Seed ecology as a determinant of population structure in some Southern African Savanna Acacia species

[摘要] ENGLISH ABSTRACT:African Acacia species are often major contributors to the progressive increase in the woodycomponent of savannas, a phenomenon commonly referred to as bush encroachment. In Hluhluwe-Umfolozi Park, the numbers of adult Acacia nilotica (L.) Willd. Ex Del. trees per hectare farexceed (by III) that of A. karroo Hayne adults. The relative dominance is reversed in the juvenilestage with A. karroo (725 ha') outnumbering A. nilotica (225 ha-I) threefold outside closedwoodlands. African acacias produce large quantities of seed and may have large soil-stored seedbanks. They suffer pre-dispersal predation by bruchid beetles and may be either wind or animaldispersed. Once dispersed they are vulnerable to post-dispersal attack.This study tested several hypotheses regarding various aspects of seed ecology of A. karrooand A. nilotica. The null hypothesis that seed ecology does not contribute to the success of A.karroo over A. nilotica, was tested.Acacia karroo trees were smaller (mean basal diameter: 7.8 cm) than A. ni/otica trees(mean basal diameter: 18.5 cm) on average, but produced more seeds (A. karroo mean: 1628; A.nilotica mean: 992) for a given basal diameter size class. It was found that A. karroo showed lessbruchid infestation (mean: 1.36-3.81%) than A. nilotica (mean: 14.67-86.70%) at all stages of poddevelopment with a proportion of A. karroo seeds (7.1 %) being able to germinate after bruchidattack. Bruchid attack rendered A. ni/otica seeds unviable. There was no difference between thetwo species with regards to the soil-stored seed bank and the viability of seeds found in the soil.Acacia karroo showed higher germination levels (5.1%) and better establishment (4.9%) than A.nilotica (1.5% and 0.4% respectively). On average, there was no difference in germination levelsbetween burnt and unbumt seeds, but there was a significant difference in germination of burntseeds in both burnt (4.5%) and unbumt (2.5%) sites and unbumt seeds in both burnt (2.8%) andunbumt (4.9%) sites when considered separately.Post-dispersal predation of A. karroo seeds (21.8%) was higher than that of A. nilotica(12.7%). There was more rodent predation in tall grass areas (26.0%) than short grass (10.7%) orcanopy areas (15.2%), and most seeds were lost from unprotected control groups. Rodent presencewas a significant factor in unexplained seed disappearance.The ability of A. karroo to germinate easily and the low levels of beetle predationexperienced by this species seemed to be its main advantage over A. nilotica as an encroachingspecies in Hluhluwe-Umfolozi Park.

[发布机构] Stellenbosch University