Modelling tropical forest responses to elevated CO2 in a nutrient-limited environment: JULES simulations at the AmazonFACE site

The AmazonFACE programme (Free-Air CO2 Enrichment) will subject areas of old-growth forest in the Amazon basin near Manaus to elevated (+200ppmv) CO2 concentrations to determine the effects on vegetation.  While elevated CO2 is assumed to have significant fertilisation effects, both increasing productivity and reducing transpiration, this needs to be measured in the field.  The fertilisation effects may not be as great as predicted due to limitations from the available nutrients, and soil in this part of the Amazon is known to be poor in phosphorus.  The programme will be supported by modelling, including a version of JULES (the Joint UK Land Environment Simulator) called JULES-CNP which includes phosphorus dynamics and its interactions with the nitrogen and carbon cycles.  Here this version has been especially set up to work in the forest near Manaus by using observation-based soil chemistry and weather data.  JULES shows large increases in productivity (and in litter, soil carbon and respiration from the soil) with elevated CO2 without the phosphorus limitations, but significantly smaller increases in productivity when the phosphorus dynamics and interactions are included.  The current JULES setup and configuration can be used with experimental investigation in AmazonFACE to inform future model improvements.