Signatures of autumn deluges revealed during spring drought in a semi-arid grassland

Increases in extremely large precipitation events (deluges) and shifts in seasonal patterns of water availability with climate change will both have important consequences for ecosystem function, particularly in water-limited regions. While previous work in the semi-arid shortgrass steppe of northeastern Colorado has demonstrated this ecosystem's strong sensitivity to growing season deluges, our understanding of ecosystem responses to deluges during the dormant season is limited. Here, we imposed experimental 100 mm deluges (~ 30% of mean annual precipitation) in either September or October in a native C₄-dominated shortgrass steppe ecosystem to evaluate the impact of this post-growing season shift in water availability during the autumn and the following growing season. Soil moisture for both deluge treatments remained elevated compared with ambient levels through April as spring precipitation was atypically low. Despite overall low levels of productivity with spring drought, these deluges from the previous autumn increased aboveground net primary production (ANPP), primarily due to increases with C₄ grasses. C₃ ANPP was also enhanced, largely due to an increase in the annual C₃ grass, Vulpia octoflora, in the October deluge treatment. While spring precipitation has historically been the primary determinant of ecosystem function in this ecosystem, this combination of two climate extremes-an extremely wet autumn followed by a naturally-occurring spring drought-revealed the potential for meaningful carryover effects from autumn precipitation. With climate change increasing the likelihood of extremes during all seasons, experiments which create novel climatic conditions can provide new insight into the dynamics of ecosystem functioning in the future.