A test of the seasonal availability of water hypothesis in a C3/C4 mixed grassland

Abstract Understanding how cool-season C3 and warm-season C4 grasses will respond to climate change is critical for predicting future grassland functioning. With warming, C4 grasses are expected to increase relative to C3 grasses. But, alterations in the seasonal availability of water may also influence C3/C4 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 during the latter half of the growing season (July-September), effectively shifting spring rainfall to summer rainfall. As expected, this shift in precipitation seasonality influenced patterns of soil water availability, leading to increased soil respiration in the summer months and sustained canopy greenness throughout the growing season. Despite these responses, there were no significant differences in C3 aboveground net primary production (ANPP) between the seasonally shifted treatment (SEAS) and the plots that received ambient (AMB) precipitation. This was likely due to the high levels of spring soil moisture present before rainout shelters were deployed that sustained C3 grass growth. However, in plots with high C4 grass cover, C4 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 C3 vs. C4 species, altering the dynamics of carbon cycling and canopy albedo in this extensive semi-arid grassland.