AN ANALYSIS OF BALLS EMPIRICAL-MODEL OF STOMATAL CONDUCTANCE

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 COa 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 C02 concentration. The model was tested against data from experiments in which the responses of stomatal conductance and COa 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 Balls model is a useful starting point for the 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. © 1993 Oxford University Press.