Height and crown allometry of native tree species across a climatic gradient in the Soutpansberg region, South Africa

Ecosystem services provided by forests worldwide strongly depend on individual stem diameter, height and crown allometric relations at single tree level. However, most allometric studies have been established in temperate forests of the northern hemisphere and only few research has been conducted in the diverse forest and woodland systems of southern Africa. In this study we investigated how height and crown allometry of native trees are driven by wood density, local climate, competition and the different species native to biomes occurring along a climatic gradient in the Soutpansberg region. Allometric scaling coefficients for crown length and crown diameter mostly followed Metabolic Scaling Theory, whereas tree height behaved more like predicted by Constant Stress Similarity and height to crown base showed even smaller coefficients. We found only minor and variable impact of wood density on tree height and crown metrics. Incorporating species as a model parameter revealed better model predictions compared to climate and competition. Nevertheless, when tested across all species, enhanced climatic water availability and competition led to significantly taller trees, with longer crowns and higher crown bases, but smaller crown diameters. Within the same species, climatic water availability had a positive effect on all variables including crown diameter, indicating that apart from turnover of species composition, also intra-specific phenotypic plasticity plays a role in driving height and crown allometry. In light of a changing climate and related species shifts in southern Africa the negative effect of reduced water availability suggests reduced tree heights and crown sizes in the future. The provided allometric equations could be of use for biomass related inventories and local forest management in the Vhembe Biosphere Reserve.