where species approach their range limits. Furthermore, because trees are long-lived, early evidence of forest response to climate change should be found in the younger understory regeneration layers. The seedling stage is a critical bottleneck and species responses to current understory conditions shape the future overstory. The overall goal of our research was to characterize tree regeneration trends within ecotonal forests and test how climatic, environmental, and biotic mechanisms are driving these trends.
We established plots at 125 sites across the upper Great Lakes region, which contains a broad forest ecotone where temperate and boreal tree species reach their northern and southern range limits, respectively. Mean annual temperature ranges 3.2 °C and precipitation varies by 33% across sampling sites. Relationships among tree regeneration drivers and seedling and sapling species abundances were examined using multivariate Redundancy Analysis (RDA) and variance partitioning to quantify total, unique, and shared explanatory relationships among ecosystem drivers.
Results/Conclusions
RDA models revealed numerous significant unique and shared explanatory relationships. Seedling and sapling full models explained 26%-36% of the variance in understory composition. Climate, soil resources, overstory trees, and competing vegetation and browse pressure all had significant effects and each individually explained 9-24% of understory variance. Although the unique effect of climate was significant, two-thirds of its influence was via indirect relationships with these other factors. The explanatory role of climate increased slightly and overstory effects and competition decreased from the seedling to the sapling layer. Species level responses showed differential sensitivities to temperature and precipitation. Sugar maple abundance was positively related to annual precipitation while red oak responded positively to summer temperature. Temperate red maple and the boreal conifers balsam fir and white spruce increased in abundance with decreasing summer temperature. These results indicate that compositional shifts will be sensitive to multiple climate variables and their relationships with other forest drivers. Understanding these trends and underlying mechanisms will facilitate the development of appropriate conservation and management strategies in the context of climate change.
