Comparative morphology and allometry of winged diaspores among the Asian Sterculiaceae

Abstract

The morphology and allometry of the winged diaspores of 25 species in the Asian Sterculiaceae were studied. The diaspores of pioneer trees are characterized by light mass and large wings. Although their parent tree heights are low in most cases, these light diaspores are easily blown upward and widely dispersed. The diaspores of Heritiera, a genus mainly found in primary forest, can be divided into groups with large and small wings by their values of the square-root of wing loading. The frequency distribution of mature tree heights for trees with the large- and small-winged diaspores are skewed into high and low statures, respectively. Therefore, the tree size at maturity may play a vital role in determining the morphology of the diaspores. Among the large-winged diaspores, shape is almost retained, despite size change. Consequently, larger diaspores have more mass per unit of wing area, suggesting that larger diaspores fall more quickly than smaller ones. Diaspore mass is approximately proportional to the nut volume. The enlargement of the nut clearly appears to be beneficial for seedling establishment and survival under a closed canopy. However, increasing nut volume reduces the potential for wind dispersal. The fact that large-winged diaspores were always lighter than their small-winged counterparts suggests that the heavier diaspores with small wings are not dispersed, preferentially by wind. Consequently, the present study supports the hypothesis made by Suzuki & Ashton (1996): that the evolution of small wings appears to have arisen mainly due to a reduction in tree height and an increase in nut size.