Relationship Between Crown Architecture and Branch Orientation in Rain Forest Trees

Abstract

Measurements of the dimensions and angles of branches on saplings of 58 species in rain forests of NE Australia and Central America showed that architectural groups defined in terms of developmental rules differ in branch shape and angle. Species were classified into four groups based on visual assessments of leaf arrangement and trunk form. The three plagiotropic groups (with planar leaf arrangements) had more nearly horizontal branches than the orthotropic group (with more three dimensional leaf arrangements). The plagiotropic groups each had a significantly lower ratio of branch depth to width than the orthotropic group, with branch width defined as the horizontal breadth of the branch foliage and depth defined as the thickness of the foliage perpendicular to the branch axis and the width measurement. The three plagiotropic groups differed in the angle of the branch tip relative to the overall branch. However, individual saplings of the different groups showed some overlap in branch characteristics. Branch angle and depth to width ratio were correlated and were related to light level, the latter assessed for three Tasmanian rain forest species. Both plagiotropic and orthotropic species had more erect and three dimensional branches in open environments than in shade. Thus, the architectural groups differ in branch geometry in common environments, but also exhibit plasticity in response to light.