Long term grassland productivity responses to changes in the amount and timing of rainfall are mediated via shifts in community stability

Rainfall is a major determinant of aboveground net primary productivity (ANPP) in grasslands, but warming-driven intensification of the hydrological cycle is changing the amount, timing and seasonality of rainfall, including more extreme events. The relative importance of these changes for biodiversity over decadal scales, and the plant community mechanisms underlying associated productivity responses, are currently unresolved. To address these knowledge gaps, we undertook a 10-year rainfall manipulation experiment in a mesic grassland in New South Wales, Australia. We used rain shelters to achieve five contrasting rainfall treatments: (i) ambient, (ii) ambient +50%, (iii) ambient -50%, (iv) reduced frequency (cumulative ambient rainfall applied once every 3 weeks) and (v) summer drought (no rain during the Austral summer).Inter-annual differences in ANPP were best explained by the amount of rainfall relative to atmospheric moisture demand (i.e. precipitation/potential evapotranspiration (P/PET), or aridity), with growing season aridity being a stronger predictor of ANPP (R2adj 0.52) than either annual rainfall (R2adj 0.26) or annual P/PET (R2adj 0.44). Changes in the size, frequency and seasonality of rainfall events were associated with shifts in community composition, loss of diversity and a reduction in the proportion of native species. Reduced stability of ANPP in response to summer drought was mediated via reduced species asynchrony – reflecting a large decline in species richness - and a decline in dominant species stability. At the other end of the scale, greater ANPP and overall stability of productivity under increased rainfall was mediated via increased asynchrony and stability of dominant grasses, despite a reduction in species diversity. Our study uniquely emphasises the importance of accounting for seasonal drivers of moisture demand when predicting productivity responses to changes in rainfall regimes and highlights the critical importance of species asynchrony and dominant species’ stability for maintaining productivity in the face of climate change.