title: Responses to atmospheric CO2concentrations in crop simulation models: a review of current simple and semicomplex representations and options for model development.
abstract: Elevated atmospheric CO2concentrations ([CO2]) cause direct changes in crop physiological processes (e.g. photosynthesis and stomatal conductance). To represent these CO2responses, commonly used crop simulation models have been amended, using simple and semicomplex representations of the processes involved. Yet, there is no standard approach to and often poor documentation of these developments. This study used a bottom-up approach (starting with the APSIM framework as case study) to evaluate modelled responses in a consortium of commonly used crop models and illuminate whether variation in responses reflects true uncertainty in our understanding compared to arbitrary choices of model developers. Diversity in simulated CO2responses and limited validation were common among models, both within the APSIM framework and more generally. Whereas production responses show some consistency up to moderately high [CO2] (around 700 ppm), transpiration and stomatal responses vary more widely in nature and magnitude (e.g. a decrease in stomatal conductance varying between 35% and 90% among models was found for [CO2] doubling to 700 ppm). Most notably, nitrogen responses were found to be included in few crop models despite being commonly observed and critical for the simulation of photosynthetic acclimation, crop nutritional quality and carbon allocation. We suggest harmonization and consideration of more mechanistic concepts in particular subroutines, for example, for the simulation of N dynamics, as a way to improve our predictive understanding of CO2responses and capture secondary processes. Intercomparison studies could assist in this aim, provided that they go beyond simple output comparison and explicitly identify the representations and assumptions that are causal for intermodel differences. Additionally, validation and proper documentation of the representation of CO2responses within models should be prioritized.
authors.affiliation: Division of Soil and Water management, Department of Earth & Environmental Sciences, KU Leuven, Celestijnenlaan 200E, po 2411, B-3001, Heverlee, Belgium.
authors.forename: Eline
authors.identifier: 0000-0002-1247-6183
authors.initials: E
authors.lastname: Vanuytrecht
authors.affiliation: CSIRO Agriculture and Food, 306 Carmody Road, St Lucia, Qld, 4068, Australia.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Thorburn
mesh_terms: D002244:Carbon; D002245:Carbon Dioxide; D000069599:Crop Production; D008962:Models, Theoretical; D009584:Nitrogen; D010788:Photosynthesis
chemical_list: D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen
keywords: 
  APSIM
  ; N dynamics; climate change; crop modelling; elevated CO2 concentration; model improvement; modelled crop response