Integrating silviculture with hemlock woolly adelgid mitigation: crown dynamics of Tsuga canadensis (L.) Carrière 10 years after thinning

Pre-emptive silvicultural treatments can enhance host-tree vigor as a component of an integrated management approach to mitigate invasive species’ impacts. Hemlock woolly adelgid (HWA, Adelges tsugae Annand, 1928) threatens the health and existence of eastern hemlock-dominated forests. We propose that increasing canopy light exposure via silvicultural thinning can reduce hemlock’s vulnerability to HWA. Three stands with high hemlock densities on the Allegheny National Forest were selected for thinning. There was no HWA infestation and half of each stand was thinned. Circular plots were established surrounding 96 hemlock “subject trees” targeted for crown release on 3–4 sides. Similar criteria were used to select 90 hemlocks in untreated plots. We visited subject trees pre-thinning and 5 and 10 years post thinning and measured the following: diameter, total height, live crown ratio, and crown area. Temporal changes in diameter were not significant between treatments, but trees in thinned plots averaged 5 cm larger after 10 years. Live crown ratio was maintained in thinned stands and decreased after year 5 in controls. Crown area increased in thinned plots and decreased in controls. Residual basal area significantly influenced growth but varied between treatments and among stands.