The contribution of belowground processes to tree diversity effects on ecosystem functioning

Many studies found evidence for a positive relationship between biodiversity and ecosystem functioning in a large variety of systems. However, there is still debate about the mechanisms that drive these relationships. Insight in ecological linkages between above- and belowground components is critical to understand the functioning of terrestrial ecosystems. Recent observational and experimental research showed positive tree diversity effects on productivity and it has also been shown that tree diversity can improve the resistance of forests to pests. But whether the effects are driven by aboveground or belowground processes and through which mechanisms is largely unknown, especially at larger spatial scales. Moreover, it is known that the magnitude of effects and causal pathways might change along environmental gradients according to the ‘stress gradient hypothesis’. Changing climatic, biotic, and edaphic conditions and associated stress intensity at the continental scale, thus may distinctly affect biodiversity-functioning relationships and the driving mechanisms across forests. Furthermore, there is still much controversy regarding the potential role of diversity in reducing the vulnerability of forests to a changing climate. Here we explored the relative importance of above- and belowground mechanisms driving tree diversity effects on ecosystem functioning in European forests. The study was conducted across a network of permanent forest plots in Europe, established through the FunDivEUROPE project. The network was designed to test the effects of diversity on ecosystem functioning in mature European forests, spanning much of the continent’s bioclimatic gradient. To test whether the effect of tree diversity on ecosystem functioning is driven by above- or belowground processes, we modelled causal pathways using structural equation modelling. We explored the linkages for a set of ecosystem functions, for which diversity effects have been observed.status: publishe