Using stable oxygen isotopes to quantify the water source used for transpiration by native shrubs in the San Luis Valley, Colorado USA

An understanding of the water source used by phreatophytic desert shrubs is critical for understanding how they function and respond to man-caused groundwater drawdowns. Shrubs can use primarily groundwater, precipitation recharged soil water, or a mixture of the two. If shrubs use primarily groundwater, a water table decline may reduce water availability and lead to high plant mortality. However, if shrubs can acquire precipitation recharged soil water, then groundwater decline could have less impact on plants. This study took place in the San Luis Valley, a large, arid, high elevation closed basin in south-central Colorado. We examined stable oxygen isotopes in precipitation, soil water from several depths, groundwater and plant xylem water to identify the likely water sources for the three most abundant shrubs in the valley: Sarcobatus vermiculatus (Hooker) Torrey, Chrysothamnus nauseosus (Pallas) Britton subsp. consimilis (Greene) Hall & Clements, and Chrysothamnus greenei (Gray) Greene. C. greenei is not known to be phreatophytic while S. vermiculatus and C. nauseosus may be phreatophytic. Mean annual San Luis Valley precipitation during the two years of study was 121 mm, with 67% occurring during the summer monsoon season of July through September. We found differences in water acquisition patterns by species, season, and along a depth to water table gradient. C. greenei only occurred in sites with a water table > 2.0 m deep, and utilized only soil water recharged by precipitation. At sites with a water table less then 2 m depth, S. vermiculatus and C. nauseosus utilized soil water from the top 0.5 m and shallow groundwater during the pre-monsoon and monsoon periods. A more complex water use pattern was found at sites with a water table deeper then 2 m. S. vermiculatus and C. nauseosus used both deep soil water and groundwater during 1996. During the pre-monsoon period in 1997, both shrubs utilized predominantly groundwater. However, during the 1997 monsoon season both species switched to utilize primarily precipitation recharged water acquired from the upper 0.3-0.4 m of soil. This is the first report that C. nauseosus can utilize summer precipitation. Our results support the hypothesis that plants utilize more summer rain recharged soil water in regions receiving a substantial proportion of annual precipitation during the summer.