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.
Pinus patula is the most intensively planted conifer in the tropics and sub-tropics. The increased proportion of corewood that results when rotation ages of pine plantations are shortened has become a wood quality factor of growing concern worldwide. The purpose of this study was to develop empirically based models for predicting the flexural properties of the wood produced from relatively young Pinus patula trees. Models were based on the properties of standing trees and their effectiveness was evaluated at board, tree and compartment levels. Sample material was obtained 1 from 170 Pinus patula trees, 16-20 years old, established in 17 compartments on the Mpumalanga escarpment of South Africa. Multiple regression models were developed which managed to explain 68%, 60% and 95% of the variation in the dynamic modulus of elasticity (MOE) on individual boards, trees and compartments levels respectively. At compartment level, 80% of the variation in the 5 th percentile MOR value could be explained by the model. Sensitivity analyses showed that site index at base age of 10 years, acoustic time-of-flight, wood density and ring width were influential variables in the MOE models. The models indicated that tree slenderness during early growth seems to play a major role in determining the dynamic MOE and MOR of lumber. This is in agreement with Euler's buckling theory and the bending stress theory. The results from this study indicated that the MOE dyn and MOR of lumber can be accurately predicted on especially a compartment level. The predictive models developed can be used as management tools to improve operational decisions around tree breeding, silvicultural practices and rotation ages.
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