Coast redwood, a popular landscape tree in Southern California, is exposed to altered environmental conditions compared to its native habitat, where precipitation and fog drip are essential sources of water. In the semi-arid climate of the Los Angeles Basin, where few tree species grow naturally, cultivated coast redwood trees are irrigated, yet exposed to unusually high atmospheric vapor pressure deficit (VPD). As coast redwood trees are reported to have poor stomatal regulation in their native habitat, high VPD may cause a loss of xylem hydraulic conductivity due to air embolism. Observations that irrigated coast redwood in the Los Angeles Basin are difficult to grow led to the following questions: Is VPD the major driver of sap flow rate in this species? Does weak stomatal regulation lead to excessive air embolism of xylem, and water stress? If so, what are the relative roles of atmospheric and soil water stress? To address these questions, we measured sap flux in the stems of 5-9 trees at three irrigated locations in the Los Angeles Basin. We also assessed the hydraulic properties and percent loss of hydraulic conductivity of xylem of branches using laboratory methods.
Surprisingly, the major factor that explained temporal variability in sap flux was photosynthetically active radiation (PAR). Sap flux increased linearly with PAR, contrary to our hypothesis that VPD would drive most of the temporal variability in sap flux. We examined the residuals of the relationship between PAR and sap flux to evaluate stomatal response to VPD. Unlike the stomatal response reported for coast redwood in its native habitat, we observed stomatal closure at high VPD. Seasonal water stress was indicated by the decline of xylem hydraulic conductivity from May through August, when the highest sap flow rates were measured, with recovery from October through January, when the sap flow rates were lower. We attributed water stress to atmospheric rather than soil moisture stress, as soil moisture values remained high at all sites throughout the study period. The results of this study imply that coast redwood receives adequate irrigation in these urban sites, but the stomatal response to other meteorological variables limits its performance in this semi-arid environment.