From canopies to streams: Consequences of hemlock woolly adelgid-induced forest changes for benthic invertebrates

Hemlock woolly adelgid (Adelges tsugae) is an invasive insect causing widespread decline of eastern hemlock (Tsuga canadensis) in the central Appalachians.  Hemlock is a foundation species and is prominent in forested riparian areas of this region.  Here, hemlock’s canopy provides thick shading and acidic leaf litter to streams, altering conditions for aquatic organisms.  Following the decline of hemlock, several broadleaved plants are expected to participate in its replacement including sweet birch, American beech, and rhododendron.  These species differ from hemlock in patterns of canopy cover, leaf abscission, and leaf physicochemical properties – all of which are known to impact benthic invertebrate communities.  The goal of this study is to examine temporal patterns of benthic invertebrate communities colonizing hemlock, birch, beech, and rhododendron litter in streams with a riparian canopy dominated by hemlocks or deciduous trees.  In fall, litter from hemlock, birch, beech, or rhododendron was sealed in mesh bags and placed in streams to monitor invertebrate colonization.  In the following winter, spring, summer, and fall, bags were collected, litter was weighed, and invertebrates were counted and identified.  Litter from beech and rhododendron broke down more slowly than hemlock and birch.  Overall, the abundance and diversity of invertebrates colonizing litter from beech and rhododendron was greater than that of hemlock and birch (P<0.05).  Additionally, there were differences in the composition of invertebrate feeding groups colonizing litter.  These results demonstrate how benthic invertebrate communities may react to changing riparian canopies and litter resources of streams – a consequence of hemlock woolly adelgid-induced forest changes.