Development and evaluation of individual tree- and stand-level approaches for predicting spruce-fir response to commercial thinning in Maine, USA

Commercial thinning is a widely used silvicultural method in the spruce-fir (Picea-Abies) forests of the northeastern USA, but limited work has been done on predicting both short- and long-term individual tree- and stand-level responses to this treatment. Based on annual inventories of permanent plots from a replicated thinning experiment in Maine, USA, stand- and individual tree-level treatment response functions for spruce-fir stands were developed and evaluated. The developed thinning modifiers were shown to significantly improve predictions of annual stand-level basal area growth and mortality as well as species-specific, individual tree-level annual diameter increment, height to crown base increment, and mortality. For these response variables, duration and magnitude of the response functions were significantly influenced by thinning intensity and to a lesser extent by thinning method. In contrast, modifiers for stand-level dominant height increment as well as tree-level height increment did not show significant improvement when compared to the baseline model. When the developed modifiers were included in stand- and individual-tree growth models, a significant improvement in prediction over the baseline models was achieved, but the individual-tree approach was superior to predicting long-term response to various alternative thinning treatments. Overall, the study highlights the advantage of including thinning modifiers in growth and yield models, even for relatively shade-tolerant species, and the effectiveness of an individual-tree approach in structurally diverse and mixed-species stands.