Reducing rainfall amount has a greater negative effect on the productivity of grassland plant species than reducing rainfall frequency

The productivity of semiarid Australian grassland ecosystems is currently limited by water availability and may be impacted further by predicted changes in rainfall regimes associated with climate change. In this study, we established a rainfall manipulation experiment to determine the effects of reduced frequency (RF; 8 days between water events) and reduced magnitude (RM; 50% reduction in amount) of rainfall events on the physiology and above-and below-ground growth of five grassland plant species with differing traits. Native C4 grasses exhibited the highest productivity in well watered, control (Cont) conditions, as well as in RF and RM treatments. The RF treatment generally had little effect on total biomass, rooting distributions or photosynthesis, suggesting species were relatively tolerant of reduction in the frequency of rainfall events. However, the RM treatment had a negative effect on total biomass and physiology, and generally resulted in a shift towards shallower rooting profiles. Overall, the reduction in biomass was greater in RM than RF, suggesting that rainfall magnitude may be a more important determinant of grassland productivity and composition than the frequency of rainfall events under future climates.