Invasive forest defoliator contributes to the impending downward trend of oak dominance in eastern North America

Disturbance by non-native insect species can be an important ecological driver shaping long-term changes in vegetation and plant species composition. While impacts of gypsy moth (Lymatria dispar L.) outbreaks in North American forests have been extensively studied, the results are quite inconsistent, particularly with respect to the amount of tree mortality associated with defoliation. In this study, we integrate geographical data describing historical gypsy moth defoliation with forest inventory data collected by a national forest inventory programme to quantify regional impacts across several million hectares of forest land in the northeastern US. While observed increases in host tree mortality rates and decreases in growth rates associated with defoliation were expected, the study also indicates that this overstory mortality, coupled with ongoing declines in oak regeneration, will result in a long-term reduction of oak density in defoliated areas. Eventually, these impacts will likely contribute to regional shifts in tree species composition and forest succession pathways. Gypsy moth outbreaks thus appear to exacerbate ongoing declines in young oak age classes in the region.