Neighborhood analysis of tree growth in mixed Mediterranean forests

Understanding how forests respond to disturbances and, more importantly, ensuring that ecosystem services are preserved is crucial under a climate change scenario. In this perspective, biodiversity is fundamental since it is essential for the stability of ecosystem functions and the services they support. First, some of the underlying ecological processes in the link between Biodiversity and Ecosystem Functions (BEF) must be explored. To achieve this, we have integrated the IDENT experiment with neighbourhood analysis to answer specific questions about BEF relationships. In a first case study, we used the Random Forests method to analyse the relative importance of competition and functional diversity in predicting tree growth. The neighbourhood competition index (NCI) characterised competition, whereas the neighbourhood functional dispersion index (FDis) described functional diversity. Both indices were calculated using species functional traits. The same predictors were studied by the linear regression and model selection in a second case study to assess their effect at the species level under the water stress-gradient. Both studies found that: (1) the neighbourhood competition is mediated by functional traits (asymmetric competition) and not dependent on equivalent competitors (symmetric competition), and (2) NCI is a better predictor than FDis. Specifically, the first study found that hierarchical competition based on functional traits related to competition for light best explains the tree growth variation. The second study, in contrast, found that hierarchical competition based on functional traits related to water transport capacity best explains the growth of the target species. Additionally, the second study found that the neighbourhood competition is stronger when species grow in soil with a high water supply. Overall, our findings highlight the importance of functional traits in the BEF relationship study, as these affect interactions between species and, as a result, forest biomass.