Tsuga canadensis (L.) Carr. (eastern hemlock), a long lived and shade tolerant coniferous tree species native to eastern North America, is currently threatened by the hemlock woolly adelgid (Adelges tsugae Annand, HWA). This exotic, invasive insect poses a serious threat to T. canadensis stands throughout their native range. The loss of this unique tree species is often coupled with numerous ecological consequences. HWA-induced mortality has exceeded 95% in some forest types, and will likely continue to decimate T. canadensis populations in the coming years. Shortly after HWA was found in Great Smoky Mountains National Park (GSMNP), long-term monitoring plots were established across a gradient of T. canadensis associated forest communities to monitor HWA impacts and stand decline. Our objectives were to (1) determine how T. canadensis populations have changed in conjunction with HWA infestation, and (2) document how HWA has affected vegetative community dynamics following five to six years of infestation in GSMNP. We assessed T. canadensis associated overstory communities and determined that crown density, top dieback, and T. canadensis stem density has significantly declined over the observational period. Furthermore, we found that midstory T. canadensis trees were experiencing greater rates of decline than those of the overstory. Even with these losses, overstory and midstory community composition did not significantly change from 2003 to 2008/09. Many studies have documented rapid pulses of understory vegetative productivity in conjunction with T. canadensis defoliation, which often allows increased amounts of solar radiation to reach the forest floor. Although we observed significant T. canadensis mortality at most plots sampled, understory species composition did not significantly change following HWA infestation. We attribute this lack of species response to the overwhelming abundance of Rhododendron maximum in the mid/understory, which restricts increased light from reaching the forest floor despite the formation of small overstory gaps. However, we found that understory composition significantly varied among ecological groupings, and differences were largely associated with soil properties and elevation. Overall, results of this project suggest that T. canadensis may largely disappear from southern Appalachian forests resulting from infestations of HWA. Although community composition of T. canadensis associated stands has not yet been altered, changes are likely to occur as duration of infestation progresses.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal