Modeling crown structural responses to competing vegetation control, thinning, fertilization, and Swiss needle cast in coastal Douglas-fir of the Pacific Northwest, USA

Crown structure is a key variable influencing stand productivity, but its reported response to various stand factors has varied. This can be partially attributed to lack of a unified study on crown response to intensive management or stand health. In this analysis of several coastal Douglas-fir (Pseudotsuga menziesii var. menziesii [Mirb.] Franco) branch datasets, a significant treatment effect of fertilization, thinning, pre-commercial thinning, varying levels of vegetation control, and intensity of a foliar disease (Swiss needle cast, caused by Phaeocryptopus gaeumannii (T. Rohde) Petr.) were all found to influence several key crown structural attributes. Maximum branch size and total and non-foliated crown radii were found to be the most dynamic and sensitive crown variables to the various stand factors as no treatment effects were found for the number of branches within an annual segment or branch angle. When the data sets were combined and used to develop a single predictive equation, treatment effects were largely accounted for by changes in bole and crown size as mean bias was relatively low despite the large range in tree ages examined (4–450 years at breast height). While crown structure is highly variable and sensitive to a variety of stand factors, general empirical equations perform quite well and should be better integrated into models of forest growth and yield.