Influence of Colophospermum mopane on surface soil properties and understorey vegetation in a southern African savanna

The influence of Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Leonard on surface soil properties and understorey vegetation was investigated in a semi-arid southern African savanna in Bulawayo, Zimbabwe. Soil samples were collected (depth 0-10 cm) under and outside small, medium and large canopied mopane trees in open woodland on shallow sandy loam soils. The concentration of soil nutrients beneath trees increased with tree size. Soils beneath trees had higher fertility than between tree interspaces. Trees seem to be the major suppliers of nutrients to the understorey vegetation in the crown zone in the form of litter. Soil nutrient concentrations outside canopies of small, medium and large trees were significantly similar, implying that tree size had no significant influence on soil nutrient concentrations in the tree interspaces. Soils were sandier and slightly acidic under canopies of medium and large trees compared to small trees, which had slightly alkaline soils. Soils in the tree interspaces had significantly higher silt and clay content than beneath trees suggesting loss of soil nutrients from surface soils through erosion in the former. Standing herbaceous biomass was significantly lower in the tree interspaces than in the below-crown microhabitats. Species richness and diversity increased significantly with canopy distance from the tree bole. The results suggest that C. mopane has a positive influence on soil fertility in its environment and the fertility improvement beneath trees is not at the expense of soil fertility in the tree interspaces.