Collaborative Research: An Isotopic Record of Response to Climate Change in Spines of Saguaro Cactus

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Wildlife species in the Sonoran Desert depend on resources supplied by large, water-storing cactuses and other succulents during periods of annual drought. The flowers and fruits of the giant saguaro cactus, for instance, provide water, nutrients and energy to numerous insects, birds and mammals during the driest periods of the year. Growth and reproduction of saguaro are extremely sensitive to climate variability and change, with potentially important impacts on dependent communities. This project develops a novel approach for assessing climate fluctuations, growth, stress responses, and reproduction of saguaro using information recorded in the isotope composition of saguaro spines. Variation in the oxygen and carbon stable isotope ratios in saguaro spines records patterns of water uptake and evaporation and changes in photosynthetic processes associated with water stress. Because spines are produced at the growing tip of the cactus and retained on the plant for up to 100 or more years, a long-term, high-resolution record of water balance and photosynthetic stress is potentially available for any saguaro plant. The age of spines is verified using radiocarbon (14C) dating techniques. This new project will construct precisely dated, high resolution isotope records from spines grown and retained throughout the life of saguaro plants across climate gradients in Arizona. The project hypothesizes that that 1) oxygen isotope ratios of spines record the source and amount of water to saguaro plants and variation in evaporative conditions; and 2) carbon isotope ratios reflect water stress effects on photosynthesis. Comparison of annually-resolved isotope records from spines with growth rates, flower/fruit production, and climate records covering the last 50 to 80 years will reveal saguaro's response to local and regional environmental variation. In addition to two post-doctoral students, the proposal will fund original undergraduate research. The project will develop a curriculum unit on cactus-water relations for an existing K-12 outreach program.