Vegetation change over fifty years in humid grasslands of KwaZulu-Natal (Acocks's sites)

Abstract

Eighty three of Acocks's sites, originally surveyed about 50 years ago, were resurveyed in 1996 to determine the extent of grassland change in the humid grasslands of KwaZulu-Natal. Sites were relocated using 1:10 000 scale ortho-photos and present land cover was determined for each site. Forty six of the sites that were still under original grassland were further examined to determine present species composition. A survey method was designed that would emulate Acocks's data and comparisons were drawn between original and present species composition. These differences were then analysed together with some environmental variables to try to determine the factors which had the most influence on the change and which environment and management factors are related to the present variation in composition among sites. Of the 83 sites, 26 had changed from natural vegetation to some other form of agriculture such as forestry or cultivation. Most of this change had occurred in the Natal Mistbelt Ngongoni Veld where large areas are forested. Cultivation is found predominantly in the communal areas where subsistence, cultivation practices are employed. The remainder of the sites had changed significantly in terms of their species composition. The most pronounced change had occurred in areas under communal tenure although significant changes had occurred in the commercially farmed areas. The direction of change was also more consistent towards species that commonly predominate in heavily grazed areas in the communal areas compared to the commercial areas. The exact reasons for this were unclear but this could possibly be attributed to heavier stocking rates in the communal areas. Change in floristic composition was also more pronounced at lower altitudes where the mean annual rainfall is lower and the mean annual temperature higher. This could possibly be a result of the vegetation at lower altitudes being less stable and thus less resistant to change. Basal cover differed significantly between commercial and communally grazed sites. Lower basal cover was found in the communal sites where intensive grazing limits the growth of individual tufts. Number of species found at each site did not differ significantly between communal and commercially grazed sites. This study was also a practical implementation of the resurveying of Acocks's sites and the original data set was found to be a useful baseline data set to determine coarse long-term changes in the vegetation.