Impacts of neighborhood species richness on individual-tree sizes evolve through time under heterogeneous environment in BEF-China main experiment

Background/Question/Methods

A positive relationship between species richness and plant biomass production has frequently been found in biodiversity and ecosystem functioning (BEF) research. Niche complementarity, facilitation and sampling effects are the main mechanisms that have been proposed to drive this relationship. However, experimental analyses and rigorous testing of the contribution of these mechanisms in diverse forest ecosystems is extremely rare. In forests tree-tree interactions take place on a local scale, and thus, the local neighborhood strongly determines the vitality and productivity of individual trees. The main objective of our study is to analyze how local neighborhood (total size and species richness) affects the growth of individual trees in the early stage of a large-scale forest BEF experiment with heterogeneous site conditions and a large species pool (BEF China). The experiment was established in the species-rich subtropical region of China in 2009. The effects of the size of neighboring trees and neighborhood species richness on total height and stem diameter of about 6700 individual trees of 24 species over four years (2010-2013) were tested using mixed effects model (ASReml) after taking account of potential confounding factors such as plot, topography, and number of neighbors.

Results/Conclusions

Our results showed that species identity of focal trees strongly determined individual-tree sizes. Until the fifth growing season, the size of neighboring trees did not influence tree height. In contrast, stem diameter was significantly affected by neighborhood size after the fourth growing season, but not before. The impact of local neighborhood species richness on individual tree height became marginal during the fourth growing season and significant during the fifth growing season and was generally negative. Stem diameter was not yet affected by neighborhood species richness. However, the prediction of models showed that the response of focal tree size to local species richness is species-specific (i.e., positive, neutral or negative relationship depending on species identity of focal trees). Significant tree-tree interactions seem to occur after the fourth growing season. Light intercept is priority at this early stage because height of focal trees did not vary with increasing neighbor sizes. Productivity of certain tree species can benefit from higher diversity but only after tree-tree interaction established. This indicates that for certain tree species, higher neighborhood species richness already leads to positive net tree-tree interactions at this early stage. We conclude that small-scale environmental heterogeneity and a diverse local neighborhood promotes the individual-tree size and thus tree productivity of certain species.