Potential impacts of eastern hemlock (Tsuga canadensis) demise on aquatic macroinvertebrates in a Georgia Appalachian stream

Abstract

Tsuga canadensis (eastern hemlock) may face ecological extinction in Georgia due to the invasion of Adelges tsugae (hemlock woolly adelgid). We were concerned that hemlock demise would impact stream invertebrates in three ways: tree death, leaching into streams of imidacloprid pesticide to control adelgids, and a future influx of hemlock wood into streams. We hypothesized that only the latter would have a major impact. To test hypotheses, we sampled riffle habitats in a Georgia headwater stream above, in, and below a stand of hemlocks kept alive with insecticide. To mimic a mass hemlock die-off, ~0.06 m3/m2 of variously-sized dead hemlock wood—10 times the ambient supply of dead wood in the stream—was added to 11 of 21 plots. We sampled macroinvertebrates and environmental conditions at all plots for one year before and one year after wood additions. We found that several invertebrates were associated with the center stream reach, presumably because of the presence of living hemlock trees. We found no evidence that past insecticide treatments had harmed invertebrates. Benthic invertebrates responded most strongly to additions of dead hemlock wood. We predict that the major impact of hemlock demise on stream ecosystems will be the future wood influx.