AN EASY PATH FROM PRECIPITATION AMOUNT TO δ18O VALUES IN THE SPINES OF COLUMNAR CACTUSES, CARNEGIEA GIGANTEA

Terrestrial tropical and subtropical climate proxies >200 years-old and annually resolved are relatively rare compared to temperate climate proxies. Columnar cactuses grow in many treeless regions of the tropics and subtropics and grow durable spines in sequence that provide a relatively high-resolution and long-duration bioarchive of information concerning plant physiology and local climate. In 2008, we measured stem-diameters, the δ¹⁸O of local precipitation, stem-water and newly grown spine-tissue from randomly selected and naturally occurring saguaro cactuses in Tucson, Arizona, to validate the linkages between rainfall and spine-tissue δ¹⁸O. In the hot/dry season before the North American Monsoon (NAM), drought and transpiration resulted in mean stem-diameter reductions up to 37%. Transpiration led to increased stem-water δ¹⁸O (~4‰) and spine-tissue δ¹⁸O (~4‰). After the onset of the NAM, stem-diameters increased with decreases in stem-water and spine δ¹⁸O (~11‰ and ~8‰, respectively). The difference may be partially accounted for by rapid recharge with rainfall that is more negative than winter rains. Following the monsoon, there is an autumn drought, also reflected in reduced stem-diameters and increased stem-water δ¹⁸O. Preliminary data suggests that recharge with winter rainfall increases stem-diameters and decreases stem-water and spine δ¹⁸O, resetting the cycle for the next year. During the growing season of spines (March to October), there is a significant inverse relationship between mean stem-diameter and stem-water δ¹⁸O (simple linear regression, adjusted r² = 0.97, P < 0.010). Likewise, there is a significant inverse relationship between mean-stem diameter and spine-tissue δ¹⁸O (simple linear regression, adjusted r² = 0.99, P < 0.049). Over the entire spine growing season (March to November) the difference between mean spine tissue δ¹⁸O and mean stem water δ¹⁸O (~25‰) is close to the theoretical value for biological fractionation (~27‰), and demonstrates the fidelity of spine tissue δ¹⁸O in recording stem water δ¹⁸O. Future work includes modeling and calibration of the δ18O in spine-tissue so that it can be used as a precipitation proxy in treeless regions lacking other annually resolved terrestrial climate proxies.