Functional trait diversity is a stronger predictor of multifunctionality than dominance: Evidence from an Afromontane forest in South Africa

Studies on how biodiversity influences ecosystem multifunctionality (EMF) help elucidate ecological mechanisms (e.g. niche complementarity and selection) underlying provision of multiple ecosystem services. While it is acknowledged that biodiversity contributes to EMF, the relative importance of functional traits diversity (niche complementarity) and dominance (selection effects) for EMF needs further investigation. To address this gap, we analysed how tree species diversity influences EMF, using data on species functional traits (specific wood density, specific leaf area and maximum plant height) and four ecosystem functions (carbon storage, habitat quality, forage provision and rockfall protection) in an Afromontane forest in South Africa. We tested the hypotheses that (i) trait diversity rather than dominance would link species richness to EMF; and (ii) diversity rather than species richness effects would increase with the level of EMF. For all possible scenarios of EMF indices, functional trait diversity metrics, especially functional divergence and functional richness correlated positively with EMF. On the other hand, functional dominance also influenced EMF, but played limited role in mediating EMF response to species richness, when compared with functional diversity. Results further revealed that total diversity effects, not species richness effect, generally increased with the level of EMF. In summary, we show that species richness does not fully capture the functional contribution of different species. Compared to dominance, trait diversity had significant advantage in explaining biodiversity-EMF relationship, stressing a greater role of niche complementarity as mechanism underpinning delivery of multiple functions. We argue that functional dominance reflects more the competitive dominance of traits and species within a given community and therefore is more likely to have greater effects on single functions than on multifunctionality.