Throughout western North America, semiarid and arid basins are likely to experience changes in the timing and amount of precipitation, which may alter regional water budgets. As water becomes increasingly limiting, accurate estimates of groundwater outflow from native plant evapotranspiration will be critical to managing water resources in basins with shallow water tables. Phreatophytes can contribute significantly to total groundwater outflow on a watershed scale. Some phreatophytes may also acquire soil water recharged by precipitation, which may reduce or supplement their groundwater use. As a result, groundwater use by phreatophyte communities may vary both spatially and temporally in response to seasonal or long-term changes in growing season precipitation. We conducted a rainfall manipulation experiment in the San Luis Valley, Colorado to investigate the response of four common native phreatophyte species to: (1) the present summer monsoon rainfall regime, and (2) increased and decreased total summer rainfall. We measured xylem pressure potential to assess plant physiological responses, and compared stable oxygen isotope signatures of plant xylem water to surface (0-15 cm) and sub-surface (15-30 cm) soil layers and groundwater.

Results/Conclusions

Our results indicate that plant response to temporal variation in soil water availability differed by species and plant functional type, across treatments. The grasses Sporobolus airoides and Distichlis spicata primarily used precipitation-recharged soil moisture, but can shift their uptake between surface and sub-surface layers. The shrub Ericameria nauseosa relied heavily on groundwater throughout the growing season. The shrub Sarcobatus vermiculatus used groundwater, but incorporated more precipitation from upper soil layers as it became available in late summer. These results suggest that native phreatophyte species will vary substantially in their sensitivity to forecast hydroclimatic changes. This may lead to changes in plant community composition that alter basin-scale groundwater use by native plants. Results of this work could inform models for managing groundwater in this region, and may have implications for water-limited regions worldwide.