In the summer of 2015, as California was in its fourth consecutive year of drought, we launched a citizen science flash sampling project to obtain a unique data set capturing the drought response of blue oak (Quercus douglasii) across its entire range. Widespread reports of drought stress, defoliation, and mortality across the state motivated our labs to seek participants to sample trees at local sites (N=46) during the peak of the 2015 dry season (September-October) to assemble a dataset to a) test for spatial patterns in tree water status and drought stress as a function of regional climate and landscape position and b) provide a baseline to quantify future patterns of mortality and recovery. By combining physiological and isotopic measurements of trees (N=557; leaf morphology, canopy health, xylem water stable isotopes, carbon and nitrogen stable isotopes) with a landscape hydrology model (Basin Characterization Model, BCM) that captures the effects of climate and soils on water deficits, our study seeks to improve the ability to predict how warmer, drier conditions will impact the performance of Q. douglasii across its geographic range.
Canopy brownness/bareness of Q. douglasii, estimated visually by participants, was correlated with drought intensity, measured as the difference between 2015 climatic water deficit (CWD) and 1981-2010 baseline CWD (r2=0.13, p<0.05). Our 2015 flash sampling sites captured the variation in water deficit across California, with southern Sierra Nevada foothill sites having the highest relative change, and northern sites having the lowest relative change. Xylem water stable isotopes indicate varied sources of water for Q. douglasii individuals across its geographic range (𝛿18O: -3 to -12 ; 𝛿2H: -39 to -88), and aid in predicting plant performance across climatic and topographic gradients. Further analyses with local meteoric water lines and groundwater stable isotopes will differentiate between use of deeper vs. more shallow sources of water and will inform predictions of future responses of Q. douglasii to warmer and drier conditions, and to extreme events (e.g. drought), that may lead to shifts in plant water availability.
Integration of BCM projections with this physiological and isotopic data, in supplement to data obtained between the 2014 and 2016 drought years at three Northern-Central Californian sites, will identify mechanisms underlying the responses of tree species in California oak woodlands to spatial and temporal variability in water deficits.