Seed bank and understorey species composition in a semi-arid environment:: The effect of shrub age and rainfall

Understorey vegetation in patches of Retama sphaerocarpa shrubs in semi-arid environments is dependent on the overstorey shrub life history. Community structure changes with shrub age as a result of physical amelioration of environmental conditions by the canopy and organic matter accumulation in the soil. We investigated the effect of the canopy on understorey species diversity in the field and its relationships with the soil seed bank under 50 shrubs from 5 to 25+ years old, and compared species composition in the field in a wet and a dry year. Species composition of the soil seed bank under R. sphaerocarpa shrubs did not differ significantly with shrub age, but seed density increased as the shrubs aged. In the field, community composition changed with shrub age, increasing species richness in a process that depended on the amount of spring rainfall. Our results suggest that the soil seed bank is rather uniform and that the shrub canopy strongly selects which species appear in the understorey. There were seeds of many species present under both young and old shrubs but which only established under old shrubs. This showed dispersal was not limiting species abundance and suggested that the canopy was an important sorting factor for species present in the understorey. Less frequent species contributed the most to patch diversity, and rainfall effectively controlled species emergence. Understorey community composition depended on multiple interspecific interactions, such as facilitation by the shrub and competition from neighbours, as well as on dispersal processes. Facilitation in this environment is a key feature in the structuring of plant communities and in governing ecosystem functioning. (C) 2000 Annals of Botany Company.