An Analysis of Ball's Empirical Model of Stomatal Conductance

Abstract

We analysed Ball's empirical model of stomatal conductance (Ball, Woodrow and Berry, 1987). The original interpretation was found to be flawed, and a new one is proposed which views the model as a description of the relationship between CO2 flux rate and stomatal conductance, rather than as a model of stomatal conductance alone. It is shown that this model is useful for describing changes in intercellular CO2 concentration. The model was tested against data from experiments in which the responses of stomatal conductance and CO2 flux density to humidity and temperature were measured in leaves of Hedera helix. It was found that the responses to temperature and humidity are not treated in a satisfactory way in the model whereas the response of the model to other variables is realistic. An alternative to Ball's model is described and tested. It is concluded that Ball's model is a useful starting point for development of simulation models to be used as submodels in canopy and regional models. However, as any empirical model, it is of no use for defining causal relationships.