Maintenance of Eastern hemlock forests: Factors associated with hemlock vulnerability to hemlock woolly adelgid

Eastern hemlock (Tsuga canadensis [L.]) is the most shade-tolerant and long-lived tree species in eastern North America. The hemlock woolly adelgid (Adelges tsugae) (HWA), is a nonnative invasive insect that feeds on eastern hemlock and Carolina hemlock (Tsuga caroliniana Engelm.). HWA currently is established in 17 eastern states and is causing tree decline and wide-ranging tree mortality. Our data from West Virginia and Pennsylvania suggest that hemlock crown vigor (a ranking of amount of live crown) relates to a predictable pattern of hemlock vulnerability at light and moderate levels of HWA infestation. We found that crown variables, such as live crown ratio and crown density and transparency, are accurate predictors of hemlock decline; more vigorous trees appear to be less vulnerable to HWA. Thus, silvicultural thinning treatments may be a means for reducing stand densities and increasing crown vigor in colder areas where climate may slow HWA spread.