Understory Composition of FiveTsuga canadensisAssociated Forest Communities in Great Smoky Mountains National Park

Abstract

Tsuga canadensis (L.) Carrière (eastern hemlock), a long-lived, coniferous tree species native to the southern Appalachian Mountains, is currently experiencing rapid defoliation and mortality resulting from recent invasions of hemlock woolly adelgid (Adelges tsugae Annand, HWA), an exotic, invasive insect. Prior to HWA infestation, T. canadensis often developed unique microclimates and exerted a strong functional influence upon the landscape. Therefore, its loss will likely impact forest structure, composition, and future successional trends throughout the Region. Research has shown that overstory T. canadensis trees are currently experiencing severe decline. However, the early effects of this decline on understory vegetation have received little study. We investigated the impacts of HWA upon T. canadensis-associated understory vegetation in Great Smoky Mountains National Park (GSMNP) by resampling understory vegetation on 29 permanent plots established 5 – 6 years previously across five forest communities in GSMNP: hemlock, montane cove, montane oak/hickory (Quercus spp.(Carya spp.)), acid hardwood, and northern hardwood. We found that understory composition has not yet been drastically altered by overstory decline and mortality caused by HWA, likely due to the heavy dominance of Rhododendron maximum (L.) (rosebay rhododendron) in many forests. This ericaceous shrub forms a buffer between the increased light passing through the declining overstory and the low light environment of the forest floor.