Aspects of the seed ecology of Acacia karroo in the Eastern Cape, South Africa

Abstract

Seed ecology may play a role in the widespread increase of Acacia karroo in savanna and grassland. Accordingly, fecundity, predation, mortality by fire, dispersal by livestock, and seed longevity were studied. Seed production per tree (P of pod production, pods per tree) was positively related to tree height, negatively related to tree density, and was greater for sites receiving run-on. Predispersal predation by bruchids, cerambycids, and wasps depleted seed production by 18%. The ant Messor capensis effectively eliminated surface seed except during the early dry season following seed shed. Rodent predation of seed was minimal. Fires >100 °C killed most exposed seed but a thin layer of soil increased survival and maintained germinability. A greater proportion of ingested seeds passed through cattle (57%) and sheep (50%) than through goats (15%). 50% of seeds were recovered within 48 h. During passage, soft seeds were digested, some hard seeds were softened, but most were egested as hard seeds; germinability was not increased. Cattle were confirmed as dispersal agents. Using experimental seed banks, it was shown that buried or surface-exposed seeds did not persist for longer than a year, but 24% of surface-shaded seeds persisted for two years. Most seeds germinated but most buried seeds died before emergence. Seed persistence in deep shade should facilitate invasion into grassland. Seed banks of A. karroo did not exceed 18 seeds m^-2 because of the above-described effects. Seed availability (fecundity, dispersal by cattle, seed longevity) is concluded to contribute to the invasiveness of A. karroo, but use of fire may offer partial control of seed availability.