Oecologia (1991) 88 : 430434 Oecologia 9 Springer-Verlag 1991 Differential utilization of summer rains by desert plants James R. Ehleringer, Susan L. Phillips, William S.F. Schuster, and Darren R. Sandquist Department of Biologyand Stable Isotope Ratio Facility for Environmental Research, University of Utah, Salt Lake City, UT 84112, USA ReceivedJuly 10, 1991 / AcceptedJuly 15, 1991 Summary. Seasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge pre- cipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent peren- nials exhibited a complete dependence on summer pre- cipitation. Herbaceous and woody perennial species si- multaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipita- tion in southern Utah is evenly partitioned between win- ter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially respond to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure. Key words: Hydrogen isotope ratio - Desert ecology - Water source - Climate change Water is the limiting factor for plant growth in desert environments and primary productivity is often linearly dependent on precipitation input (Whittaker and Niering 1975; Hadley and Szarek 1981; Ehleringer and Mooney 1983). The survival of perennial species through extended drought periods is a product of both the ability of roots to acquire remaining soil moisture and the ability of shoots to tolerate water stress (MacMahon and Schimpf 1981; Ehleringer 1985; Smith and Nowak 1990). Plant species within desert ecosystems are sensitive to episodic events (1 10 years) such as prolonged droughts or un- usual wet periods, which can result in marked physiologi- Offprint requests to: J.R. Ehleringer cal changes and dramatic shifts in community composi- tion (Stockton and Meko 1975; MacMahon and Schimpf 1981 ; Turner 1990). Analyses of how desert plant com- munities respond to gradual changes over extended periods of time (> 50 years) are more difficult to obtain. Yet, these are the critical data needed in order to predict how desert ecosystems might respond to the anticipated climate changes associated with a doubling of atmo- spheric CO2 (e.g., changes in precipitation patterns) as predicted by global circulation models (Schlesinger and Mitchell 1987). To begin to address this critical issue, we measured the stable isotopic composition of xylem sap and water stress levels of the dominant species in a desert scrub commu- nity in southern Utah. This was done to determine the patterns and sources of water in relation to current pre- cipitation and environmental conditions. The study re- gion was chosen because the deserts throughout this part of North America currently receive approximately equal amounts of precipitation from winter (Pacific fronts) and summer (monsoon) storms (Markham 1970; Sellers and Hill 1974). Global circulation models of anticipated cli- mate change in response to a doubling of atmospheric CO2 concentration predict that the amount of winter- spring precipitation in this region should remain con- stant, but that the amounts of summer precipitation should increase (Schlesinger and Mitchell 1987). Long- term analysis of precipitation records over the past 100 years for this part of North America indicate that summer precipitation has been gradually increasing while winter precipitation has remained constant (McDonald 1958; yon Eschen 1958; Williams 1979). During the annual hydrologic cycle of these deserts, soils are recharged by winter-spring precipitation; the gentle rains associated with winter storm systems allow for greater penetration of the precipitation into the soil and cool temperatures reduce evaporative and transpira- tional water losses (Caldwell 1985). Precipitation from these frontal storms has an average hydrogen isotope ratio (6D) of approximately -90%o at interior desert sites (Winograd 1985; Ingraham and Taylor 1991; Fla-