- A natural experiment on plant acclimation: lifetime stomatal frequency response of an individual tree to annual atmospheric CO2 increase. Wagner, F., Below, R., Klerk, P.D., Dilcher, D.L., Joosten, H., Kürschner, W.M., Visscher, H. Proc. Natl. Acad. Sci. U.S.A. (1996)