Explosive seed dispersal of the rainforest treeTetraberlinia moreliana(Leguminosae – Caesalpinioideae) in Gabon

Abstract

Explosive or ballistic seed dispersal has been found in many plant species, genera and families (Ridley 1930); in both 'annuals and perennials, herbs and trees, and in open and shaded, wet and dry habitats' (Stamp & Lucas 1983). In the Leguminosae, explosive dehiscence 'is found most frequently in herbs (. . .), but also occurs in high forest trees, from which large seeds lacking dispersal devices are thrown many metres away' (van der Pijl 1982: 155). Little information is available about explosive dispersal by trees, although this dispersal method occurs frequently in specific forest types. For example, 'leguminous trees with explosively dehiscent pods dominate most primary and secondary dry forest and woodland types in the Zambesian Regional Centre of Endemism' (Bingham 1994). Unpublished observations by scientists of the Department of Plant Taxonomy in Wageningen have shown that explosive dispersal is a common feature of caesalpinioid trees in the African rainforests. In this paper, some observations and experiments on the explosive dispersal of the large rainforest tree Tetraberlinia moreliana Aubreville are described. The research was carried out in Rabi, Gabon, at 10 55' S and 90 51' E, and at c. 40 m altitude. Rabi has an average annual rainfall of 2166 mm (1987-1993), and a dry season from June to September. The prevailing wind direction is SSW. The distribution of T. moreliana is confined to west Gabon and a small area in south-west Cameroon (Rietkerk et al. 1995). T. moreliana can attain a height of 51 m. The crown has the shape of a hemisphere, and can emerge above the surrounding canopy. The pods are woody, and only grow on the crown surface, beyond the outermost leaves. The longitudinal axis of a pod usually has a horizontal orientation, like a pennant, but can point in any horizontal direction