Climate change is intensifying the hydrologic cycle globally, resulting in more extreme weather events, ranging from extended droughts to more frequent large precipitation events, or deluges. Arid and semi-arid ecosystems are expected to be particularly responsive to increases in deluges because, in these systems, ecological processes are largely controlled by the pulsed nature of individual precipitation events. Prior research suggests that the shortgrass steppe is most responsive to spring precipitation inputs, however deluges can occur throughout the growing season, so their projected increase in frequency could alter this relationship.
We designed a field experiment to test how the shortgrass steppe of Northeastern Colorado responds to deluge events, and to determine if deluge timing within the growing season differentially influences ecosystem function. A 70 mm deluge event was applied to replicated plots in either early, mid, or late summer. Otherwise all plots, including controls, received ambient precipitation during the growing season. Throughout the growing season, we monitored several ecosystem responses, including soil moisture, soil respiration, canopy greenness, as well as leaf water potential, leaf length, and flowering of the dominant grass, Bouteloua gracilis. At the end of the growing season, we measured above- and below-ground net primary production (ANPP & BNPP).
Results/Conclusions
A single deluge significantly increased ANPP, BNPP, and growing season soil respiration relative to control treatments. Increased belowground production was driven by increased root biomass at depth (10 – 20 cm). We found that mid-season (early July) deluges increased ANPP most effectively, but there was significant visual plot green-up in response to deluges at all times of the growing season. Soil respiration initially increased with all deluge applications, but the early and mid-season deluges led to the greatest increases in growing season soil respiration. B. gracilis water potential also increased with all deluge treatments, but a significant increase in leaf length and flowering occurred only with the mid-season deluge. We conclude that this semi-arid ecosystem is most responsive to the mid-season deluges, coinciding with optimal growing temperatures. However, our results also demonstrate that a single large rain event can significantly affect many ecosystem processes, including those that are measured only at the end of the growing season.