Tree species and environment associations within hemlock-silverbell stands treated for hemlock woolly adelgid in Great Smoky Mountains National Park1

Abstract The hemlock-silverbell (Tsuga canadensis-Halesia tetraptera) association is endemic to several small stands in federally protected lands in the southern Appalachian Mountains and is currently threatened by the hemlock woolly adelgid (Adelges tsugae). Pre-infestation stand structure was still evident at the time of the study but hemlock mortality and associated vegetation responses portend rapid changes in this system. In order to guide potential future restoration efforts and impact assessments, a study was implemented to determine the structure of hemlock-silverbell stands and to identify environmental variables associated with differences in stand composition in areas of Great Smoky Mountains National Park treated to control hemlock woolly adelgid. Halesia tetraptera and T. canadensis exhibited even-aged and uneven-aged stand structures, respectively. Forest composition varied as a function of environmental variables within the hemlock-silverbell association with overstory H. tetraptera relative density positively correlated with higher water availability (Beers aspect and slope) and overstory T. canadensis was negatively correlated with Beers aspect. Relative density of H. tetraptera and T. canadensis in the understory were similarly associated with higher and lower productivity conditions, respectively. In the understory, Acer pensylvanicum relative density was negatively associated with that of understory T. canadensis. The results suggest that partial canopy removal resulting from the loss of T. canadensis due to hemlock woolly adelgid may benefit aggressive tree species such as A. pensylvanicum. The United States Park Service vegetation management policies, here and in other natural areas where non-native invasive species are controlled (but native species responses are unaccounted for), will need to address this phenomenon if T. canadensis restoration is to be accomplished in the event adelgid control can be achieved.Abstract The hemlock-silverbell (Tsuga canadensis-Halesia tetraptera) association is endemic to several small stands in federally protected lands in the southern Appalachian Mountains and is currently threatened by the hemlock woolly adelgid (Adelges tsugae). Pre-infestation stand structure was still evident at the time of the study but hemlock mortality and associated vegetation responses portend rapid changes in this system. In order to guide potential future restoration efforts and impact assessments, a study was implemented to determine the structure of hemlock-silverbell stands and to identify environmental variables associated with differences in stand composition in areas of Great Smoky Mountains National Park treated to control hemlock woolly adelgid. Halesia tetraptera and T. canadensis exhibited even-aged and uneven-aged stand structures, respectively. Forest composition varied as a function of environmental variables within the hemlock-silverbell association with overstory H. tetraptera relative density positively correlated with higher water availability (Beers aspect and slope) and overstory T. canadensis was negatively correlated with Beers aspect. Relative density of H. tetraptera and T. canadensis in the understory were similarly associated with higher and lower productivity conditions, respectively. In the understory, Acer pensylvanicum relative density was negatively associated with that of understory T. canadensis. The results suggest that partial canopy removal resulting from the loss of T. canadensis due to hemlock woolly adelgid may benefit aggressive tree species such as A. pensylvanicum. The United States Park Service vegetation management policies, here and in other natural areas where non-native invasive species are controlled (but native species responses are unaccounted for), will need to address this phenomenon if T. canadensis restoration is to be accomplished in the event adelgid control can be achieved.