The effects of topo-edaphic variables and cultivation on the woody vegetation of Weenen Nature Reserve, KwaZulu-Natal

Abstract

The soil types and woody vegetation of Weenen Nature Reserve (WNR), KwaZulu-Natal, were described. Mispah, overlying shale, and Shortlands, overlying dolerite were the most common of 18 the soil forms encountered. Six vegetation types were identified by TWINSPAN which differed in slope, aspect, land-use history, species richness and tree density. Vegetation types ranged from species-rich, dense Coddia rudis-Dombeya cymosa Closed Woodland overlying dolerite and occurring on steep north-east facing slopes that had never been cultivated, to previously cultivated, species-poor Acacia sieberiana Open Woodland on flat topography, overlying shale. Ordination analyses further described the main associated floristic-environmental gradient from steep slopes supporting a high diversity of broadleaved species through to species-poor, flatter areas with an abundance of microphyllous plants. A second main gradient described was from uncultivated sites on steep slopes of usually dolerite-derived soils through to flat, previously cultivated land on shallow soils overlying shale. Topo-edaphic variables and previous cultivation, which were confounded, were therefore the main determinants of floristic variation in WNR, whereas fire had no influence. Half of the dominant woody species exhibited a reverse-J size structure, indicating relatively constant population change. A number of species had a preponderance of individuals in the smallest (0.51.5m height) size class, owing possibly to recruitment during the exceptional wet season of 1995/6, and/or the inability of individuals to escape the fire-trap. Several, mostly Acacia, species appear to have recruited intermittently. The vegetation changes of this system will have an affect on browse availability. The woody vegetation of WNR appears to be changing in response to successional processes and management influences.


Keywords: carrying capacity; classification; environmental gradient; ordination; savanna


African Journal of Range and Forage Science 2001, 18(2&3): 75-91