Functional diversity, functional composition and functional β diversity drive aboveground biomass across different bioclimatic rangelands

Abstract

There has been a growing interest in understanding how biodiversity affects the functioning of ecosystems, while, it is still unclear how and to what extent different aspects of biodiversity are related to ecosystems function. We tested the relationship between aboveground biomass (AGB) and various aspects of functional diversity (functional richness (FRic), functional evenness (FEve) and functional divergence (FDiv)), functional β diversity and functional composition. Our study areas were located in four different bioclimatic regions including desert-steppe, steppe, semi-steppe, and Mediterranean rangelands extending along a productivity gradient created by the elevational-rainfall variations in the central Zagros Mountains of Iran.The results showed that FDiv positively predicted AGB in more productive (i.e., Mediterranean) and negatively in less productive rangelands of desert-steppe, steppe and semi-steppe. Functional β diversity predicted AGB only in more productive regions in semi-steppe and Mediterranean rangelands. Functional β diversity decomposed into turnover and nestedness, with turnover being more important in more productive and nestedness being more important in less productive ecosystems. The results also showed that the effects of FD and functional β diversity on AGB are generally directed by animal grazing through homogenizing community-wide trait composition in less productive ecosystems and inducing shifting mosaics across locations in more productive ecosystem.The results indicate that different components of FD were significantly related to AGB and the strength and direction of these effects vary with the different components studied and the bioclimatic regions under the study. In higher productive ecosystems, both functional diversity and functional β diversity are likely to contribute more to AGB through the complementary use of the resources. In less productive ecosystems, however, the functional identity of dominant species and traits would drive AGB.