Modelling CO2-enrichment effects using an interactive vegetation SVAT scheme

A new version of the ISBA-A-gs (interactions between soil, biosphere, and atmosphere, CO2-reactive) scheme is employed to simulate the observed plant growth and energy and water budget of perennial ryegrass swards grown in highly ventilated plastic tunnels. The model is modified in order to represent the greenhouse conditions, and best-fit vegetation parameters are proposed for two nitrogen fertiliser supplies. The simulations are performed in ambient or doubled CO2 concentration, and with a 3°C increase in air temperature in doubled [CO2] conditions. It is shown that in the case of perennial ryegrass, the ratio between leaf area index and active biomass depends on both nitrogen supply and climate conditions. A simple parameterisation of this variable is proposed, based on a plant nitrogen decline model. The enhanced model still needs a small number of parameters (only five) and is able to describe leaf area index, the total biomass, the nitrogen use, and the dry matter yield. Finally, a control 10-year simulation using the new version of ISBA-A-gs is performed and a sensitivity study shows that the model is able to simulate contrasting responses of leaf area index to CO2 or nitrogen enrichment.