Bifurcation Ratios and the Adaptive Geometry of Trees

Abstract

In several recent studies the branching system of trees was assumed to constitute a geometric series with a constant branching ratio indicative of the adaptive geometry of a species. However, in a population of young cottonwood trees (P. deltoides), branching ratios varied significantly within the crown of individual trees and among experimental groups of different growth vigor. With a simple mathematical model, based on the symmetric branch system of the tropical tree Tabebuia rosea, with increasing size of a tree, the geometric (exponential) increase in branch number per order must be reduced, if a physiologically optimum leaf area per terminal branch is to be maintained. Also, branching ratios based on centripetal branch ordering are not predictive of tree development and ignore information essential to characterize the architectural model of a tree. Bifurcation ratios thus appear to be unsuited as indicators of the adaptive geometry of trees.