Growing tall vs growing wide:: tree architecture and allometry of Acacia karroo in forest, savanna, and arid environments

In order to investigate how environmental factors other than light availability affect tree architecture, differences in branching architecture and allometry were analysed in populations of Acacia karroo Hein. from three different environments in South Africa: forests, savannas and arid-shrublands. Factors such as fire and herbivory have a large effect on tree life history in certain environments and are likely to have selected for trees that have different architectures from those of forest trees, whose major limitation is light assimilation. Significant differences were found in stem architecture and branching architecture between trees in each environment. Compared with forest trees, trees in savannas had an elongated growth form with small canopy and leaf areas, and tall, thin, unbranched trunks. Trees in and areas showed opposite trends with wider canopies, and increased lateral branching. Savanna trees had significantly smaller spines than trees in other environments, and both forest and savanna trees showed delayed reproduction. These differences are probably related to a trade-off between an architecture geared towards rapid height-gain and one promoting lateral spread, and can be explained with reference to the different selective pressures in each environment. In forests, vertical and horizontal growth are both important. However, in savannas there is a great pressure for rapid vertical growth to escape fires, while in and areas a defensive, lateral growth form is selected for. Savanna trees and and karoo trees have evolved architectures that are more extreme vertically and laterally than the range of architectures displayed in a forest community.