Sensitivity of a semi-arid grassland to extreme precipitation events

2021 Summer.Includes bibliographical references.Rising global temperatures due to climate change are intensifying the hydrological cycle, resulting in larger and more frequent extreme rain events, or deluges. Dryland ecosystems are predicted to be especially sensitive to this change in precipitation pattern because, in these water-limited systems, ecological processes are largely controlled by infrequent water pulses and deluges represent an extreme precipitation pulse. Therefore, my dissertation examined how the semi-arid shortgrass steppe of eastern Colorado responds to deluge timing and size. Using field experiments, I applied deluge events to grassland plots that varied in seasonal timing (early, middle, or late growing season) and magnitude (moderate – extreme event sizes), and quantified ecosystem response. I also conducted an observational study to determine if these plot-level results could be scaled to the larger shortgrass steppe landscape. I identified natural deluges in the historical precipitation record and related spatial variation in precipitation to post-deluge canopy greenness via satellite imagery. My field experiments showed that the shortgrass steppe is extremely responsive to large rain events, with most measured variables exhibiting a substantial increase following an applied deluge event. Measured variables included soil moisture, soil respiration, and above and belowground net primary production (ANPP & BNPP), as well as growth and flowering of the dominant grass species, Bouteloua gracilis (blue grama). However, response magnitude depended on both deluge timing and size. The shortgrass steppe was most responsive to a mid-growing season (July) deluge, and ecosystem processes generally increased linearly with increasing deluge size, with limited evidence for response saturation. My observational study exhibited similar patterns at the landscape-scale, suggesting that these experimental plot-level results can be scaled to the larger shortgrass steppe landscape, despite greater variation in soil texture, grazing regime, and plant community. Overall, my dissertation research suggests that semi-arid ecosystems could be well-adapted to the increase in rainstorm size and frequency predicted with climate change; however, the magnitude of the ecosystem response depends on intra-seasonal precipitation patterns, including deluge timing and size. These findings have important implications for predicting both local ecosystem services (e.g., forage production) and global carbon cycling under altered precipitation regimes