Growth releases of three shade-tolerant species following canopy gap formation in old-growth forests

Abstract

Questions: Do the number, duration and magnitude of growth releases following formation of natural, fine-scale canopy gaps differ among shade-tolerant Thuja plicata, Tsuga heterophylla and Abies amabilis? What is the relative importance of tree-level and gap-level variables in predicting the magnitude and duration of releases? What does this tell us about mechanisms of tree species coexistence in such old-growth forests? Location: Coastal British Columbia, Canada. Methods: We estimated the timing of formation of 20 gaps using dendroecological techniques and extracted increment cores from all three species growing around or within gaps. Using a species- and ecosystem-specific release-detection method, we determined the number of trees experiencing a release following gap formation. We quantified the duration and magnitude of individual releases and estimated the influence of tree-level and gap-level variables on these release attributes. Results: Eighty-seven per cent (304 of 348) of all trees experienced a release following gap formation. T. heterophylla and A. amabilis experienced higher magnitude and longer duration releases than T. plicata. The effect of diameter on the duration of releases varied among species, with T. heterophylla and A. amabilis experiencing decreasing, and T. plicata experiencing increasing, duration of releases with increasing diameter. The effect of growth rate prior to a release on the magnitude of releases varied among trees of different diameters, with the slowest growing and smallest individuals of all species experiencing the most intensive releases. Conclusions: Our results provide detailed information on the number, duration and magnitude of growth releases of the above three species following gap formation. Differences in response to canopy gaps suggest differences in how these species ascend to the canopy strata. T. plicata may be less dependent on gaps to reach the canopy. Differing strategies for ascending to the canopy strata may be important in facilitating coexistence of these three species in old-growth forests of coastal British Columbia.