Linking Hydraulic Responses to Gas Exchange and Water Stress in California Oaks

Marissa Ochoa, University of California, Los Angeles, UCLA Ecology & Evolutionary Biology

Camila Medeiros, University of California, Los Angeles, Department of Ecology and Evolutionary Biology

Victoria L. Sork, University of California, Los Angeles, Department of Ecology and Evolutionary Biology

Lawren Sack, University of California, Los Angeles, Department of Ecology and Evolutionary Biology

Across the globe species in Mediterranean-type ecosystems face mortality because of continued alterations in temperature and precipitation regimes from climate change. Plant hydraulic traits are useful measures of plant adaptations to drought. Studies across oak (Quercus) species have established that leaf traits are involved in their ability to tolerate plant drought tolerance. Here, we tested relationships of leaf hydraulic conductance and stomatal conductance with climatic aridity for nine Quercus species grown at the California Botanic Garden (formerly known as Rancho Santa Ana Botanic Garden) in Claremont, California. We sampled eight Quercus species, 3 – 4 individuals per species representing the sections Quercus, Protobalanus, and Lobatae, from the months of March through June of 2019.

To obtain leaf hydraulic and stomatal conductances, well-hydrated and dehydrated leaves representing a range of water potentials were collected and used for each method. Leaf hydraulic conductance was measured using the evaporative flux system and was calculated as the flow rate through the leaf divided by leaf water potential and normalized by leaf area. Stomatal conductance was measured using a porometer on the abaxial side of the leaf. After each measurement, leaf water potential was obtained using a pressure chamber and curves were obtained as the plot of conductances versus negative water potential. By plotting curves representing vulnerability to embolism, we were able to quantify leaf hydraulic vulnerability and stomatal vulnerability to stomatal closure. We present the relationships of hydraulic traits with climatic aridity. Overall, these results suggest high variability within this genus that can be explained by climatic adaptation.