Factors affecting subcanopy growth following variable-density thinning in second-growth conifer forests on the Olympic Peninsula in Western Washington

Little is known about which stand and tree factors influence the ability of subcanopy trees to respond to release; however, this information is important to managers interested in accelerating development of late-successional structural characteristics in second-growth forests  We examined basal area growth response five years following variable-density thinning in an effort to determine the effect of thinning and local environment on the release of subcanopy western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata ex. D. Don) at six blocks of the Olympic Habitat Development Study in western Washington.  Release was measured as the difference between average annual basal area growth 5-years prior to and following treatment. Preliminary results indicate that subcanopy trees retained in a uniformly thinned matrix grew significantly better than those in unthinned patches for both western hemlock (N= 169, P = 0.0293) and western redcedar (N=94, P = 0.0395).  Factors related to the ability of western hemlock in the thinned matrix to increase in growth after release include measures of crown fullness (P = 0.0113) and crown crowding (P = 0.0039).  Western redcedar release in the thinned matrix was influenced by initial tree dbh (P = 0.0362), local crowding (e.g., total basal area within 10 m, P = 0.0102) and relative age (P = 0.0030).  Our results indicate that subcanopy western hemlock and western redcedar retain the ability to respond rapidly when overstory competition is reduced and thus suggest that thinning can be an effective tool to accelerate growth of subcanopy trees.