Potential impacts of Tsuga canadensis (L.) Carr. demise on macroinvertebrates in a Georgia Appalachian stream

Background/Question/Methods

Tsuga canadensis (L.) Carriere (eastern hemlock) is in decline and may face ecological extinction in Georgia due to the invasion of Adelges tsugae Annand (hemlock woolly adelgid, HWA).  It is predicted that mass hemlock deaths will result in an excess of coarse woody debris (CWD) entering Appalachian streams.  We predict that this future change in CWD volume in streams will have important consequences to stream invertebrates: a significantly richer community of invertebrates with greater biomass is expected at sites with wood additions.  To test this hypothesis, we have initiated a CWD introduction study in 21 riffle habitats of Billingsley Creek, a third-order stream in the Chattooga River watershed of Rabun Co., Georgia.  To imitate a mass hemlock die-off, ~0.006 m³ of variously-sized dead hemlock wood—roughly 10 times the ambient woody debris in the stream per m²—was added to 11 sites.  A t-sampler and tethered pieces of wood substrate were used to sample macroinvertebrates at all sites for a year before and after wood additions. Community abundance and biomass is being compared on the family and genus levels using ANOSIM. 

Results/Conclusions

Current data suggests an impact of wood additions that appears seasonally in sediment samples but not on wood piece samples and on individual taxa that were identified as indicators.  Potential impacts of pesticide to control HWA and of loss of canopy cover were also investigated and impacts were not significant.