Background/Question/Methods Defoliation outbreaks are recurring, large-scale disturbances that influence the productivity and composition of temperate forests over long time-scales. We simulated defoliation outbreaks with a new extension for the forest disturbance and succession model, Landis-II, to better understand the long-term consequences of defoliation on forest carbon (C). Our disturbance extension recreates defoliation spatial and temporal patterns as well as species-specific tree growth and mortality responses to accumulated stress. The European gypsy moth (GM, Lymantria dispar L.) invaded our study area, a managed forest in the central Appalachian mountains of western Maryland, U.S.A, in the 1980s. We simulated forest dynamics for 400 years with and without GM defoliation disturbance to compare above-ground C dynamics expected in the absence of GM with those following introduction. Baseline disturbance simulations included periodic timber harvests and the native defoliator forest tent caterpillar (Malacosoma disstria Hbn.). Disturbance simulations were run individually and with GM, to examine how multiple defoliators with shared hosts interact with human management to alter disturbance response.

Results/Conclusions Our simulations show that the introduction of GM disturbance changes the trajectory of forest species composition, facilitating increases in non-host species that would not otherwise occur. Forest C is temporarily reduced following individual outbreaks, and long-term mean C accumulation declines to a lower steady-state, once GM enters the landscape. Changes in forest C storage are even more pronounced when a native defoliator has periodic outbreaks as well, particularly when outbreaks synchronize. However, compositional shifts are muted in the presence of two defoliators due to subtle differences in host preferences. The gradual compositional changes observed diverge from management targets, suggesting that harvesting strategies may need to be revised. Interacting insect disturbances have significant consequences for forest C storage and should be incorporated into our growing understanding of C dynamics at landscape scales.