Impact of the hemlock woolly adelgid on understory plant communities in Tsuga canadensis forests

Background/Question/Methods Tsuga canadensis (eastern hemlock) is a shade tolerant species commonly associated with multiple forest types in eastern U.S. forests. The species often develops unique microclimates beneath its canopy that can exert a strong functional influence on understory species composition.  Throughout its native range, however, T. canadensis is experiencing rapid defoliation and mortality due to infestations of an exotic, invasive insect known as hemlock woolly adelgid (Adelges tsugae, HWA). HWA has decimated hemlock stands throughout New England and the mid-Atlantic States since the late 1980s, however, its spread into the southern Appalachian Mountains is relatively recent. The objective of this study was to analyze understory plant communities of forest types associated with T. canadensis and determine if these communities have changed following the arrival of HWA, and subsequent T. canadensis mortality. We compared understory species on permanent vegetative plots sampled in 2003 and 2008/9 in five forest types throughout Great Smoky Mountains National Park where hemlock is commonly found: hemlock, montane cove, montane oak-hickory, acid hardwood, and northern hardwood.
Results/Conclusions Preliminary results indicate that understory species composition significantly differed among forest types (P = 0.004). Species richness varied among forest types and was greatest in the hemlock forest types and lowest in the acid hardwood forests. Despite evidence of T. canadensis mortality on most plots, we did not observe a significant change in understory species composition following HWA infestation. This may be attributed to the abundance of Rhododendron maximum in the midstory that is restricting increased light from reaching the forest floor despite the formation small overstory gaps following the loss of T. canadensis.