A salinity gradient mediates plant diversity-function-stability relationships across a coastal marsh landscape

Ecosystem function and stability are often positively related to species richness in experiments, but in nature, abiotic gradients may over-ride or obscure the effects of richness on function and stability. In addition, community stability may be driven by distinct mechanisms along diversity or abiotic gradients and exhibit different responses to differing degrees of variability in an abiotic condition. We explored relationships between salinity, plant diversity, ecosystem function (annual biomass production), and stability of biomass over 25 years for seven coastal marsh plant communities arranged across an estuarine salinity gradient in Georgia, USA. In contrast to expectations, neither plant biomass nor its stability was related to species richness. Plant richness was positively correlated to within-year high tide salinity range and peaked at intermediate values of mean salinity. Biomass stability was negatively correlated with mean salinity but was unrelated to salinity range. Stability was driven by dominant species at sites with low richness, but asynchrony likely contributes to stability at intermediate salinity sites with higher diversity. Our results illustrate the importance of studying variability in abiotic conditions in addition to their mean and demonstrate that stability and richness may not necessarily be correlated in nature due to underlying abiotic gradients affecting both.