Past climate changes and ecophysiological responses recorded in the isotope ratios of saguaro cactus spines

The stable isotope composition of spines produced serially from the apex of columnar cacti has the potential to be used as a record of changes in climate and physiology. To investigate this potential, we measured the delta O-18, delta C-13 and (FC)-C-14 values of spines from a long-lived columnar cactus, saguaro (Carnegiea gigantea). To determine plant age, we collected spines at 11 different heights along one rib from the stem apex (3.77 m height) to the base of a naturally occurring saguaro. Fractions of modern carbon ((FC)-C-14) ranged from 0.9679 to 1.5537, which is consistent with ages between 1950 and 2004. We observed a very strong positive correlation (r = 0.997) between the (FC)-C-14 age of spines and the age of spines determined from direct and repeated height measurements taken on this individual over the past 37 years. A series of 96 spines collected from this individual had delta O-18 values ranging from 38 parts per thousand to 50 parts per thousand [Vienna standard mean ocean water (VSMOW)] and delta C-13 values from -11.5 parts per thousand to -8.5 parts per thousand [Vienna Peedee belemnite (VPDB)]. The delta O-18 and delta C-13 values of spines were positively correlated (r = 0.45, P < 0.0001) and showed near-annual oscillations over the similar to 15-year record. This pattern suggests that seasonal periods of reduced evaporative demand or greater precipitation input may correspond to increased daytime CO2 uptake. The lowest delta O-18 and delta(13)pC values of spines observed occurred during the 1983 and 1993 El Nino years, suggesting that the stable isotope composition recorded in spine tissue may serve as a proxy for these climate events. We compared empirical models and data from potted experimental cacti to validate these observations and test our hypotheses. The isotopic records presented here are the first ever reported from a chronosequence of cactus spines and demonstrate that tissues of columnar cacti, and potentially other long-lived succulents, may contain a record of past physiological and climatic variation.