Direct impact of atmospheric CO2 enrichment on regional transpiration

Plant physiological research has revealed that stomatal aperture of many plant species is reduced by CO 2 . Therefore, the question has been raised as to how transpiration will be affected if the ambient C0 2 concentration increases. This study focuses on the prediction of changes in transpiration at the regional scale (10-100 km horizontal, 1-5 km vertical). A rather detailed, coupled vegetation- Planetary Boundary Layer (PBL) model has been constructed in order to identify important processes that control such changes.The coupled model uses the well-known "big-leaf' model for the vegetation part. Surface resistance (r s ) is described by means of an up-scaled ''A-g s '' model, where stomatal conductance is related to photosynthetic rate. The background of this model for (r s ) is outlined. A new parameterization to mimic stomatal humidity responses is proposed. The parameterization prescribes a linear relation between the specific humidity deficit at the leaf surface and the ratio of the internal C0 2 concentration to the external C0 2 concentration. The resulting ''A-g s '' model simulates stomatal responses to CO 2 , light, temperature, humidity as well as their synergistic interactions. The model is tested using data for grapevines ( VitisVinifera L., cv. Airen). The model is able to simulate the photosynthetic rate and the stomatal conductance of this species satisfactorily.The PBL part of the coupled model is a 1D, first-order closure model, which takes into account nonlocal turbulent transport by means of a countergradient correction. The PBL model also simulates C0 2 fluxes and concentrations. The surface flux of C0 2 is driven by photosynthetic rate from the up-scaled ''A-g s '' model. The complete coupled model realistically simulates the state of the PBL, (r s ) transpiration, and the most important aspects of the biosphere-atmosphere interaction for extensive, homogeneous, well-watered canopies with dry leaves.Systematic sensitivity studies using the coupled model reveal that the interaction between vegetation and the PBL has a significant effect on transpiration and on (r s ) On the one hand, the PBL provides a strong negative feedback on transpiration which reduces the change in the transpiration at given change in (r s ) The influence of the PBL depends strongly on surface roughness. On the other hand, the simultaneous change of (r s ) and of the specific humidity deficit inside the canopy provides a positive feedback, thereby increasing the initial perturbation of (r s ) and transpiration. A second positive feedback mechanism is present if the optimum temperature for photosynthesis is exceeded.The main conclusion is that the interaction between the PBL and vegetation has to be taken into account if transpiration and its changes due to changing surface characteristics are to be predicted at the regional scale.