The treatment of vegetation in land surface models: implications for predictions of land-atmosphere exchange

Plant processes affect fluxes of energy, moisture and CO2 between the land and the atmosphere. Land surface models need to correctly represent the vegetation functioning and its response to environmental conditions. Due to anthropogenic carbon emissions rising, and global warming, plant processes are being affected and in turn modulate the terrestrial carbon sink. However, models still disagree on the response of plants to changing conditions. This work analyses how vegetation is treated in two land surface models: the Joint UK Land Environment Simulator (JULES) and Carbon Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land (CTESSEL). The aim is to analyse how environmental variables control the vegetation processes at daily and seasonal timescales at present day climate and the changes that arise in a scenario of double atmospheric CO2 and higher temperature. The analyses are carried out at the leaf level and at the canopy level. To investigate the responses at the leaf level, the photosynthesis scheme used in each model was extracted, thereby providing a submodel that can be run in stand alone mode. The photosynthesis submodel provides a means to analyse the leaf level response of each photosynthesis model to environment variables as well as the internal model parameters that characterise each plant type. In JULES the environmental controls on photosynthesis are explicitly introduced by three limiting regimes: light, rubisco (carbon) or export limiting regime. In CTESSEL the carbon and light limitations are implicitly represented but there is no export limitation. Due to the lack of export limiting regime, CTESSEL presents higher sensitivity to CO2 concentration resulting in a stronger CO2 fertilization effect. The carbon and energy fluxes produced by the full land surface models were tested and compared at 10 European FLUXNET sites. The main differences between modellled carbon fluxes were found to be the treatment of soil moisture stress and the lack of export limiting regime in CTESSEL. The optimum temperature for photosynthesis in models is the result of model parameters’ dependence on temperature and the combination of limiting regimes. The optimum temperature for photosynthesis was found to be a determining element in the strength and sign of the vegetation modelled feedback to climate change