The impact of invasive insects on aboveground processes in forest ecosystems has received considerable attention over the last several decades. Invasive insects have led to the decline and removal of several dominant tree species across broad regions in eastern forests. The invasive hemlock woolly adelgid (HWA; Adelges tsugae) has impacted many eastern hemlock (Tsuga canadensis) forests with widespread foliar loss, mortality and subsequent compositional changes. While much work has been conducted on the aboveground consequences of adelgid-induced hemlock decline, less attention has been paid to the below-ground consequences. Data from several recent studies conducted in hemlock forests infested with HWA are presented to characterize the belowground ecosystem function changes resulting from the loss of hemlock. Studies examining natural decline due to HWA and simulated decline with hemlock removal examined changes in nitrogen (N) capture, N- cycling, and soil respiration. In addition, the rhizosphere was examined in hemlocks with and without HWA in forest stands to characterize fine-root ectomycorrhizal (EM) colonization and bacterial abundance.
Results suggest as hemlock declines, micrometeorological and compositional changes lead to changes in ecosystem function. Infested forests experienced significantly higher N-nitrification rates, net N mineralization rates, and resin bag capture of ammonium and nitrate than uninfested forests, although these changes were often transient, only lasting for several years due to the rapid regrowth of vegetation. Similarly, soil respiration rates declined with hemlock loss and then recovered rapidly to pre-removal levels within 2-4 years, although within plot variation was high. Adelgid infestation decreased the percentage of root material colonized by ectomycorrhizal fungi by more than 67%, possibly due to enhanced soil N levels. Rhizosphere bacterial abundance on fine roots was 25% lower on adelgid-infested versus uninfested trees, and roots of adelgid-infested trees contained significantly less carbon. Chemical analysis of fine roots from adelgid-infested versus uninfested trees revealed that percent carbon was significantly lower in infested hemlock stands, but that adelgid infestation did not affect percent nitrogen. Findings suggest that aboveground feeding by adelgid on hemlock has belowground consequences on ecosystem function and rhizosphere processes far from the site of localized damage that further magnify the impacts of hemlock loss.