Responses of sagebrush-dominated ecosystems to increased precipitation intensity

Martin C. Holdrege, Karen Beard and Andrew Kulmatiski, Department of Wildland Resources and the Ecology Center, Utah State University, Logan, UT, Kyle A. Palmquist, Department of Biological Sciences, Marshall University, Huntington, WV

Background/Question/Methods
Sagebrush-dominated ecosystems are the most widely distributed ecosystems in the western United States. Because the structure and function of these ecosystem types are closely tied to temperature and precipitation, it is necessary to understand how sensitive these systems are to changes in climate. In addition to changes in temperature and total precipitation, increases in precipitation intensity are widely forecasted, but potential impacts are not as well understood. Ecohydrological models allow for the examination of ecological responses across climate scenarios and broad spatial and temporal scales. Multiple studies have used ecohydrological models to assess sagebrush ecosystem responses to climate change, but did not isolate the impact of increased precipitation intensity. Our objective was to understand how soil water availability and biomass of key plant functional types respond to fewer, larger precipitation events, and how these responses vary across sites with differing climates. We used an individual-based plant simulation model (STEPWAT2) to simulate the response of sagebrush-dominated ecosystems to increased precipitation intensity at sites that span the climatic envelope of these ecosystems. We increased precipitation intensity by decreasing the frequency of precipitation and increasing mean event size, without altering total annual precipitation.
Results/Conclusions
Increased precipitation intensity decreased transpiration from shallow (< 10 cm) soils. In contrast, transpiration increased from deeper soil layers, especially in more arid sites. Total transpiration across all soil layers increased with precipitation intensity in arid sites with smaller changes (both positive and negative depending on site) in more mesic sites. Reflecting increases in deeper soil water availability, biomass of shrubs increased with precipitation intensity, with the strongest response found in arid sites. Grasses, which are more shallowly rooted, did not exhibit consistent responses to increased precipitation intensity across sites. Results are in agreement with the hypothesis that larger precipitation events increase shrub growth by increasing water availability, and ‘pushing’ water deeper into the soil, and that this could contribute to shrub encroachment. Response differences with aridity are consistent with experiments in grasslands that suggest positive effects of increased precipitation intensity on water availability (less interception and evaporation from shallow soil) dominate in arid sites, while negative effects (run-off, deep drainage, surface drying between precipitation events) dominate in mesic sites. Our results suggest that understanding sagebrush ecosystem responses to climate change requires accounting for precipitation intensity, and that responses to increased precipitation intensity will be strongest in arid sites.