Diverse marsh plant communities are more consistently productive across a range of different environmental conditions through functional complementarity.
Understanding the influence of biodiversity on ecosystem functionality is crucial in modern ecosystem management, especially with regard to the resistance and resilience of ecosystems to future environmental changes. In this study, we assessed the effects of three different environmental regimes on the relationship between diversity and biomass production among marsh plants in comparison with a control treatment to elucidate the underlying classes of proximate mechanisms. We subjected assemblages of up to 23 marsh plant species to four different treatments (control, drought, salt, shade) for 4 months. We examined the treatment effect on the relationship between species diversity and biomass production and explored the underlying mechanism. Biomass production in the manipulated treatments showed a stronger positive effect of biodiversity than the control because of greater declines of biomass production in low diversity mixtures. This effect was owing to an increasingly positive complementarity effect, i.e. a benefit of most species, with increasing diversity, particularly in shade treatment. The selection effect, i.e. a benefit for few species at the expense of the others, was increasingly negative with increasing diversity and dominance by species with lower than average monoculture biomass. The variability of biomass production decreased with increasing species richness in all treatments. Synthesis and applications. We show that the productivity of diverse marsh plant communities is more consistent across a range of environmental conditions than that of depauperate communities and that this unexpectedly resulted from complementarity rather than selection effects. Our results demonstrate that loss of vegetation diversity reduces the average biomass production across a range of environmental conditions and emphasizes the importance of maintaining species-rich biotic assemblages, especially in the face of global change.