Analysis of radial growth responses to changes in stand density for four tree species

Enhancement of radial-growth by reducing stand density is a common silvicultural practice. Traditional methods of analysis based on stand level variables, such as stand mean diameters, overestimate the effects of reduced density on radial-growth due to ʹtechnical growthʹ, i.e. increases in the mean diameter caused by the thinning operation itself through removal of small trees. A new method for analyzing radial-growth responses in forest stands based on samples of the largest trees within populations is proposed, and the hypotheses tested that: (1) reducing stand density enhances radial-growth; (2) radial-growth response varies with site and changes with the release period, i.e. the period of time after thinning; (3) radial-growth response varies among tree species. Based on studies of Picea abies (L.) Karst. it is suggested that water supply influences radial-growth responses significantly. Tree species differ greatly in their ability to enhance radial-growth when stand density is reduced. For Pinus sylvestris L. and Acer pseudoplatanus L. the responses are very limited. P. abies (L.) Karst. showed the greatest response to thinning while Fagus sylvatica L. showed an intermediate response. These differences may be ascribed to differences in strengths of intraspecific competition, strategies of photosynthate allocation or inherent growth inertia. The method of analysis applied in this study can be used to quantify radial-growth responses of tree species relative to the length of release periods and site.