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

Abstract

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 δ^18 O and δ^13 C isotope record from the spines of five saguaro cacti was created using bomb radiocarbon and semi-annual variations in δ^13 C. Once dating errors in the composite record are corrected, mean annual spine δ^18 O 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 parts per thousand) in the maximum annual spine δ^18 O are positively correlated (P<0.01) with the Southern Oscillation Index (SOI). We attribute these decreases to enhanced winter rainfall associated with the El Nino phase of the El Nino-Southern Oscillation. Minimum annual δ^13 C 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.