Soil water content variability drives productivity responses of a model grassland system to extreme rainfall events under elevated CO₂

Climate change is significantly altering rainfall patterns globally and will likely cause increases in extreme rainfall events. Grassland systems are particularly vulnerable to these changes as their productivity is strongly mediated by soil water content (SWC). SWC mean and variability are driven by the amount of rainfall received as well as the distribution through time of that rainfall. In this study, we used a model grassland system in a controlled glasshouse experiment to identify whether SWC mean or variability is a stronger driver of productivity. We then examined how extreme rainfall events alter this driver and the resulting effect this has on productivity and biomass allocation under ambient and elevated carbon dioxide (CO₂). Rainfall amount was held constant, but distribution through time varied (control, one in 20 years event, one in 100 years event). SWC variability was a stronger driver of productivity (mesocosm biomass) than SWC mean, with increasing extreme rainfall event magnitude resulting in greater SWC variability. Surprisingly, elevated CO₂ only had a small effect on these productivity and biomass allocation responses which may be due to the relatively small CO₂ difference tested. Our results suggest that distribution of rainfall in time is an important driver of grassland productivity and that increases in extreme rainfall events, for a given total rainfall, will result in reduced grassland productivity.