What was done
The authors used the ecosys crop growth model in an attempt to simulate wheat biomass production in response to elevated CO2 at low and high soil moisture contents.  As key inputs, the authors considered stomatal resistance to vary directly with the air-to-leaf CO2 concentration gradient, inversely with leaf carboxylation rate, and exponentially with leaf turgor.

What was learned
Model predictions of wheat biomass were fairly consistent with observed values measured in a FACE experiment conducted on spring wheat grown at atmospheric CO2 concentrations of 350 and 500 ppm near Maricopa, Arizona, USA.  At ambient CO2, for example, biomass (g m^-2) was observed to be 1361 ± 97 and 1856 ± 145 under low and high soil moisture regimes, respectively, while corresponding simulated values were 1390 and 2050.  Likewise, at elevated CO2 observed biomass values were 1604 ± 151 and 2044 ± 167 under low and high soil moisture regimes, respectively, while corresponding simulated values were 1710 and 2240.  Consequently, the observed CO2-induced percentage biomass increases were 18% and 10% under low and high soil moisture regimes, respectively, while the corresponding simulated increases were 23% and 9%.

What it means
The authors demonstrated that the ecosys crop growth model did a good job of simulating the greater CO2-induced percentage growth increase in spring wheat under limiting vs. non-limiting soil moisture conditions.  Consequently, and because of the basic nature of the plant physiological processes simulated in the model, the authors believe it can be used to project the productivity responses of many additional crops to atmospheric CO2 enrichment under a wide range of soil, management and climate conditions, thus helping to better forecast agricultural responses to the ongoing rise in the air's CO2 content.