Plant Water Relations during Drought-Induced Dieback in Greenbark Ceanothus (Ceanothus spinosus) in the Santa Monica Mountains

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This experiment investigated Ceanothus spinosus dieback in the natural landscape of Pepperdine University’s campus in Malibu, California. We measured midday water potential, gas-exchange rates (photosynthesis and transpiration), chlorophyll fluorescence, and native embolism levels of stem xylem, comparing irrigated controls on campus to plants undergoing severe water stress during unprecedented drought in southern California. The water potentials for naturally occurring C. spinosus (-6.6 MPa) was much lower than for irrigated controls (-3.4 MPa). Water stressed plants experience 47.2% embolism in their stem xylem that blocks water transport to leaves compared to 12.6% embolism in irrigated controls. This high embolism level matched previously obtained vulnerability curves to water stress-induced embolism. These findings were consistent with the observation of much lower photosynthetic rates, transpiration rates, and chlorophyll fluorescence parameters in water stressed versus control plants. Taken together, these data suggest the historic drought between 2012 and 2016, in southern California, was the ultimate cause of the observed dieback and whole plant mortality on the Pepperdine University campus. This pattern likely extends to the entire range of the Santa Monica Mountains and may represent vegetation response to erratic changes in climate now emerging in southern California.