Detecting bimodality in plant size distributions and its significance for stand development and competition

Bimodal distributions in plant size, with a major mode of small and minor mode of large-size plants, have been reported for a range of single species stands in different growing conditions. The occurrence of bimodality has implications for the dynamics of competition within a stand and potentially for stand productivity. However, deduction of the existence of bimodality has been by visual assessment of histograms of a single measure of plant size which can lead to differences of opinion about its existence. We show that the bivariate distribution of plant height and weight is more informative about stand structure than univariate distributions. We demonstrate how sub-populations of small- and large-size plants can be identified using a bivariate mixture distribution fitted using evolutionary computation. For the multiple datasets we analyse, a bimodal distribution fits in preference to unimodal or trimodal distributions. Small- and large-plant sub-populations, respectively, form a lower and upper canopy. The numbers of plants in these canopies change during stand development and vary with initial spacing, contrary to reports for other species. Early in stand development, large plants show spatial separation between themselves but spatial association with small plants, and as stands develop the number of large plants declines markedly but they remain spatially separated between each other. Bivariate analysis of height and weight, and spatial analysis of individuals of different sizes, provides a more comprehensive description of stand structure than that obtained in previous studies.