The importance of Salvadora australis in relation to tree thinning in preserving herbaceous plants in a semi-arid Colophospermum mopane savanna

Abstract

The southern African Mopane savanna is a water-limited ecosystem and an increase in woody plant abundance often results in the suppression of herbaceous plants. In an attempt to counter this effect, the thinning or even total clearing of all woody plants is a common practise in South Africa. Since the outcome of woody plant removal is determined by both negative and positive responses to tree removal, a study was conducted to establish if all tree woody species have the same impact on the herbaceous layer. The study area featured a dense stand of Colophospermum mopane, interposed with isolated colonies of Salvadora australis, each forming its own characteristic habitat. Seven plots (65 m×180 m) were subjected to different intensities of tree thinning, ranging from a totally cleared plot (0%) to plots thinned to the equivalent of 10%, 20%, 35%, 50% and 75% of the leaf biomass of a control plot (100%) with a tree density of 2711 plants ha −1. The S. australis-habitat, in comparison to the larger C. mopane-habitat, supported a superior cover of herbaceous plants. The S. australis-habitat supported a larger proportion of perennial grasses, acting as valuable seed resources of these and other herbaceous plants. Differences in the soil nutrient status and soil water between the C. mopane- and S. australis-habitats are likely determinants of these differences. A poor ground cover existed at high tree densities of the larger C. mopane-habitat. A rapid colonization of bare ground by herbaceous plants resulted with increasing intensity of tree thinning of the major C. mopane-habitat, but the opposite appeared to happen on the S. australis-habitat. The S. australis trees are considered critical in maintaining conditions favourable for the persistence of the associated herbaceous component. Thinning or removal of S. australis is therefore considered undesirable.