The perplexing drought response strategy of Eastern US oaks: Extending site-level conclusions to regional scales

Background/Question/Methods: The many ecological, economic, and functional benefits of Eastern US oak (Quercus) species are endangered by an ongoing decline in their relative abundance. While still a matter of debate, the decline of oaks is probably caused by multiple drivers, including fire suppression, increasing precipitation, and episodic mortality, that have direct or indirect links drought, and how oaks and co-dominant species respond to hydrologic stress. A holistic, mechanistic understanding of the drought-response strategies of oaks and their neighbors is critical for understanding the historic and future fates of these keystone species. The overall objective of this study is to revisit long-held assumptions about the mechanisms by which oaks respond to drought stress, with a focus on linking our results to current eco-physiological theory. We specifically address the question: “Compared to neighboring co-dominants, are oaks more or less sensitive to drought, and why?,” We will consider a diverse range of traits and response variables linked to drought, with a particular emphasis on traits describing tree hydraulic function.

Results/Conclusions: We find that the gas exchange and growth of oaks is often unresponsive to soil drought, whereas maples and other mesophytic species close stomates more quickly as soil water declines. However, we also demonstrate that the sensitivity of stomatal conductance to atmospheric drought (e.g. rising vapor pressure deficit, or VPD) is greatest for trees like oaks that experience relatively little soil moisture limitation. As a result, oaks will likely experience greater limitation to gas exchange in future climates characterized by higher VPD. The stomatal response is insufficient to prevent oaks from experiencing low leaf water potentials and small safety margins, which can increase the risk of xylem cavitation and hydraulic failure. Surprisingly, oaks adopt this strategy despite the fact that they have xylem that are especially vulnerable to cavitation (e.g. higher “P50” – the water potential associated with a 50% loss in conductivity). Using a combination of cross-site synthesis, literature meta-analysis, and regional-scale analysis of information contained in tree-ring networks and forest inventories, we show that while oaks are likely to grow faster and sequester more CO₂ during drought than their neighbors, often, they are often also more likely to die. We end by discussing how what we’ve learned about oak drought response strategy can inform efforts to restore and regenerate oaks across the region, under current and future climate regimes.