Invasions by two non-native insects alter regional forest species composition and successional trajectories

• We examine impacts of beech bark disease (BBD) and hemlock woolly adelgid (HWA) on forests in the US.
• Beech and hemlock mortality rates increase following invasion by BBD and HWA, respectively.
• Beech and hemlock net growth rates decrease following invasion by BBD and HWA, respectively.
• Beech and hemlock show apparent compensatory growth in response to mortality in the other.
• Our results suggest that BBD and HWA invasions may modify future species compositions.

While invasions of individual non-native phytophagous insect species are known to affect growth and mortality of host trees, little is known about how multiple invasions combine to alter forest dynamics over large regions. In this study we integrate geographical data describing historical invasion spread of the hemlock woolly adelgid, Adelges tsugae, and beech scale, Cryptococcus fagisuga, with regional forest inventory data collected by the US Forest Service’s Forest Inventory and Analysis program to quantify the individual and combined impacts of these pest species. This analysis indicates that regional impacts of these insects on their hosts occur surprisingly slow but act to change regional forest succession pathways. Because beech and hemlock commonly co-occur in eastern North American forests, invasions by the two pest species are altering the current and future composition of large forest regions through their impacts on these two late-successional species. Such results demonstrate how forest insect invasions can profoundly modify forest dynamic processes, resulting in long-term changes in forest ecosystems.