Stable Isotopes in the Spines of Columnar Cactus: a New Proxy for Climate and Ecophysiological Research

There are relatively few annually resolved climate proxies in arid and semi-arid regions. Columnar cactuses are common in these regions and the stable isotopes of carbon and oxygen in durable spines record variations in rainfall, humidity and ecophysiology as they grow in series along the sides of cactuses. Despite their spines, columnar cactuses provide important ecosystem resources and services in drought prone areas, however, the impact that long-term climate variability and infrequent storms (El Niño or tropical storms) have on the ecology and ecophysiology of columnar cactus is less clear. Stable isotopes in trees and corals serve as useful proxies of climate and ecophysiological information, but for cactus we lack the most rudimentary information about the isotopic systems and their links to the environment. Here, we present an isotopic framework that begins with developing semi-empirical mechanistic models of δ¹³C, δ¹⁸O and δ²H variation in saguaro cactuses that link physical and physiological fractionation factors in stem water and spines to rainfall and humidity. We also review a novel method for determining the age of spines, an important step in developing useful chronologies of isotopic variation in spines. The mechanistic models combined with local climate records enhance our understanding of isotopic variation in daily and annually dated spine δ¹³C and δ¹⁸O records and explain the statistical association of δ¹³C and δ¹⁸O in spines with rainfall, vapor pressure deficit, and El Niño enhanced winter rains. While there are still some challenges to overcome, we expect that isotopic spine series will be used as climate proxies to answer questions regarding regional climate variability or to enhance current models of past and future climates. Likewise, ecophysiologists can use the isotopic spine series in conjunction with gas exchange or carbohydrate studies to look at reproductive or biological responses to changing environments