VARIATIONS IN SEED SIZE AND SHAPE IN RELATION TO DEPTH OF BURIAL IN THE SOIL AND PRE-DISPERSAL PREDATION IN ACACIA-NILOTICA, A-TORTILIS ANDDICHROSTACHYS CINEREA

Little is known about the persistence of seeds in the soil of spinesce nt trees/shrubs in the savannas of South Africa. Thompson at al. (1993 ) show that seed size and shape can be used to predict long-term persi stence (> 5 years) in the soil at the species level, with persistence related to ease of burial. Furthermore, pre-dispersal seed predators h ave been shown to be influenced by seed size/shape at the species leve l. We determined seed mass, seed shape ratio and the variance in seed dimensions (based on axiometric dimensions) in relation to depth of bu rial for three common spinescent savanna trees/shrubs, Acacia nilotica , A. tortilis and Dichrostachys cinerea. These species are hard-seeded and form at least short-term persistent (1-5 years) soil seed banks. There was a consistent tendency for seed size within species to increa se with depth of burial, which is interpreted as showing that larger s eeds within species may have greater longevities in the soil than smal ler seeds. In most cases seeds tended to greater sphericity (using bot h measures) with depth, except for A. nilotica which showed no differe nce. Current season's seeds predated by endophagous bruchid beetles te nded to be smaller (in terms of volume), and for A, tortilis, more asp herical. The size/shape of seeds, which directly affects ease of buria l, appears to be a useful barometer in determining persistence in the soil, but it is largely through burial that persistence is confered.