Colorado State University Fort Collins, Colorado, United States
Abstract: The shortgrass steppe (SGS) is a biome that covers ~20 million ha of land in the rain shadow of the Rocky Mountains, and most ecosystem functions are water-limited, even in years with normal precipitation. Thus, the SGS is particularly sensitive to changes in precipitation amounts and patterns. Climate change is expected to increase drying as well as the frequency of large rain events (deluges), with the SGS expected to respond strongly to each of these. We sought to assess how both spring drying and deluges interact to affect ecosystem functions in the SGS, as well as how regrowth after canopy removal (simulating grazing) was influenced by such changes in precipitation patterns. To address these questions, we established rain-out shelters to reduce spring precipitation inputs into a native grassland at the USDA-ARS Central Plains Experimental Range in northeastern Colorado. We estimated aboveground net primary productivity (ANPP) directly before adding a mid-July deluge of 60mm, and then compared pre- and post-deluge ANPP as well as the regrowth of grasses from the pre-deluge harvested plots. Responses in soil respiration and canopy greenness were also assessed.
Although ANPP was similar at mid- and late-growing season sampling times between the ambient and reduced spring precipitation plots, regrowth after the deluge varied depending on spring precipitation inputs. Plots with ambient spring precipitation and a deluge had greater regrowth than plots with drier springs plus a deluge. Ambient plots with no deluge regrew the least. This interactive effect of spring precipitation inputs and a midseason deluge has important implications for agricultural management, especially for grasslands used for livestock grazing. Canopy greenness and soil respiration responses to the deluge also varied depending on spring precipitation history – a midseason deluge stimulated these responses in drier spring plots sufficiently to match ambient plots. But the combination of a wet spring with a deluge increased canopy greenness and soil respiration the most. We conclude that a midseason deluge may compensate for the lost precipitation earlier in the season and such deluges may be particularly important in grazed grasslands.
