Using the spatial grain of regeneration to select harvestable tree species in subtropical forest

Abstract

Abstract We describe a graphic model for selecting tree species suitable for harvesting of pole-sized stems. Pole size stems are the most harvested size class by rural users of African forest. Increasing harvesting pressure on this size class has created an urgent need for tree species selection and harvest off-take strategies that differ from large-scale and canopy tree-logging processes. Methods for selecting species suitable for harvesting are constrained in their complexity by the need to use available data that are typically from single survey, size-class frequency distributions (SCDs) of population structure. Using SCD data sets from three subtropical Afromontane forests in South Africa, we use the spatial scale (grain) of regeneration of the harvestable fraction of pole-sized stems to identify species that can be harvested. In fine-grained species the sub-canopy individuals are well represented beneath the canopy of mature conspecifics and thus display high levels of natural regeneration over a small spatial scale. Fine-grained species are suitable for harvesting because there is a large supply of pole-sized trees whose exploitation has potentially less effect on forest structure and composition than the use of coarse-grained species. Furthermore, because the abundant fine-grained species may experience strong neighbourhood effects in time, and their stem densities are likely to be regulated by density-dependent factors, judicious exploitation of these species is arguably an extension of the natural process of density-dependent self-thinning. In contrast, coarse-grained species regenerate over large areas and at low densities and their use requires careful management and should probably be restricted. Unfortunately, most species being targeted for harvesting are coarse- to medium-grained and appear to regenerate at levels insufficient for replacement. Using the spatial grain of regeneration to select harvestable species does not provide estimates of sustainable harvesting rates. These critical harvesting limits (stems ha−1 per annum) are set from experience. Incorporated into the management process, this model provides a simple means of ensuring that levels of natural regeneration are maintained without the need for intervention to maintain either tree species diversity, or potential economic value for pole production, in heavily used subtropical forests.