Multi-year ecosystem response to hemlock woolly adelgid infestation in southern New England forests

The introduced hemlock woolly adelgid (HWA) (Adelges tsugae Annand) has generated widespread tree decline and substantial mortality of eastern hemlock (Tsuga canadensis (L.) Carriere) throughout the eastern United States. To assess the magnitude of ecosystem response to this disturbance, we conducted a multi-year study of forests with and without damage from HWA. Infested forests had significantly higher HWA-induced foliar loss and significantly lower forest floor C:N ratios and soil organic matter than uninfested forests. There were no significant soil temperature differences among stand types, although infested stands did have lower forest floor soil moisture than uninfested stands. Net nitrification and net N mineralization rates were significantly higher in infested versus uninfested forests by the second and third year of this study, respectively. In addition, total N pools and resin bag capture of NH4 and NO3 were significantly higher in infested versus uninfested forests throughout this study. Increases in N were likely due to a combination of factors including enhanced decomposition, reduced uptake of water and N by declining trees, sparse understory vegetation, and N-enriched throughfall from infested canopies. These results confirm that invasive pests can initiate substantial changes in ecosystem function soon after infestation occurs, prior to substantial overstory mortality or understory reorganization.