It can be argued that introduced pests and pathogens of native forest trees have been the most profound agents of change in forests of the northeastern US over the past century. The pace of introductions of novel pests and pathogens has accelerated in recent years, presumably in response to increased globalization of trade. Climate change can be expected to exacerbate the problem – both in terms of the rate of emergence of new pests and disease agents, and the abilities of host populations to respond. We have little ability to predict the identities of either the introduced pests and pathogens or their specific tree hosts. Models of forest dynamics, however, can allow us to categorize the long-term impacts on both tree population dynamics and forest ecosystem processes, based on the nature of the effects on individual hosts, and the pattern of spread of the insect or disease both regionally and within stands.
Results/Conclusions
I will summarize results of simulations of the effects of beech bark disease and Hemlock wooly adelgid on long-term dynamics of northeastern forests. The two agents differ dramatically in rate of spread and in the mode of their effects on growth, mortality, and reproduction of host trees. Predicted impacts on successional dynamics, and rate of recovery from outbreaks, in both cases appear to be dependent on the relative abundance of the host at the time of the outbreak. With the wooly adelgid, very high rates of mortality in stands with high relative abundance of the host, and a rapid, regionally-moving wave of outright mortality of adults, should allow more rapid recovery of hemlock populations (through initiation of secondary succession). In contrast, where the hemlock is co-dominant with a close competitor, and mortality initiates gap phase replacement by other shade tolerant species, recovery of host abundances to pre-outbreak levels may require millennia. This rate of recovery is particularly problematic under a changing climate. Beech bark disease presents fundamentally different dynamics, with relatively low rates (and slow development) of canopy tree mortality. Analysis of FIA data from northeastern states, however, shows dramatic declines in tree growth rates, large drops in aboveground biomass (of both beech and other species), and large shifts in tree size structures in response to the disease. Our models suggest little long-term recovery of either host populations or forest carbon without the emergence (either naturally or through human intervention) of some form of biological control.
