Anderegg, William R. L., Wolf, Adam, Arango-Velez, Adriana, Choat, Brendan, Chmura, Daniel J., Jansen, Steven, Kolb, Thomas, Li, Shan, Meinzer, Frederick, Pita, Pilar, Resco de Dios, Víctor, Sperry, John S., Wolfe, Brett T., and Pacala, Stephen
Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbon‐maximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle. W.R.L.A. acknowledges funding for this research from NSF 1714972 and from the USDA National Institute of Food and Agriculture, Agricultural and Food Research Initiative Competitive Programme, Ecosystem Services and Agro-Ecosystem Management, grant no. 2017-05521. We thank T. Brodribb and one anonymous reviewer for their insightful reviews, B. Medlyn and Y.S. Lin for sharing data and R. Norby for providing Vcmax data for several species. We appreciate the assistance from Marion Feifel in collecting data of leaf photosynthetic parameters of five European tree species. S.L. acknowledges financial support from the China Scholarship Council (CSC). VRD acknowledges funding from a Ram on y Cajal fellowship (RYC-2012-10970). B.T.W. was supported by the Next Generation Ecosystem Experiments-Tropics, funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research. DJC acknowledges funding from the National Science Centre, Poland (NN309 713340).