Silvicultural canopy gaps improve health and growth of eastern hemlocks infested with Adelges tsugae in the southern Appalachian Mountains

•Sustainability of eastern hemlock is threatened by the invasive insect Adelges tsugae.•Canopy gaps improved crown health and radial growth of infested understory hemlocks.•Tree competition index and pest density were predictive of crown decline post-release.•Results suggest silvicultural release can complement biological control of A. tsugae. The hemlock woolly adelgid (HWA, Adelges tsugae Annand) is an invasive forest insect threatening the sustainability of eastern hemlock (Tsuga canadensis (L.) Carrière) in eastern North America. In this study, cutting small gaps in the forest canopy above understory eastern hemlocks was evaluated as a potential tool for use in the integrated pest management of HWA. Two gap sizes and two gap creation methods (felling vs. girdling trees) were assessed for effects on HWA density, hemlock crown health, hemlock growth, and regeneration of competing tree species, at three different latitude groups in the southern Appalachian Mountains. In all three latitude groups, the treatment that removed the most canopy competition (Large Fell) increased the production of new hemlock shoots, reduced the percentage of dead shoots, and increased basal area increment of target hemlocks relative to unreleased trees. Smaller gaps and those that removed fewer competing live trees (Girdle treatments) also improved hemlock health in the southernmost latitude group. Positive effects of canopy gaps on hemlock crown condition and growth occurred despite similar or higher HWA densities on gap-released vs. unreleased trees. In regression analysis, more than half the variability in crown transparency change and basal area growth post-treatment was explained by the competition index of live trees remaining, average HWA density index, and tree size (dominant tree height and hemlock dbh). In the southernmost latitude group, the regeneration density of deciduous species increased significantly in the three years following gap creation and were highest in the Large Fell treatment. Results support the hypothesis that the increased availability of sunlight (and possibly other limited resources) resulting from canopy gaps (radius approximately ¼ to ½ dominant tree height) improves the physiological tolerance of eastern hemlock to HWA infestation and could complement biological control by providing a more stable source of new shoots with adelgid prey for predatory insects.