Complementarity as a mechanism of coexistence between functional groups of grasses

1 Increasing functional diversity often leads to an increase in ecosystem productivity in the form of overyielding. While the mechanisms (i.e. complementarity or facilitation) that underlie overyielding provide strong insights into species coexistence and community assembly, they are rarely tested. In subalpine grasslands, traditional management through manuring and hay‐making results in intermediate productivity that is associated with high functional diversity. This functional diversity results from the coexistence between conservative plant species (with slow growth rates, low specific leaf area) and exploitative species (with fast growth rates, high specific leaf area). 2 We hypothesized that overyielding occurs among these two functional groups and tested whether complementarity or facilitation can explain overyielding. Using three perennial grass species per functional group, we compared single and mixed functional group mesocosms at low and intermediate levels of fertilization to test the occurrence of overyielding. Additionally, we measured the outcomes of biotic interactions among these two functional groups by manipulating plant density. 3 After two growing seasons, we found evidence of overyielding under intermediate levels of fertility. Overyielding was associated with a reduction of competition intensity when both functional groups were grown together. These results suggest that complementarity, as evidenced by a decrease in competition intensity, rather than facilitation, explains the observed overyielding. Indeed, we found evidence for complementarity for light and modification of nutrient use as possible mechanisms for the overyielding. 4 Synthesis. Complementarity between functional groups might be an important mechanism enhancing functional diversity, particularly in harsh environments at intermediate rather than low fertility.