Topo-edaphic, floristic and physiognomic gradients of woody plants in a semi-arid African savanna woodland

The variation in topo-edaphic conditions, woodland composition and physiognomic structure of a semi-arid savanna woodland, namely the 59 400 ha Klaserie Nature Reserve, was studied with multivariate techniques. Spatial variation in the texture and chemistry of both the A- and B- horizons was substantial. One gradient of the A-horizon was of increasing clay, silt, organic matter, pH and conductivity while a second gradient was of increasing rockiness, slope and gravel content, associated in part with upper landscape positions. Physiognomic structure of the woodland also displayed substantial spatial variation. Variation in tree density had the greatest effect on total canopy volume, and the density of coppice and dead individuals were closely correlated. Topo-edaphic variation, after the effect of vegetation composition had been accounted for, had a direct effect on woodland structure, as clay content of soils or rockiness influenced shrub density, and catenal position together with silt- or gravel-content of soils influenced coppice density. Ordination analyses identified that the included topo-edaphic variables accounted for a significant amount of floristic variation, but also showed that important environmental variables were omitted. Floristic variation of woody elements was of the nature of a large number of relatively equivalent, independent gradients rather than of a few simple primary gradients. Ten woodland types were identified by TWINSPAN which differed in their topo-edaphic and physiognomic character. Distinctive woodland assemblages were found on rocky outcrops or alluvial areas, on deep, sandy soils or on clay- or silt-rich soils. Colophospermum mopane formed the densest woodlands. Characteristics of the B-horizon were often dissimilar to those of the A-horizon, and seemed important for woodland composition.