The importance of extreme rainfall events and their timing in a semi-arid grassland

Abstract

Climate change is intensifying the hydrologic cycle globally, increasing both the size and frequency of extreme precipitation events, or deluges. Arid and semi-arid ecosystems are expected to be particularly responsive to this change because their ecological processes are largely driven by distinct soil moisture pulses. However, since soil moisture, air temperature and plant phenology vary throughout the growing season, deluges will likely have differing impacts on these systems depending on when they occur. We conducted a field experiment to assess how the seasonal timing (early, middle or late growing season) of a single deluge (70-mm precipitation event) altered key ecological processes in the semi-arid shortgrass steppe of North America. Regardless of timing, a single deluge stimulated most ecosystem processes, but a deluge at mid-season caused the greatest increase in soil respiration, canopy greenness, above-ground net primary production (ANPP) and growth and flowering of the dominant plant species Bouteloua gracilis. In contrast, below-ground net primary production (BNPP) was insensitive to deluge timing, with a consistent BNPP increase in all the deluge treatments that was almost twice as large as the ANPP response. This BNPP response was largely driven by enhanced root production at the 10–20 cm, rather than 0–10 cm, soil depth. Synthesis. In a semi-arid ecosystem, a single deluge can have season-long impacts on many ecosystem processes, but responses can be mediated by event timing. Therefore, predicting responses of semi-arid ecosystems to more dynamic precipitation regimes, and subsequent impacts on the global carbon budget, will require knowledge of how deluge magnitude, frequency and timing are being altered by climate change.