The role of rainfall and predators in determining synchrony in reproduction of savanna trees in Serengeti National Park, Tanzania

Abstract

Summary We examined the factors determining synchrony in reproduction in nine Acacia and six other tree species in the Serengeti ecosystem. We test two hypotheses: (i) an abiotic hypothesis where the primary determinant of synchrony is an adaptation to water availability; and (ii) biotic hypotheses where these adaptations to water can be further refined by additional adaptations to avoid predators, or attract seed and fruit dispersers. Flowering and fruiting were recorded monthly for individually marked trees during 1997–2004. Flowering in different species occurs semi‐annually, annually or, in the case of one species, once every 2 years. For most species synchrony of flowering was correlated with seasonal rainfall, with lags related to the mean height of the species; small species flowered during the rains while larger species flowered in the dry season. Fruiting seasons occurred at the end of the rains irrespective of the flowering season. Most species showed flowering synchrony greater than expected from the distribution of rainfall. This may be related to avoidance of insect seed predators through predator satiation. Two Acacias showed multi‐annual fruiting (masting), possibly as a predator avoidance mechanism. Acacia tortilis has two flowering seasons: a dry season flowering with early abortion of pods and a wet season flowering producing successful fruits. Two species of Commiphora appeared to be synchronized so as to attract birds that disperse seeds. Acacia tortilis produced indehiscent pods attractive to ungulates, possibly to kill bruchid beetles during digestion and so increase seed viability. Our results suggest that synchrony in these trees is caused by a strong interaction between abiotic and biotic factors. Closely related species have different reproductive patterns of synchrony that seem to be adapted to different combinations of rainfall, predators and dispersers. Rainfall is the primary determinant but the activities of predators and dispersers increase the degree of synchrony.