Experimental Evidence Supporting the Seasonal Availability of Water Hypothesis in a Mixed C₃/C₄ Grassland

Understanding how cool-season C-3 and warm-season C-4 grasses will respond to climate change is critical for predicting future ecosystem functioning in many grasslands. With warming, C-4 grasses are expected to increase relative to C-3 grasses, but alterations in the seasonal availability of water may also influence C-3/C-4 dynamics because of their distinct seasons of growth. To better understand how shifts in the seasonal availability of water can affect ecosystem function in a northern mixed-grass prairie in southeastern Wyoming, we reduced early season rainfall (April-June) using rainout shelters and added the amount of excluded precipitation later in the growing season (July-September), effectively shifting spring rainfall to summer rainfall. As expected, this shift in precipitation seasonality altered patterns of soil water availability, leading to a 29% increase in soil respiration and sustained canopy greenness throughout the growing season. Despite these responses, there were no significant differences in C-3 aboveground net primary production (ANPP) between the seasonally shifted treatment and the plots that received ambient precipitation, likely due to the high levels of spring soil moisture present before rainout shelters were deployed that sustained C-3 grass growth. However, in plots with high C-4 grass cover, C-4 ANPP increased significantly in response to increased summer rainfall. Overall, we provide the first experimental evidence that shifts in the seasonality of precipitation, with no change in temperature, will differentially impact C-3 versus C-4 species, altering the dynamics of carbon cycling in this geographically extensive semi-arid grassland.