# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 4 Discussion  
### Canopy Stomatal Conductance  
Land components of climate and carbon models require accurate descriptions of the stomatal control of canopy energy exchange, evapotranspiration, and carbon exchange because land surfaces provide a continuous feedback of latent and sensible heat fluxes to the atmosphere, which drives weather and climate [45]. The method developed in this study expands the usefulness of controlled environments for improving land surface models because it allows the measurement of responses of canopy-level G\({}_{\text{S}}\), an essential control of canopy gas exchange, to environmental variables. In this study, G\({}_{\text{S}}\) was measured when surface radiation forcing (R\({}_{\text{net}}\)), boundary layer forcing (T\({}_{\text{air}}\) & D\({}_{\text{Bulk}}\)), surface layer feedbacks (g\({}_{\text{A}}\)), and soil moisture were held constant during the photoperiod. Furthermore, the use of well-watered plant stands grown at constant light reduced much of the environmental variability that confounds estimates of G\({}_{\text{S}}\) in natural ecosystems, such as periodic drought, the diurnal change in solar radiation, or short, temporal fluctuations in radiation due to cloud cover. Moreover, the carbon and water vapor fluxes measured in this study do not include significant contributions from soil respiration and evaporation as compared to field measurements. Canopy G\({}_{\text{S}}\) was derived from direct measurements of surface G\({}_{\text{SFC}}\), g\({}_{\text{A}}\), and energy balance (R\({}_{\text{net}}\), LE, and P) in controlled environments. Once the IR transducers were positioned above the canopy, chamber CO\({}_{2}\) concentration was manipulated to alter stomatal conductance, which in turn resulted in corresponding changes in sensible heat flux and the canopy-air temperature difference. Canopy g\({}_{\text{A}}\) was obtained radiometrically from the slope of a plot of H vs. \(\Delta\)T\({}_{\text{IR}}\) (Figure 7), and the offset correcting for differences between T\({}_{\text{R}}\) and T\({}_{\text{aero}}\) was determined. The radiometric method presented here differs from other methods for calculating G\({}_{\text{S}}\) because canopy g\({}_{\text{A}}\), E\({}_{\text{can}}\), and canopy-to-air temperature differences are measured directly. This avoids the complexity of methods for scaling leaf level observations to the canopy scale because these must integrate responses of leaf stomatal conductance to vertical profiles in radiation, temperature, humidity, and wind speed within a canopy. Separating g\({}_{\text{A}}\) from the measured canopy G\({}_{\text{SFC}}\) permits the determination of the physiologically controlled canopy-scale G\({}_{\text{S}}\), which is equivalent to the \"big-leaf\" stomatal conductance, where the stomatal conductances of individual leaves of the canopy act in parallel, and the vertical gradients in temperature and humidity are averaged by the aerodynamic T\({}_{\text{Aero}}\) and D\({}_{\text{Aero}}\). The strength of this approach is that canopy G\({}_{\text{S}}\) responses are measured at the correct scale for predicting ET in future elevated CO\({}_{2}\) and climate change scenarios. Furthermore, estimates of canopy ET made using the measured G\({}_{\text{S}}\) also account for the feedback of Ds on ET, as shown by the decoupling coefficient. In the chamber, g\({}_{\rm A}\) was set constant by the air flow rate provided by its recirculation fans. However, the g\({}_{\rm A}\) established in the chamber was different for each species when measured at the same turbulent field provided by the chamber fans. The g\({}_{\rm A}\) for the wheat (5.5 mol m\({}^{-2}\) s\({}^{-1}\) or 7.5 s m\({}^{-1}\)) and for the soybean (2.5 mol m\({}^{-2}\) s\({}^{-1}\) or 16.5 s m\({}^{-1}\)) canopies were within the range of typical aerodynamic conductances of field crops (ranging from 3.2-10 mol m\({}^{-2}\) s\({}^{-1}\) or 4-13 s m\({}^{-1}\); [46]).