Litter source determines macroinvertebrate colonization in Appalachian headwater streams

The impending loss of eastern hemlock (Tsuga canadensis) due to hemlock woolly adelgid (Adelges tsugae) induced mortality has profound implications for future forest communities.  The adelgid is a non-native, invasive herbivore, and eastern hemlock is highly susceptible. Eastern hemlock is considered a foundation species and is the only conifer to occupy forested riparian zones of central Appalachia; its eventual replacement by broadleaf species will impact resources for benthic macroinvertebrates of headwater streams.  Conifer litter generally decomposes less rapidly than broadleaf litter, providing a more stable substrate for colonization by aquatic macroinvertebrates. However, its value as a nutritional resource is poor relative to deciduous litter.  In this study, aquatic macroinvertebrate colonization of leaf litter from eastern hemlock and three co-occurring riparian species, including sweet birch (Betula lenta), American beech (Fagus grandiflora), and great laurel (Rhodendenron maximum) was assessed using mesh litter bags (N=72 per species). In December 2011 litter bags were placed in six headwater streams of two overstory vegetation types (hemlock-dominated and deciduous-dominated), and macroinvertebrate colonization was monitored. Bags were collected sequentially, and here we present data from two collections, representing macroinvertebrate colonization over the winter (December through March) and spring (March through May).  Litter bag contents were sorted, and macroinvertebrates were counted and identified to family.  During the winter months there were no differences in macroinvertebrate colonization based on litter type or based on riparian vegetation.  During the spring macroinvertebrates colonized beech litter more readily (F_{15, 121}=2.39; P<0.05) than the remaining species, but there were no differences based on overstory vegetation.  Additional collections in the summer will provide a more complete documentation of temporal and spatial macroinvertebrate colonization of primary litter sources in forested headwater streams of central Appalachia, and provide insight into the long term effects of adelgid-induced loss of eastern hemlock on benthic macroinvertebrate communities.