Vegetation patterns in the Kalahari affected by Acacia erioloba: the importance of the regeneration niche

In arid ecosystems with a discontinuous shrub or tree layer, distinct groups of plant species are associated with the area under woody perennials and open interspaces respectively. The processes behind such patterns are not completely understood. According to the species pool concept which species can be found in a habitat is in the first place determined by the ability of species to disperse to that habitat, but abiotic and biotic factors form a habitat specific environmental sieve that determines which of the arriving species can establish and maintain a population there. For plants the regeneration niche is expected to be of major importance for filtering species from the species pool. Being largely immobile plants can track habitat mainly by seed dispersal and germination cueing. Because canopy and matrix contrast strongly in abiotic conditions plants are expected to have adapted to either one of them due to various trade-offs. As the process of germination basically forms the only possibility for a plant to �choose� precisely under which conditions it will grow, germination behaviour is expected to have evolved to direct germination and establishment to favoured habitat. The aim of this study was to determine the relative importance of various regenerative traits (seed dispersal, persistence and germination) on the origin and maintenance of vegetation patterns affected by Acacia erioloba in the Kalahari. The seed rain of the two most common fleshy-fruited canopy species (Lycium bosciifolium, Solanum capense) was found to be concentrated under acacias and practically absent in the open matrix. Seed traps under artificial perches received higher seed rain than seed traps under trees which suggests that the availability of perches for birds is a main reason for concentration of the seed rain under trees. No significant differences between the number of seeds collected from the soil and from traps excluding predators were observed for matrix and canopy. Except for S. capense where under sapling acacias the number of seeds collected from the soil was less than from traps, suggesting more predation under saplings than under mature trees. No differences between all available canopy and matrix species were found in persistence based on depth distribution of seeds in soil. In an ordination canopy and matrix plots separated clearly for vegetation, but not for seed bank data. Several matrix species were more common under trees in the seed bank. Matrix species common in the seed bank are anemochorous or small-seeded while canopy species common in the seed bank are zoochorous with larger seeds. Matrix species can thus disperse more widely to both microhabitats, but environmental filtering restricts growth to the matrix. Germination responses to abiotic factors that vary between microhabitats were studied in the lab. Degree of association with acacias did not affect the ability to germinate under water stress. Annuals associated with acacias germinated more slowly than annuals found in the matrix. Germination of all fleshy fruited perennial Acacia-associated species and two annual Acacia-associated species was inhibited by temperatures resembling un-shaded conditions compared with temperatures resembling shaded conditions. A sod transplant experiment showed that the total number of seedlings was significantly reduced in canopy sods transplanted to the matrix compared with a control. There was no difference in seedling number between matrix sods transplanted to the canopy and a control. Germination behaviour provides a likely explanation: germination of canopy species is prevented in the matrix because soil dries out too fast for slow germinating seeds to germinate and by inhibition by matrix temperatures. However, the only matrix species common enough to allow statistical testing showed a negative response to transplanting to the canopy. Thus dormancy breaking or germination of matrix species may also be inhibited by canopy conditions. In an additional experiment emergence of sown seeds of canopy species L. bosciifolium and Pollichia campestris was only observed under canopies. This study supports the hypothesis that dispersal and filtering by abiotic factors at the germination and early seedling stage is of major importance for the maintenance of vegetation patterns affected by Acacia erioloba. The transplant and sowing experiment suggest that germination behaviour provides a mechanism for maintenance of the observed vegetation pattern that overrules dispersal patterns. Dispersal can be seen as an ultimate cause for the observed patterns by directing the evolution of germination behaviour, which provides a present day mechanism for the maintenance of vegetation pattern.