Diversity and production in an Afromontane Forest

This contribution evaluates the effect of forest structure and tree species diversity on plot productivity and individual tree growth in the unique Knysna forests in Southern Africa using mapped tree data from an observational study that has been re-measured over a period of 40 years.

The effects of tree species diversity and forest structure on tree growth and forest production are evaluated on three levels of resolution: a) the forest community (canopy,sub-canopy species),b) the subplots (number of trees per ha,skewness of the diameter distribution,diameter coefficient of variation) and c) the immediate neighborhood of selected reference trees ("Mingling","Dominance",Aggregation" and "Size Variation").

An analysis of the community level identified two distinct clusters,one including dominant/canopy species with the highest growth rates and a greater variation of growth,and another cluster which includes the remaining subcanopy species which have a smaller maximum size and lower rates of growth. The area-based structure variables on plot level have a highly significant effect on total basal area growth. However,the effects of forest density and species richness on productivity were not straight forward. Maximum basal area production of about 0.75 m²/ha/year is achieved at medium levels of richness (around 20 species per ha) and medium levels of density (around 30 m²/ha basal area) using percentile regression estimates. The relative "Dominance" of a selected reference tree had a highly significant effect on individual tree growth on all investigated species. Other neighbourhood structure variables were only occasionally significant or not significant at all.

This contribution presents a new theoretical framework for analysing natural forests that includes community,plot and neighborhood variables of forest structure and diversity,and a first specific analysis of the structure and dynamics of the Knysna Afromontane Forest,based on a unique set of longterm observations. The species-area (SAR) model developed in this study,represents a new general approach that can be used to derive a common standard of tree species diversity for different plot sizes,the species richness per hectare.