Ehleringer, J.R., R.D. Evans, and D. Williams. 1998. Assessing sensitivity to change in desert ecosystems - a stable isotope approach, p. 223-237. In H. Griffiths (ed.), Stable Isotopes Integration of Biological, Ecological, and Geochemical Processes. BIOS Scientific Publ., Oxford.

Growth and gas exchange of plants in arid zones are driven primarily by periodic pulses of moisture and nutrient availability. Both seasonal and inter-annual changes in precipitation occur in response to long-term weather cycles causing direct impacts on productivity, competitive displacement, and mortality (Comstock and Ehleringer, 1992; Smith et al., 1997). Deviations from mean climatic conditions might be expected to increase under predicted climate change scenarios, particularly given the disequilibrium between terrestrial and oceanic regions as radiative forces increase (Rind et al., 1990). Surface disturbance caused by land-use change has resulted in significant changes in soil nitrogen levels of aridland ecosystems (Belnap, 1995; Evans and Belnap, 1997) and this may be further exacerbated by variations in monsoonal precipitation patterns.

In this chapter, we explore how analyses of stable isotope ratio patterns in arid land soils and plants helps us better understand ecosystem dynamics in response to changes in moisture and nitrogen availability. Arid land ecosystems on the Colorado Plateau in the western United States provide an opportunity to examine plant response to the potential sensitivity of ecosystem components to variations in summer and winter moisture inputs along moisture clines, while at the same time exploring the impact of different land-use patterns on nitrogen availability.

To facilitate modeling approaches for estimating productivity it would be ideal if all vegetation components responded equally to seasonal variations in resource input. More realistically, it would be reasonable to expect that different vegetation components could be aggregated into functional groups, such as by life form, with members of the same life form responding similarly to seasonal pulses in moisture and nitrogen. While stable isotopes may not contribute directly to estimating productivity, stable isotope studies will contribute directly to understanding mechanistic aspects of the constraints on productivity in different species and/or functional groups, thereby complementing more traditional approaches (Ehleringer et al., 1993). Carbon isotopes in organic material provide an estimate of the extent to which different gas exchange components affect productivity (Farquhar et al., 1989). Hydrogen and oxygen isotopes in xylem waters contribute directly to quantifying use of monsoonal and winter water sources (Ehleringer and Dawson, 1992). Nitrogen isotope ratios provide a quantitative estimate of the balance between nitrogen inputs and losses from the soils and species-specific patterns of nitrogen use, particularly in response to disturbance (Evans and Ehleringer, 1993, 1994; Evans and Belnap, 1997). Together stable isotopes provide an opportunity to assess ecosystem dynamics and the sensitivity of these vegetation components to change and to complement more traditional gas exchange and biomass-assessment approaches.