Potential impacts on stream macroinvertebrates of an influx of woody debris from Eastern hemlock demise

Abstract

Eastern hemlock (Tsuga canadensis) may face ecological extinction in Georgia due to hemlock woolly adelgid (Adelges tsugae) invasion. Hemlock demise is predicted to lead to a large influx of hemlock wood into streams. We sampled invertebrates in Billingsley Creek, a Georgia Appalachian headwater stream, to test the hypothesis that this influx of coarse woody debris (CWD) will shift community composition by diversifying and adding to ecosystem structure. To mimic a mass hemlock die-off, ~600 m3/ha of variously sized dead hemlock wood was introduced to 11 of 21 plots. We sampled macroinvertebrates and environmental conditions at all plots for 1 year before and 1 year after wood introductions. Benthic invertebrates responded positively but marginally to introduced wood in terms of abundance, biomass, and diversity. Results suggested that CWD is a limiting resource for invertebrate abundance and biomass in Billingsley Creek. Plots with introduced wood collected significantly more benthic organic matter (63.2 g/m2) than plots without introduced wood (20.6 g/m2) in December, soon after leaf-fall (P = 0.049), which may help explain this response. Several shredder taxa were indicators of introduced wood. We predict that a future influx of deadwood will alter community composition without posing a threat to its constituents.