Branch growth and crown form in old coastal Douglas-fir

Abstract

Theoretical and empirical methods were applied to the analysis of branch growth and crown form in old trees of coastal Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii). Conventional use of allometric models proved insufficient in explaining the variability found in the relationship between branch diameter and length. In order to infer the potential pattern of branch growth, a maximum relationship was empirically estimated by selecting maximum points in an incremental fashion. This relationship asymptoted to infinity, suggesting that factors such as damage and die-back may modify the branch diameter–length relationship before an asymptotic maximum length is reached. The difference between the observed branch length and the estimated maximum branch length was found to be a good measure of the degree of branch damage. The degree of damage increased with increasing branch diameter, suggesting that large diameter branches have experienced more extensive damage and/or die-back. This trend was stronger in dead branches than in live branches. A conceptual model of branch growth is proposed where damage and die-back modify the branch diameter–length relationship, severe damage and die-back lead to branch death, and recovery helps to restore the relationship in live branches. The vertical distribution of branch diameters within the crown was analyzed to characterize crown form. An allometric model describing the maximum relationship between branch height and branch diameter was fit to each tree. Two types of distribution patterns were identified: the conical and rounded. The frequency distributions of branch diameters at three crown levels (upper, mid and lower crown) were also used to characterize crown form. Three crown types were characterized: the mature crown, old crown, and stressed crown. These crown types corresponded well with the vertical distribution patterns of maximum branch diameters. The two criteria are suggested as a useful tool for characterizing crown form in old coastal Douglas-fir for purposes of forest management.