The influence of tree thinning on the establishment of herbaceous plants in a semi-arid savanna of southern Africa

Abstract

The investigation was conducted on an area covered by a dense stand of Colophospermum mopane. Seven plots (65 m × 180 m) were subjected to different intensities of tree thinning, ranging from a totaly 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 2 711 plants ha−1. Thinning was completed during 1989 and a study on the establishment of herbaceous plants in response to reduced competition from the woody plants was conducted during the 1989/1990, 1990/1991 and 1991/1992 seasons. A poor ground cover, mainly characterized by bare ground and the dwarf grass Oropetium capensis, existed in the 50%, 75% and 100% plots. This state proved to be relatively independent of rainfall, typical of an induced or pseudo-drought condition. It can be concluded that the tree density of the 50% plot (mean: 3 620 ETTE ha−1, where 1 ETTE = mean leaf volume of a 1.5 m single-stemmed tree = 500 cm3 leaf volume) had already exceeded the critical point of detrimental grass-tree competition. Colonization of bare ground by herbaceous plants increased from increased levels of tree thinning. Annual grasses were the main colonizers of bare ground, while perennial grasses on(v constituted a small proportion of the grass species composition. The successional order of establishing grasses following thinning was Tragus berteronianus, Brachiaria deflexa and Aristida species, terminating in a state where Enneapogon cenchroides predominated. Perennial grasses showed a general lack of successional trends.