Bergwind fires and the location pattern of forest patches in the southern cape landscape, South-Africa

Abstract

A hypothesis is developed that environmental factors (rainfall and substrate) determine the potential limits of forest distribution, but that actual forest location pattern is determined by the fire pattern, which in turn is determined by the interaction between prevailing winds during dry periods and terrain physiography. The warm-temperate, mixed, evergreen forests of the study area occur in few large but isolated patches on the coastal platform and river valleys, but in many small patches in the dissected mountains. Late arrival of europeans in the area due to many deep and steep gorges through the coastal platform, and the early control over timber cutting and forest clearing prevented man from influencing the location pattern of the forests. Rainfall throughout the study area is high (900-1200 mm per annum, and expected to be higher in the mountains). The strike of geological formations cuts across forest patches. The forests persist on both deep and very shallow, rocky soils and the soils are similar both inside and outside the forest. The study has shown that forests on the coastal platform persisted in topographic shadow areas of the gusty, hot, desiccating north-westerly fohnlike berg winds which are common during autumn and winter. Bergwind direction is locally changed due to barriers posed by the position and forms of the mountain ridge to the windward (northern) side of the forests, and is channelled through valleys running from the mountains. Fires associated with the berg winds would bum with higher frequency in zones in the landscape where forest is currently absent. The wind-fire pattern would furthermore cause calm conditions and a lower frequency of fire in localities where the forests have survived. A graphic model is presented to indicate the likelihood of forest persistence in topographic positions in relation to berg wind direction. The study also related under-storey differences and the presence of seed of the legume tree Virgilia divaricata Adamson, and of charcoal in the litter layer of Witelsbos forest to such a berg wind fire which occurred an estimated 230 years ago. Forest can therefore recover from episodic; extreme fires, but disappears from areas where fires occur at high frequencies. The results have implications for the interpretation of species-diversity patterns in the landscape in relation to disturbance and recovery, for the application of prescribed bums in catchments management, for the development of fire protection plans for commercial forestry, and for understanding the spread and control of invasive alien plants.