Application of Loreau & Hector's (2001) paritioning method to complex functional traits

In 2001, Loreau & Hector proposed a method to calculate the effect of biodiversity on ecosystem-level properties that distinguished selection effects from complementarity effects. The approach was designed, and has been widely used, for the study of yield in mixed-species situations taking into account the relative abundance of each species in ecosystem-level yield. However, complex functional traits commonly used to integrate ecosystem-level properties cannot be analyzed like yield data because the weighted contribution of each species is not determined by its relative abundance. We adapted the original method by clearly identifying ecologically meaningful weighting coefficients to represent species specific contributions to ecosystem function. We applied the adapted method of analysis to tree foliar carbon isotope composition in an experimental plantation in order to test the influence of species richness on plot water use efficiency (WUEplot). The appropriate weights for the WUEplot of each species are leaf CO2 assimilation rate. We observed a large range of WUEplot and biodiversity effects among plots. The absence of a significant selection effect on WUEplot indicated that the overall net biodiversity effect was primarily driven by a complementarity effect. The net biodiversity and complementarity effects were mostly negative, suggesting that interspecific interactions resulted in a decrease in the ratio between carbon acquisition and transpiration at the ecosystem level. The application of the method to complex components of ecosystem functioning provides important new insights for the practical and conceptual aspects of functional biodiversity research.