Predicting of regional transpiration at elevated atmospheric CO2: influence of the PBL vegetation interaction.

A coupled planetary boundary layer (PBL)-vegetation model is used to study the influence of the PBL-vegetation interaction and the ambient CO2 concentration on surface resistance rs and regional transpiration E. Vegetation is described using the big-leaf model in which rs is modeled by means of a coupled photosynthesis-resistance model. The PBL part is a one-dimensional, first-order closure model. Nonlocal turbulent transport is accounted for by means of a countergradient correction. The PBL model also describes CO2 fluxes and concentrations, which are driven by photosynthesis of the canopy. A number of sensitivity analyses are presented in which the behavior of rs and E at an atmospheric CO2 concentration representative for the present-day situation is compared to their behavior under an approximately doubled CO2 concentration. The results reveal a positive atmospheric feedback on rs, by which an initial increase of rs, due to changes in ambient CO2 concentration, is magnified. The stomatal humidity response appears to be the key factor here: if rs increases, the air within the canopy dries out, which causes the stomata to close further. The PBL enlarges the effect of this positive feedback loop. The model suggests plants with a C4 photosynthetic pathway to be less sensitive to the humidity-mediated positive feedback than plants with a C3 photosynthetic pathway. Another important aspect of biosphere-atmosphere interaction is the negative feedback of the PBL on transpiration. It is concluded that the interaction between PBL and the 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. This conclusion applies to modeling studies as well as to extrapolation of results from plant physiological research or from small-scale field plots to the regional scale.