Abstract: Factors affecting forest drought response include competition, climate, site and tree characteristics. Northern California spans a variety of site conditions and includes habitat for diverse conifers with different adaptations and requirements to explore the effects of these factors. This study used annual 13C discrimination and growth metrics to assess drought resistance and resilience in two coastal and four montane conifer species, and the environmental factors that affect these drought responses. Using 54 study sites (9 sites per species and 540 trees total), we found large differences in drought survival strategies between coastal and montane species. Generally, coastal species had lower drought resistance and resilience than montane species. Isotopic data indicated that coastal species regulated stomata during drought while montane species did not, suggesting that coastal species may be more vulnerable to long-term droughts while montane species may be more vulnerable to severe droughts. Isotopic data for coastal species indicated physiological resilience, suggesting the potential for drought recovery if drought conditions are infrequent. While coastal species had similar drought responses influenced by climate, site conditions, and competition, drought responses in montane trees varied markedly with species, site conditions, habitat moisture availability, and competition. The six conifers in this study weathered the 2012 – 2015 drought in Northern California with reasonably high resistance and resilience, yet our results suggest that trees may be at risk for increased stress and mortality in the event of longer or more frequent, severe droughts.
