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While plant δ¹⁵N values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of δ¹⁵N values from two Mojave Desert populations ofEncelia farinosato clarify sources of population‐level variability.

We leveraged > 3500 foliar δ¹⁵N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ¹⁵N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ¹⁵N, intra‐plant fractionations, and relationships between soil and plant δ¹⁵N values.

We observed extensive within‐population variability in foliar δ¹⁵N values and found plant age and foliar %N to be the strongest predictors of individual δ¹⁵N values. There were consistent differences between root, stem, and leaf δ¹⁵N values (spanningc. 3‰), but plant and bulk soil δ¹⁵N values were unrelated.

Plant‐level variables played a strong role in influencing foliar δ¹⁵N values, and interannual relationships between climate and δ¹⁵N values were counter to previously recognized spatial patterns. This long‐term record provides insights regarding the interpretation of δ¹⁵N values that were not available from previous large‐scale syntheses, broadly enabling more effective application of foliar δ¹⁵N values.

 

While tree rings have enabled interannual examination of the influence of climate on trees, this is not possible for most shrubs. Here, we leverage a multidecadal record of annual foliar carbon isotope ratio collections coupled with 39 y of survey data from two populations of the drought-deciduous desert shrub Encelia farinosa to provide insight into water-use dynamics and climate. This carbon isotope record provides a unique opportunity to examine the response of desert shrubs to increasing temperature and water stress in a region where climate is changing rapidly. Population mean carbon isotope ratios fluctuated predictably in response to interannual variations in temperature, vapor pressure deficit, and precipitation, and responses were similar among individuals. We leveraged the well-established relationships between leaf carbon isotope ratios and the ratio of intracellular to ambient CO 2 concentrations to calculate intrinsic water-use efficiency (iWUE) of the plants and to quantify plant responses to long-term environmental change. The population mean iWUE value increased by 53 to 58% over the study period, much more than the 20 to 30% increase that has been measured in forests [J. Peñuelas, J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597–608 (2011)]. Changes were associated with both increased CO 2 concentration and increased water stress. Individuals whose lifetimes spanned the entire study period exhibited increases in iWUE that were very similar to the population mean, suggesting that there was significant plasticity within individuals rather than selection at the population scale.