A classification and analysis of crown form in green ash (Fraxinus pennsylvanica)

Abstract

Forty-five boulevard green ash (Fraxinus pennsylvanica var. subintegerrima (Vahl) Fern.), both male and female trees, were selected and numerous morphometric measurements were obtained. Multivariate and univariate techniques applied to the data suggested three crown shape classes based on the pattern of crown diameter changes. The crown shape classes were identified as broad and rounded (group C) to tall and conical (group A), with an intermediate category (group B). Although tree age was not significantly different, group C trees tended to be larger with larger crown areas and basal main-stem diameters compared with groups A and B, which were not different. A visual rating analysis of the woody frame revealed that group A trees tended to have an excurrent form, whereas group C trees were predominately decurrent. When comparing sexes, it was found that male trees were larger, with larger crown areas than females, and they had a greater ratio of crown area to basal stem diameter. The crowns of male trees were generally branchier, with more foliage per unit crown volume than females. It appears that, in terms of growth and form, male trees are probably better suited for use as boulevard trees than female trees. Differences among groups and between sexes are discussed in relation to factors like apical control and reproductive sinks.