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

The sensitivity of an operational CO2-responsive land surface model (the ISBA-A-gs model of Meteo-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.