Ecophysiology of two Sonoran Desert evergreen shrubs during extreme drought

Recent drought across the desert Southwest US may strongly affect the physiological functioning of evergreen desert species that maintain leaves through these dry periods. In July 2002 we compared the ecophysiological performance of the open-canopied, small-leaved creosotebush (Larrea tridentata) to the dense-canopied, more broad-leaved jojoba (Simmondsia chinensis) growing on a ridge-top, eastand west-facing slopes to assess how differences in leaf habit and exposure affect these species’ ability to withstand severe drought. Standardized precipitation index (SPI) from 100 years precipitation data showed July 2002 rainfall was normal, but the 12-month period ending July 2002 was of extreme aridity (SPI1⁄4 2.71). Predawn water potentials were extremely low, and lower in jojoba ( 8.1 to 9.5 MPa) than in Larrea ( 6.6 to 9.4 MPa). Chlorophyll fluorescence showed jojoba had more consistent photochemical performance than Larrea across three exposures, having higher optimal photosystem II (PSII) efficiency (Fv/Fm), lower light-adapted PSII yield, and better ability to thermally dissipate light energy (i.e. higher NPQ), especially in inner-canopy jojoba leaves. These findings suggest jojoba may better cope with high light under drought conditions, and that canopy shading in jojoba may reduce high-light stress. Moreover, lower Fv/Fm, NPQ and high PSII yields in Larrea was similar between eastfacing and ridge-top locations, but achieved levels similar to jojoba on west-facing exposures. These findings suggest these drought-tolerant evergreens rely on different physiological mechanisms to cope with high light under extreme drought conditions. Published by Elsevier Ltd. Recent severe drought has induced an extensive pulse of adult plant mortality over broad ranges of elevation and latitude across the Southwest US (Bowers, 2005; Breshears et al., 2005;Miriti et al., 2007). Extensive die-back of established plants indicates this drought is more severe than normal, even for these highly waterlimited systems, andmay be the start of prolonged climatic drought which occur across this region on an inter-decadal and intercentennial basis (Swetnam and Betancourt, 1998). However, desert evergreen species have fared better than shorter-lived drought deciduous shrubs (Hamerlynck and McAuliffe, 2008; Miriti et al., 2007). Comparative studies of basic physiological performance would give insight how long-lived desert evergreen shrubs survive these infrequent climate extremes. Under water-limited conditions, desert evergreens must quench excess energy from saturating light levels, in part because prolonged stomatal closure limits carbon fixation as an effective ; fax: þ1 520 670 5550.