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 CO2 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 CO2 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.
Rationale
Fraudulent region‐of‐origin labeling is a concern for high‐value, globally traded commodities such as coffee. The oxygen isotope ratio of cellulose is a useful geographic tracer, as it integrates climate and source water signals. A predictive spatial model (“isoscape”) of the δ18O values of coffee bean cellulose is generated to evaluate coffee region‐of‐origin claims.
Methods
The oxygen isotope ratio of α‐cellulose extracted from roasted coffee beans was measured via high‐temperature conversion elemental analyzer/isotope ratio mass spectrometry (TC‐EA/IRMS) and used to calculate the δ18O value of coffee bean water. The 18O enrichment of coffee bean water relative to the δ18O value of local precipitation was modeled as a function of local temperature and humidity. This function was incorporated into a mechanistic model of cellulose δ18O values to predict the δ18O values of coffee bean cellulose across coffee‐producing regions globally.
Results
The δ18O values of analyzed coffee bean cellulose ranged from approximately +22‰ to +42‰ (V‐SMOW). As expected, coffees grown in the same region tended to have similar isotope ratios, and the δ18O value of coffee bean cellulose was generally higher than the δ18O value of modeled stem cellulose for the region. Modeled δ18O values of coffee cellulose were within ±2.3‰ of the measured δ18O value of coffee cellulose.
Conclusions
The oxygen isotope ratio of coffee bean cellulose is a useful indicator of region‐of‐origin and varies predictably in response to climatic factors and precipitation isotope ratios. The isoscape of coffee bean cellulose δ18O values from this study provides a quantitative tool that can be applied to region‐of‐origin verification of roasted coffee at the point‐of‐sale.
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