Environmental and biogeographic influences on the distribution and composition of the southern Cape forests (Veld type 4)

Doctoral Thesis


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University of Cape Town

This study aims at explaining the distribution and composition of the southern Cape forests, the largest forest complex in southern Africa. These are the only forests in southern Africa which are actively and scientifically managed for their products and values. Population growth due to forestry, agricultural and economic development and a growing tourism industry exerts increasing pressures on the natural environment of the southern Cape coast and therefore affect the dynamics and conservation of the forests. Conservation and sustained utilization of the forests require a sound knowledge of the composition, structure and dynamics of the forests. This study was aimed at an understanding of the biogeography of the forests at the landscape level in order to isolate those variables which contributed to the present distribution and composition of the forests. Determinants of the forest location pattern in the southern Cape were identified as rainfall above 500 mm, which determines the potential limits of the forests, and the bergwind fire pattern, which determines the actual forest distribution. Fires driven by the hot, dry, northwesterly, föhnlike bergwinds interacted with the terrain physiography since prehistorical times and the forests persisted in topographic shadow areas. The largest forests in the area therefore occur on the coastal platform at the foot of the mountains, in the river valleys and on the coastal scarp. Forests in the mountains, with high rainfall, are small and scattered. The results have shown that the bergwind driven fires control the distribution of forests which have important implications for the understanding of forest dynamics and for conservation management of forests in multiple-use management systems. Forest composition at the landscape level was studied by means of plant species lists. A species list for the southern Cape forests was annotated with information on the growth form, breeding system, propagule type, forest type, moisture tolerance, abundance and spread in the study area, and the distribution range in southern Africa, of each species. Analyses of the list showed that the species/family ratios for the southern Cape forest flora are very low, that woody plants have mostly fleshy propagules and herbaceous plants mostly dry propagules, and that several species have adaptations to adverse conditions. The species richness and composition, and floristic similarity and relationships were compared between the southern Cape forest flora and the floras of 13 other forests representing particular geographic regions in southern Africa. Forest size explained relatively little of the variation in species richness of the forests. Stepwise multiple regression analyses indicated that the number of dispersal corridors, the proximity to other forests and mean altitude explained most of the variation in number of woody species, whereas the number of landscape types and dispersal corridors explained most of the variation in number of herbaceous species. The high similarity between the southern Cape forest flora and those of the forests along the escarpment from the eastern Cape to northern Transvaal, and the southern attenuation of species suggest that the forests were once continuous. It is suggested that the Sundays river valley east of Port Elizabeth isolated the southern Cape forests from those to the east already during the Pliocene or earlier.