Summer precipitation pulse response in western Joshua trees (Yucca brevifolia Englm.)

Abstract The western Joshua tree ( Yucca brevifolia Englm.) is a culturally and ecologically important species of the Mojave Desert that is threatened by climate change. The North American summer monsoon provides rainfall during an otherwise dry period, and some scenarios envision a 40% decrease in monsoon rainfall by 2100. We used summer water additions and the record-setting storm, Tropical Storm Hilary, to address the following questions: (1) Can Joshua trees use summer water pulses to improve water status, photosystem health, and carbon gain? and (2) Does pre-watering gas exchange physiology vary with antecedent water conditions? Measurement campaigns were undertaken in August of 2021 and 2023, which were dry (53 mm precipitation) and wet (383 mm precipitation) water years respectively. We measured leaf water potential, photosynthetic gas exchange, and chlorophyll a fluorescence of Joshua trees near Pioneertown, CA. Net leaf level CO 2 assimilation (A net ) for Y. brevifolia saw an average increase of 2.3 μmol CO 2 m −2 s −1 five days after the precipitation pulse in 2023, showing that Joshua trees can upregulate photosynthesis in response to summer precipitation. Stomatal conductance to water vapor increased in response to water in both years, by 80% in 2021 and by 500% in 2023. Our results suggest that the average monsoon precipitation pulse of 10 mm does not induce upregulation of photosynthesis, but precipitation pulses of sufficient magnitude can induce upregulation of multiple aspects of photosynthesis even at one of the hottest times of the year. Pre-watering differences between years were negligible, suggesting that antecedent effects dissipate within three and a half months without rain. Predicted decreases in monsoon rainfall may influence future summertime carbon gain by Joshua trees in the Mojave Desert.