The effect of neighbourhood competition and species richness on individual-tree growth under heterogeneous environmental conditions

Background/Question/Methods

Biodiversity and ecosystem functioning (BEF) research yields important findings such as a positive diversity effect on resource capture and biomass production. Field experiments manipulating species richness or functional diversity contribute to a central part of these findings. However, most of these experiments make use of small-scale simplified model systems and, therefore, the minimum diversity levels required for ecosystem functioning may have been underestimated. The objective of our study is to analyse how local neighbourhood interactions and local site condition affect initial growth response of individual trees in a large-scale forest BEF experiment incorporating a broad diversity gradient and great environmental heterogeneity. The experiment was established in the species-rich subtropical region of China (BEF China). Annual  height, stem diameter and crown area growth were used to characterize the performance of 6,723 tree individuals across 25 species on 231 experimental plots. Local neighbourhood conditions were captured in terms of a competition index and the Shannon index. Topographical parameters included aspect, slope, curvature and elevation. Nitrogen content, carbon/nitrogen ratio and pH value were used to characterize soil conditions.

Results/Conclusions

Our results showed that individual tree growth rates were markedly species specific and significantly impacted by the local site conditions. Height growth was not influenced by local species diversity, whereas stem diameter and crown area growth were both positively related to local species diversity. However, effect sizes of local species diversity on growth rates were much smaller than those of the local neighbourhood competition index. The positive effect of local neighbourhood diversity on stem diameter and crown area growth of the tree saplings was probably due to the realization of niche differentiation and resource complementarities, attributable to highly heterogeneous resource and topographical conditions as well as a great local species diversity gradient. We conclude that small-scale environmental heterogeneity and a diverse local neighbourhood promotes the individual-tree growth and thus productivity.