Plant Water Use in Owens Valley, CA: Understanding the Influence of Climate and Depth to Groundwater
There is a long-standing controversy in Owens Valley, California about the potential impacts of water exports on the local ecosystem. It is currently extremely difficult to attribute changes in plant cover and community composition to hydrologic change, as the interactions between ecological and hydrologic processes are relatively poorly understood. Underlying predictions about losses of grasslands and expansion of shrublands in response to declining water tables in Owens Valley are assumptions about the differential access of grasses versus shrubs to groundwater and their physiological respones to water stress. We sought to test these assumptions with measurements of natural abundance isotope tracers in plants, soils, and groundwater, and with physiological measurements. We found that the grass species Distichlis spicata did use shallower water sources than co-occuring shrub species, particularly in late summer. However, at sites with watertable depths < 3 m we did not find evidence of water stress in the grass species. In fact, Distichlis was more resistant to water stress-induced cavitation than co-occurring shrub species, indicating greater tolerance of dry conditions. Instead, soil nutrient availability appeared to be strongly limiting grass photosynthesis and gas exchange at low nutrient sites. As has been found previously, our results showed that where water table depths exceeded 3 m, grass cover was extremely sparse across a depth to watertable gradient. We did find evidence of water stress at 6 m watertable depth in the phreatophytic shrub species Ericameria nauseousa. Vulnerability to cavitation, an important component of plant water stress response, increased with watertable depth in this species. Hence, plant hydraulic architecture showed intraspecific variation in response to water availability. Stem lignin content increased with vulnerability to cavitation, possibly because of the linkage between lignified xylem and the ability to withstand cavitation. We propose that this relationship is a potential mechanism linking ecosystem water and nutrient availability, which appear to be closely correlated in Owens Valley. Our results highlight the importance of nutrient limitation in these ecosystems, and suggest that models used to evaluate ecosystem responses to hydrologic change in this region will require explicit attention to the ecosystem nitrogen cycle and the spatial and temporal variability in nitrogen availability.