Thursday Seminar: Disentangling responses to climate change versus broader anthropogenic impacts in forests of the eastern U.S.
In the face of rapid environmental change and given the inherently long time scales of tree population dynamics, it seems inescapable that forests are increasingly in disequilibrium, lagging in their responses to anthropogenic changes in the physical and biotic environment. Anthropogenic impacts on forests of the eastern U.S. include (1) climate change, (2) changes in logging regimes, (3) the litany of introduced pests and pathogens that are profoundly changing eastern U.S. forests, and (4) changes in the atmospheric environment, particularly rates of nitrogen (N) deposition and CO2 concentrations. My research uses a spatially-explicit, individual-based model of tree population dynamics (SORTIE-ND) that is linked to continental-scale forest inventory data maintained by the U.S. Forest Service. Preliminary results indicate that the direct effects of climate change on the distribution and abundance of tree species in the eastern US will be relatively small over the next 50-100 years, in part because climate impacts on sapling and adult tree growth and survival are relatively small. Climate impacts on seedling recruitment and survival appear to be much stronger, but the current occupants of a site have both a numerical and competitive advantage over new colonists. This provides a great deal of inertia to forest structure and composition, and analyses with the model suggest that this inertia would remain even in the absence of dispersal limitation. Tree species in the region show a wide range of responses to current rates of N deposition, and in many of the species the magnitude of expected response to anticipated future reductions in N deposition is of the same magnitude as the expected response to anticipated future temperature increases. Both historical and current harvest regimes are driving successional dynamics that can be expected to result in large changes in both forest structure and composition at a regional scale. Those successional dynamics are promoting the abundance of two late successional species – eastern hemlock and beech – that are currently threatened by introduced pests or pathogens. Processes that supplement the natural mortality rates of adult trees, whether through harvesting or the actions of introduced pests or pathogens, may accelerate the effects of climate change by opening the canopy and allowing new tree recruitment. Models provide the only real tool to integrate these disparate effects, and analyses with SORTIE-ND have all the hallmarks of complex system behavior, including initial condition dependence but predictable general features of system recovery following perturbation.
Coffee and cookies will be served at 4 p.m.