Age-related development of crown structure in coastal Douglas-fir trees

Abstract

We compared crown structure among 20-, 40-, and 450-year-old Douglas-fir ( Pseudotsuga menziesii Mirb. (Franco) var. menziesii) trees, and present a conceptual model of crown development. The model is based on the idea that the tree crown can be considered a vertical chronosequence of cohorts of branches that increase in age from upper- to lower-crown. Mean branch volume increased from upper- to lower-crown following the exponential or general logistic growth curve. Maximum branch volume occurred in the lower-crown for 20- and 40-year-old trees, while it occurred in the middle-crown for 450-year-old trees. For the 20- and 40-year-old trees, branch death did not occur in the upper-most part of the crown, and branch density decreased exponentially for the lower one-half and two-thirds of the crown, respectively. For the 450-year-old trees, branch death occurred and branch density decreased exponentially for the full extent of the crown. Epicormic branches increased branch density in the lower-crown, and moderated the rate of decrease in branch density. For the 20- and 40-year-old trees, branch diameter distributions changed from an abundance of small-diameter branches in the upper-crown, to positively skewed bimodal distributions in the middle-crown, and unimodal distributions comprised of surviving large-diameter branches in the lower-crown. For the 450-year-old trees, branch diameter distributions in the upper-crown were unimodal consisting mostly of original branches. In the middle- to lower-crown, branch diameter distributions were bimodal comprised of small-diameter epicormic branches and large-diameter original branches. For the 20- and 40-year-old trees, the relationship between mean branch volume and branch density showed two distinct phases. In the upper-crown where branch death was not observed, and mean branch volume increased with decreasing height while branch density remained relatively constant. In the middle- to lower-crown where branch death occurred, mean branch volume increased while branch density decreased exponentially with decreasing height. For the 450-year-old trees, branch death occurred, and mean branch volume increased while branch density decreased exponentially with decreasing height for the full extent of the crown. The relationship between mean branch volume and branch density after the onset of branch death defined the branch self-pruning line/curve. This relationship reflected sequential changes in the population structure of cohorts of branches growing under increasingly shady conditions as the crown grows taller and new cohorts develop above old ones. As a result of the combined effects of branch growth and death, vertical distribution of branch volume shifted toward the upper-crown with increasing tree age.