Disentagling responses to climate change versus broad anthropogenic impacts in temperate forests

Background/Question/Methods

Climate change is taking place in the context of a broad suite of other anthropogenic impacts on forest ecosystems, ranging from invasive species to air pollution.  But those impacts are very difficult to predict individually.  To address the potential interactions among these processes, I have parameterized a spatially-explicit model of forest dynamics (SORTIE-ND) for the 50 most common tree species in the eastern U.S., entirely from FIA data.  This includes functions for seedling recruitment, and growth and survival of seedlings, saplings, and adult trees.  The growth and survival functions explicitly incorporate effects of competition, climate, and N deposition.  I have also developed new statistical models to characterize regional variation in harvest regimes, and the effects of a wide range of pests and pathogens. 

Results/Conclusions

Results from the model suggest that the direct effects of climate change alone on the distribution and abundance of tree species in the eastern US will be relatively small over the next 50 years, largely because direct climate impacts on adult tree survival are relatively small.  Even a simulated 6^o C temperature increase over the next 100 years produces only modest shifts in tree species presence and abundance along climate gradients.  Both productivity and aboveground biomass, however, decline by roughly 25%.  These patterns hold even in runs where dispersal limitation is removed.   Processes that supplement the natural mortality rates of adult trees, whether through harvesting or the actions of introduced pests or pathogens, accelerate the effects of climate change by opening the canopy and allowing new colonization and recruitment.   This is particularly true when the additional mortality is highly selective in terms of species and sizes of trees removed (as can be the case for both harvesting and pests).  The largest changes predicted by the model reflect expected successional trajectories under the impacts of previous and current land-use, particularly the harvest regimes.  Specifically, there has been a sea-change in forest harvest practices over the past 50 years, replacing clear-cutting and intensive harvest with both more selective and more frequent partial harvests.  Previous harvest regimes favored early successional species like red maple (Acer rubrum), but the current harvest regimes promote late successional species, specifically beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis).  But both of these species are threatened by introduced pests or pathogens (beech bark disease, and the hemlock wooly adelgid, respectively).