Past and future scenarios of the effect of carbon dioxide on plant growth and transpiration for three vegetation types of southwestern France

Abstract. The sensitivity of an operational CO₂-responsive land surface model (the ISBA-A-gs model of Météo-France) to the atmospheric CO₂ concentration, (CO₂), is investigated for 3 vegetation types (winter wheat, irrigated maize, coniferous forest). Past (1960) and future (2050) scenarios of (CO₂) corresponding to 320 ppm and 550 ppm, respectively, are explored. The sensitivity study is performed for 4 annual cycles presenting contrasting conditions of precipitation regime and air temperature, based on continuous measurements performed on the SMOSREX site near Toulouse, in southwestern France. A significant CO₂-driven reduction of canopy conductance is simulated for the irrigated maize and the coniferous forest. The reduction is particularly large for maize, from 2000 to 2050 (−18%), and triggers a drop in optimum irrigation (−30 mm y⁻¹). In the case of wheat, the response is more complex, with an equal occurrence of enhanced or reduced canopy conductance.