Modelling of stomatal density response to atmospheric CO2

Stomatal density tends to vary inversely with changes in atmospheric CO2 concentration (C-a). This phenomenon is of significance due to: (i) the current anthropogenic rise in C-a and its impact on vegetation, and (ii) the potential applicability for reconstructing palaeoatmospheric C-a by using fossil plant remains. It is generally assumed that the inverse change of stomatal density with C-a represents an adaptation of epidermal gas conductance to varying C-a. Reconstruction of fossil C-a by using stomatal density is usually based on empirical curves which are obtained by greenhouse experiments or the study of herbarium material. In this contribution, a model describing the stomatal density response to changes in C-a is introduced. It is based on the diffusion of water vapour and CO2, photosynthesis and an optimisation principle concerning gas exchange and water availability. The model considers both aspects of stomatal conductance: degree of stomatal aperture and stomatal density. It is shown that stomatal aperture and stomatal density response can be separated with stomatal aperture representing a short-term response and stomatal density a long-term response. The model also demonstrates how the stomatal density response to C-a is modulated by environmental factors. This in turn implies that reliable reconstructions of ancient C-a require additional information concerning temperature and humidity of the considered sites. Finally, a sensitivity analysis was carried out for the relationship between stomatal density and C-a in order to identify critical parameters (= small parameter changes lead to significant changes of the results). Stomatal pore geometry (pore size and depth) represents a critical parameter. In palaeoclimatic studies, pore geometry should therefore also be considered. (C) 2008 Elsevier Ltd. All rights reserved.