Impact of Hemlock Woolly Adelgid on Canopy Throughfall in Southern New England 2002

Exotic insect pests may strongly disrupt forest ecosystems and trigger major shifts on nutrient cycling, structure and composition. Although the immediate impact of these pests are frequently examined as physical disturbances (i.e., defoliation, decline in leaf area, and tree mortality) that initiate changes in ecosystem function, the insects often generate fundamental biochemical and trophic changes in tree canopies that may be equally important in altering ecosystem dynamics. Consequently, investigation of the linkages between canopy-level, ecosystem and environmental impacts may be critical for a thorough understanding of functional, structural, and compositional changes resulting from pest infestation. We sought to establish a better understanding of these linkages for the hemlock woolly adelgid (HWA), which is devastating hemlock forests in an expanding region across eastern North American and has the potential to eliminate this long-lived and extremely shade-tolerant species across much of its range. We examined the impact of the adelgid on hemlock needle chemistry and epiphytic microorganisms, litter production, and shoot growth in stands differing in their levels of infestation and linked these to shifts in canopy nutrient cycling and stand and landscape dynamics. HWA initiated major changes in canopy biomass and distribution. Whereas uninfested trees exhibit a decline in canopy biomass from the center to the periphery and a positive correlation between total needle litter and estimated biomass, infested trees support predominantly woody biomass, have significantly less total canopy biomass, produce less new foliage and exhibit no correlation between litter and canopy biomass. Foliar %N was strongly influenced by needle age and the level of infestation and was highest in young foliage supporting the highest densities of HWA. Foliar %C was unaffected by HWA or foliar age. Epiphytic microorganisms on hemlock needles exhibited little variation in abundance within canopies, but bacteria, yeasts and filamentous fungi were significantly more abundant on medium and heavily infested than uninfested trees. Throughfall chemistry, quantity, and spatial pattern were strongly altered by HWA. Beneath uninfested trees throughfall exhibits a strong gradient, decreasing in volumes from the canopy periphery to the trunk. Beneath infested trees the amount of throughfall is greatly increased, spatially unpatterned and characterized by higher concentrations of nitrogen compounds, carbon and cations. Across the southern New England landscape there is a strong south to north gradient of decreasing tree and sapling mortality and understory compositional change that correspond to the duration of infestation. Regionally, black birch, a nitrogen demanding species that is competitively enhanced by increasing nitrogen availability is profiting most from hemlock decline.