Giant cacti: isotopic recorders of climate variation in warm deserts of the Americas.

The plant family Cactaceae is considered among the most threatened groups of organisms on the planet. The threatened status of the cacti family has created a renewed interest in the highly evolved physiological and morphological traits that underpin their persistence in some of the harshest subtropical environments in the Americas. Among the most important anatomical features of cacti is the modification of leaves into spines, and previous work has shown that the stable isotope chemistry of cacti spines records potential variations in stem water balance, stress, and Crassulacean acid metabolism (CAM). We review the opportunities, challenges, and pitfalls in measuring δ 13C, δ 2H, and δ 18O ratios captured in spine tissues that potentially reflect temporal and spatial patterns of stomatal conductance, internal to atmospheric CO2 partial pressures, and subsequent patterns of photosynthetic gas exchange. We then evaluate the challenges in stable isotope analysis in spine tissues related to variation in CAM expression, stem water compartmentalization, and spine whole-tissue composition among other factors. Finally, we describe how the analysis of all three isotopes can be used in combination to provide potentially robust analysis of photosynthetic function in cacti, and other succulent-stemmed taxa across broad spatio-temporal environmental gradients.