Temporal variability in water supply controls seedling diversity in limestone pavement microcosms

1 Environmental fluctuations can hypothetically promote greater species diversity, but this has rarely been experimentally tested in plant communities. We predicted that high temporal variability in soil moisture would promote high diversity in a community of recruiting seedlings. Using a glasshouse experiment designed to imitate field conditions in species‐rich limestone pavement habitats, we examined the effects of temporal variability and total quantity of water provided on seedling species richness, evenness and composition within a single growing season. 2 Temporal variability was altered by varying the frequency at which water was added while keeping the total quantity of water constant. Low temporal variability was created by high frequencies of water addition. High variability was associated with more extreme fluctuations between droughted and flooded conditions. Each of three variability treatments was carried out using three total quantities of water added to create a gradient in total soil water availability. 3 Species richness was greatest in higher quantity and lower variability treatments. Higher total quantities of water added buffered the negative effects of higher temporal variability, and lower temporal variability buffered the negative effects of lower quantity. Species richness was negatively correlated with temporal variability in soil moisture content both in the glasshouse and in a field study. Evenness was negatively correlated with richness. Low richness, high evenness communities had low densities and were prone to extinction. Thus, neither richness nor evenness was maintained by higher temporal variabilities within a growing season. 4 Lower species richness under more temporally variable conditions was attributable to both random extinctions due to reductions in total community density and species‐specific differences in mortality. Final species relative abundances differed between water treatments, but there were no separate effects of variability in soil moisture content or total quantity of water added. Upland species were more abundant in treatments where drought was more prevalent, either due to low total quantities of water or high temporal variability in soil moisture. Spatial heterogeneity in moisture supply, or interannual fluctuations between dry and wet years, may thus contribute to coexistence of drought‐tolerant and flood‐tolerant species in the field.