Wheat Crop Biomass and Grain Yield Response to Elevated CO2 in Australia.

Response of wheat crops to elevated CO₂ has been reported to range from 5 to 40% from various FACE and controlled environment experiments.  A recent FACE experiment, conducted in a dryer and lower yielding environment in Australia reported averaged responses between biomass and ultimate grain yield of around 25% averaged over 3 years and a range of agronomic treatments including supplementary irrigation (O’Leary et al 2015 DOI: 10.1111/gcb.12830).   There was some variability with individual treatments with the measured response of biomass to elevated CO₂ at 40% under warmer temperatures from late sowing and under irrigation. However, varied simulated responses reported in that comparative study, were all much closer to the 25% measured averaged response.

Examination of individual treatments in specific years show even a larger apparent response to elevated CO₂ ranging from –5 to +70% for biomass to anthesis and from –11 to +70% for final grain yield.  An important question is how well do our suite of simulation models perform under such conditions?  None of the six models in the original study of O’Leary et al. (2015) predicted an averaged response to elevated CO₂ of greater than 30% but the predicted model range of response which varied from 17 to 28% was within the measurement error.  We propose that the CO₂ response under warmer temperatures and drought and well watered conditions be further explored experimentally and particularly from a modelling perspective.  An important modelling question is - what is the range of response from other untested models?  This might be a task suitable for the international Agricultural Model Intercomparison and Improvement Project (AgMIP) network.