Intra-annual precipitation effects on annual grassland productivity and phenology are moderated by community responses

Within ecosystems, intra-annual precipitation patterns-the variability and timing of rainfall-may be a stronger driver of net primary productivity than total annual precipitation. In particular, the amount and timing of precipitation directly affects the amount and timing of plant production, but also indirectly affects productivity via changes to plant community composition. Community response patterns may either buffer or amplify productivity responses to precipitation, as different species respond to different conditions. In a semi-arid California grassland, we experimentally tested how plant communities respond to intra-annual precipitation using rainout shelters in which we manipulated drought amount and timing (early season drought, late season drought, continuous drought and ambient precipitation) over 3 years and assessed plant responses: annual net primary productivity (ANPP), phenological timing of peak production and senescence, and community composition. Overall, early season and consistent drought treatments had lowest productivity, while late season and consistent drought treatments senesced earlier. Plots with functionally diverse communities shifted community composition and had a significant ANPP response to precipitation treatments. In contrast, communities dominated by a single resource-acquisitive grass species did not change in community composition over time and had no ANPP response to precipitation treatments. The timing of production also differed by community, however, where functionally diverse communities remained green longer (particularly under the early season drought treatment) compared to communities dominated by one grass species, which senesced earlier (particularly under the late season drought treatment). Synthesis. Our study demonstrates that drought patterns may indirectly drive ANPP via plant community responses in composition and phenology. This suggests that the combination of species composition and vegetation phenology could jointly alter ecosystem-level sensitivity to precipitation seasonality under future climate change. We show that both functional diversity and dominant stability mechanisms are in operation simultaneously, highlighting the need to understand both the context and variation in community structure to predict ANPP responses to intra-annual precipitation.