Principal Investigator

At a Glance

Plants lose water and take up carbon through stomates, and the ability to simulate their behavior under various conditions is an essential part of global climate models. The Pacala group has developed and tested a new hypothesis of stomate regulation that improves upon current models in predicting stomate behavior during drought.


Research Highlight

Tiny valves on the surfaces of leaves, called stomates (Figure 1.1.), regulate carbon gain and water loss by plants, and are thus linchpins of the global carbon and water cycles. Amazingly, a simple equation regulates stomates worldwide. This equation is backed by enormous empirical data and a 40-year-old evolutionary explanation, and controls carbon gain and water loss in all Earth System models that predict climate. It is one of the most widely accepted paradigms in ecology and has been taught for decades in introductory biology courses worldwide, including Princeton’s.

Figure 1.1. A stomate on the surface of a cucumber leaf.

Nonetheless, neither the simple model nor the evolutionary hypothesis explains observed stomatal closure during drought. Moreover, the 40-year-old evolutionary hypothesis is not consistent with the current understanding of plant competition for water, and does not include recent discoveries about damage to plant hydraulic systems during drought.

The Pacala group developed an alternative hypothesis that includes plant competition and hydraulic damage, such as impaired water flow through xylem. The new hypothesis has the same empirical support as the classical hypothesis under non-drought conditions and also predicts observed stomatal closure during drought.

A statistical test was developed to explicitly separate the classical and new hypotheses, and assembled a global data set that could be used with the test. The results unanimously support the new hypothesis over the classical hypothesis. We have since built the new model of stomatal control into our Earth System Model. Early tests imply improvements in both the carbon and hydrologic cycles, particularly in tropical forests. This work is timely because of recent studies implying that drought has reduced the Amazon carbon sink.



Anderegg, W.R.L., A. Wolf, A. Arango-Velez, B. Choat, D.J. Chmura, S. Jansen, T. Kolb, S. Li, F. Meinzer, P. Pita, V. Resco de Dios, J.S. Sperry, B.T. Wolfe, and S.W. Pacala. Stomata are regulated to manage hydraulic damage: empirical evidence and global consequences. Nature, in revision.

Anderegg, W.R.L., A. Wolf, A. Arango-Velez, B. Choat, D.J. Chmura, S. Jansen, T. Kolb, S. Li, F. Meinzer, P. Pita, V. Resco de Dios, J.S. Sperry, B.T. Wolfe, and S.W. Pacala. Plant water potential improves prediction of stomatal models. Plant Cell Environ., in review.

Chou, C., L.O. Hedin, and S.W. Pacala. Functional groups, species, and light interact with nutrient limitation during tropical rainforest sapling bottleneck. J. Ecol., in revision.

Muller-Landau, H.C, and S.W. Pacala. What determines the abundance of lianas? In A. Dobson, D. Tilman, and R. Holt, Eds., for the volume Unsolved Problems in Ecology, in press.

Ocko, I.B., S.P. Hamburg, D.J. Jacob, D.W. Keith, N.O. Keohane, M. Oppenheimer, J.D. Roy-Mayhew, D.P. Schrag, and S.W. Pacala. Two-valued global warming potential effectively captures long- and short-term climate forcing. Science, in press.

Pellegrini, A.F.A, W.A. Hoffmann, G. Durigan, A.Tourgee, and S.W. Pacala. Overlapping woody plant composition and intermediate vegetation formations across savannas and forests in the Neotropical Brazilian Cerrado. Ecography, in review.

Uyehara, I.K.U., and S.W. Pacala. The role of succession in the evolution of flammability. Theor. Ecol., in review.

Xu, X., D. Medvigy, S.J. Wright, K. Kitajima, J. Wu, L. Albert, G. Martins, S. Saleska, and S.W. Pacala. Variations of leaf longevity in tropical evergreen forests predicted by a trait-driven carbon optimality model. Ecol. Lett., in revision.