A Decade of Rainfall Manipulation Reveals Growing-Season Aridity as a Key Determinant of Grassland Productivity and...

Warming-driven intensification of the hydrological cycle is altering global rainfall patterns. However, the relative importance of changes in the amount versus timing of rainfall and the role of atmospheric drivers of moisture demand in modifying relationships between rainfall, biodiversity and ecosystem functioning are currently unresolved. To address this, we undertook a 10-year rainfall manipulation experiment in a mesic grassland in New South Wales, Australia. We used rain shelters to achieve five rainfall treatments: (i) ambient, (ii) ambient +50% (IA), (iii) ambient -50% (RA), (iv) reduced frequency (RF, cumulative ambient rainfall applied once every 3 weeks) and (v) summer drought (SD, no rain during the Austral summer). We found that inter-annual variation in ANPP was best explained by the amount of growing season rainfall relative to potential evapotranspiration (i.e., P/PET, or aridity) (R² _adj 0.52). Reductions in the amount of rainfall, particularly during summer, were associated with productivity decline, shifts in community composition and a loss of diversity. However, reducing the frequency of rain events had no overall effect on productivity, despite a loss of species diversity. Notably, treatment-related declines in diversity and/or richness were associated with both increases (IA) and decreases (SD) in temporal stability of ANPP and the stabilising role of species asynchrony, thereby highlighting the importance of species identity and associated functional traits for community stability. Our study uniquely emphasises the importance of accounting for seasonal drivers of moisture demand when predicting functional responses to changes in rainfall regimes and highlights how the ecological mechanisms underpinning community stability are influenced by changes in both the amount and timing of rainfall. These mechanistic insights can enhance the predictive capacity of Earth system models and inform targeted management strategies to offset the negative effects of future, more extreme rainfall on the ecosystem services provided by global grasslands.