TOPO-EDAPHIC, FLORISTIC AND PHYSIOGNOMIC GRADIENTS OF WOODY-PLANTS INA SEMIARID AFRICAN SAVANNA WOODLAND

The variation in topo-edaphic conditions, woodland composition and phy siognomic 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 increas ing clay, silt, organic matter, pH and conductivity while a second gra dient was of increasing rockiness, slope and gravel content, associate d in part with upper landscape positions. Physiognomic structure of th e woodland also displayed substantial spatial variation. Variation in tree density had the greatest effect on total canopy volume, and the d ensity of coppice and dead individuals were closely correlated. Topo-e daphic variation, after the effect of vegetation composition had been accounted for, had a direct effect on woodland structure, as clay cont ent of soils or rockiness influenced shrub density, and catenal positi on together with silt- or gravel-content of soils influenced coppice d ensity. Ordination analyses identified that the included topo-edaphic variables accounted for a significant amount of floristic variation, b ut also showed that important environmental variables were omitted. Fl oristic variation of woody elements was of the nature of a large numbe r of relatively equivalent, independent gradients rather than of a few simple primary gradients. Ten woodland types were identified by TWINS PAN which differed in their topo-edaphic and physiognomic character. D istinctive woodland assemblages were found on rocky outcrops or alluvi al areas, on deep, sandy soils or on clay- or silt-rich soils. Colopho spermum mopane formed the densest woodlands. Characteristics of the B- horizon were often dissimilar to those of the A-horizon, and seemed im portant for woodland composition.