A 26-year stable isotope record of humidity and El Niño-enhanced precipitation in the spines of saguaro cactus, Carnegiea gigantea

Seasonal and annual variations of rainfall and humidity are recorded in the carbon and oxygen stable isotope ratios of sequentially grown spines found on the columnar cactus, Carnegiea gigantea. A 26-year long composite δ18O and δ13C isotope record from the spines of five saguaro cacti was created using bomb radiocarbon and semi-annual variations in δ13C. Once dating errors in the composite record are corrected, mean annual spine δ18O is negatively correlated (P < 0.001) with total annual precipitation (TAP) from November through October and positively correlated (P < 0.01) with mean annual nighttime vapor pressure deficit (VPD). Year-to-year decreases (> 2‰) in the maximum annual spine δ18O are positively correlated (P < 0.01) with the Southern Oscillation Index (SOI). We attribute these decreases to enhanced winter rainfall associated with the El Niño phase of the El Niño-Southern Oscillation. Minimum annual δ13C is negatively correlated with TAP (P < 0.05) and mean nighttime VPD (P < 0.05). These results bolster proposed mechanistic models of isotopic variation in the spines of columnar cactus and demonstrate how isotopic spine series may be used as climate proxies in regions of the Americas where trees suitable for traditional or isotopic dendrochonology are absent.