Oecologia DOI 10.1007/s00442-011-2021-1 123 GLOBAL CHANGE ECOLOGY - ORIGINAL PAPER Seasonal photosynthetic gas exchange and water-use eYciency in a constitutive CAM plant, the giant saguaro cactus (Carnegiea gigantea) Dustin R. Bronson · Nathan B. English · David L. Dettman · David G. Williams Received: 29 September 2010 / Accepted: 9 May 2011  Springer-Verlag 2011 Abstract Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use eYciency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to cli- mate change. We monitored net CO 2 assimilation (A net ), stomatal conductance (g s ), and transpiration (E) rates peri- odically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tuc- son, Arizona USA to investigate environmental and physio- logical controls over carbon gain and water loss in this ecologically important plant. We hypothesized that sea- sonal changes in daily integrated water use eYciency (WUE day ) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO 2 uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE day occurred during time periods with extreme high and low air vapor pressure deWcit (D a ). The diurnal with the highest D a had low WUE day due to minimal net carbon gain across the 24 h period. Low WUE day was also observed under conditions of low D a ; however, it was due to signiWcant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D a conWrmed the relationship between D a and g s . Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succu- lent CAM plants of warm desert environments. Keywords Crassulacean acid metabolism · CAM · Columnar cactus · Sonoran desert · Transpiration · Stomatal conductance · Humidity Introduction Crassulacean acid metabolism (CAM) plants use phospho- enolpyruvate carboxylase (PEPc) to Wx CO 2 and temporar- ily store the Wxed C as malate inside cell vacuoles at night. The malate is then decarboxylated during the day, releasing CO 2 at high concentrations inside photosynthetic tissues (Osmond 1976). This unique CO 2 concentrating mecha- nism allows CAM plants in warm desert environments to carry out carboxylation and Calvin cycle reactions during daytime periods while maintaining relatively low daytime stomatal conductance and transpiration rates. As a result, many CAM species in warm deserts achieve relatively high water use eYciencies (WUE; CO 2 uptake/H 2 O lost) compared to plants that utilize the more common C3 and Communicated by Robert Pearcy. D. R. Bronson (&) · D. G. Williams Departments of Renewable Resources and Botany, University of Wyoming, 1000 E. University Dr., Laramie, WY 82071, USA e-mail: dbronson@upenn.edu N. B. English Earth and Environmental Sciences Division, Los Alamos National Laboratory, MS J495, Los Alamos, NM 87545, USA D. L. Dettman Department of Geosciences, University of Arizona, 4810 E. 4th St., Bldg #77, Tucson, AZ 85721, USA