California’s recent historic drought has provided ecophysiologists an unprecedented opportunity to investigate the physiological responses of different plant species to exceptionally severe water deficits, and to better understand the implications of such responses on the processes of carbon starvation, hydraulic failure, and drought-induced mortality. To capitalize on this opportunity, we have been monitoring the physiological performance of adult blue oaks (Quercus douglasii, Fagaceae) and valley oaks (Q. lobata) across three sites that capture extensive variation in climate and overall exposure to this drought. Since the summer of 2014, we have been taking seasonal measurements of stomatal conductance and plant water status (pre-dawn and midday shoot water potentials), and pairing them with seasonal stable isotope ratio analyses of leaf tissues (bulk leaf δ13C and leaf photosynthate δ13C). Together, these data have allowed for the assessment of how the physiological responses of these two long-lived trees varied in response to drought severity and seasonal changes in site-specific water availability, and by extension, how these responses impacted the extent of carbon fixation through this event.
Site-specific variation in soil water availability and accumulated water deficit through the drought translated into significant variation in the extent of carbon fixation between these two oak species. As hydric deficit intensified through the 2014-2016 growing seasons, both plant water status and leaf-level transpiration became increasingly impaired. Significant relationships between seasonal measurements of shoot water potential and stomatal conductance indicated that trees with greater soil water availability through the drought could support higher degrees of transpiration throughout the day. Consequently, trees that could support higher degrees of transpiration through the drought consistently fixed more isotopically-depleted sugars than trees with lower stomatal conductance. This differential extent of carbon fixation was also revealed by significant seasonal relationships between bulk leaf δ13C values and leaf photosynthate δ13C values for both species, which suggests that the carbon cost of the drought was isotopically imprinted in the carbon pools that these trees were constructing their leaves from each growing season. Taken together, these results indicate the importance of sustained water resources for these long-lived trees to survive and maintain carbon gain during such extreme climatic events, which are only expected to become more common and severe in California’s future due to climate change.