Modelling of stomatal density response to atmospheric

Stomatal density tends to vary inversely with changes in atmospheric CO 2 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 CO 2 , 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.