Forest structure in selected South African forests: edaphoclimatic environment, phase and disturbance

Analysing data from 903 permanent sample plots situated in medium-moist and moist forests in the southern Cape, South Africa, we explored factors controlling forest structure. Pronounced subcanopy stem density persistence (well-stocked subcanopy forest matrix) and stem density packing (comparatively high stem densities of relatively large-sized trees) were found in the moist, less seasonal (quasi-tropical) Tsitsikamma forests. These attributes of structure were linked to the prevailing dystrophic, less seasonal conditions and the associated metabolic vertical growth orientation. The cool, moist and seasonal (quasi-temperate) Knysna forests had lower densities of relatively large-sized trees at the canopy level (stem density intolerance). This was attributed to the lateral growth mode and extended persistence of the trees involved. The warm, seasonal (quasi-subtropical) Outeniqua forests, on relatively nutrient-rich soils, had high stem densities at the canopy level relative to the subcanopy stratum; due to a combination of low subcanopy tree persistence, fast ingrowth of trees into the canopy stratum, which were then lost to mortality before they reached large sizes (high canopy tree turnover). Persistence of the multi-species subcanopy forest matrix supported asynchronous establishment and death of individual trees. Typical for tropical-type forests, the development of trees towards maturity (phase) was associated with a spatially fine-grained disturbance regime. A metabolic performance trade-off model was developed and provided an ecophysiological framework for the interpretation of forest structure and its underlying dynamics. This explanatory model indicated causal links between intraspecific metabolic tactics of trees in response to their edaphoclimatic environment and associated attributes of forest structure. Some implications of the findings for tropical forest management are discussed.