# Radiometric Method for Determining Canopy Stomatal Conductance in Controlled Environments
## 3 Results  
### Wind and Temperature Profiles  
Average wind speed and air temperature profiles were measured at different heights above and within wheat and soybean canopies in a ventilated chamber. The mean wind speed at any given plane above the canopy was highly spatially and temporally variable, typically ranging from 0.5 to 2.4 m s\({}^{-1}\) in wheat (Figure 2A), and from 0.4 to 1.4 m s\({}^{-1}\) in soybean (Figure 2B). The average wind speed at the canopy surface was attenuated rapidly within the first few centimeters of foliage. Wind speed within the canopies was more uniform than above and was often below 0.4 m s\({}^{-1}\), reaching as low as 0.1 m s\({}^{-1}\) at the bottom of the wheat canopy.  
Vertical air temperature profiles within the growth chamber were homogeneous in an empty, dark chamber since there was no foliage to trap pockets of air, and because the surfaces within the chamber (glass and chamber walls, and the surface of the growth media) equilibrated at nearly the same temperature. When the lights were turned on, the vertical air temperature profiles within the canopy were spatially variable; air near the plants could be 1-5 \({}^{\circ}\)C higher than the reference air temperature measured above them depending on the relative magnitudes of incident radiation or wind speed within the chamber (Figure 3). These large air temperature differences within the canopies result from vertical differences in light intensity, leaf temperature, and leaf transpiration rates. Transpiration cools cooled the lower layers of the canopy to temperatures below the reference air temperature, and the uppermost leaf layers remain warmer because they are heated by the absorption of incident radiation.  
Incident PPF affected the air temperature difference above and within a wheat canopy (Figure 3A; 20-day old; [CO\({}_{2}\)] = 400 umol mol\({}^{-1}\); T\({}_{\text{air}}\) = 21 \({}^{\circ}\)C; RH = 68%; medium wind speed: 1.7 m s\({}^{-1}\)). In the dark, the top layers of foliage remained warmer than the lower layers because they were heated by sensible heat flux from the warm chamber air flowing above the canopy. During the photoperiod, the top of the canopy remained hotter than the lower leaf layers as the top layers of foliage absorbed most of the incident radiation. The air within the top 5 cm of the canopy became hotter than the reference air temperature as incident light levels increased to 1050 and 1850 umol m\({}^{-2}\) s\({}^{-1}\) (Figure 3A).  
Figure 2: Vertical wind profiles of wheat and soybean canopies measured with needle anemometers. Y-axis units are fraction of canopy height.  
At constant PPFo, the fan speed setting (high: 2.3 m s\({}^{-1}\); medium: 1.7 m s\({}^{-1}\); low: 0.8 m s\({}^{-1}\)) changed the amount of forced convection in the chamber and affected the vertical air temperature profiles above and within the wheat canopy (Figure 3B; 35-day old; PPF = 1800 \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\); [CO\({}_{2}\)] = 1200 \(\upmu\)mol mol\({}^{-1}\); T\({}_{\rm air}\) = 21 \({}^{\circ}\)C; RH = 68%). At the low chamber wind speed setting (0.8 m s\({}^{-1}\)), the upper 7 cm of the canopy was 1.5 \({}^{\circ}\)C warmer than at medium (1.7 m s\({}^{-1}\)) and high (2.3 m s\({}^{-1}\)) settings (Figure 3B). Air temperature up to 12 cm above the canopy was also heated by 0.5-1.2 \({}^{\circ}\)C by the warm foliage at the low wind speed. This suggests a threshold in turbulence in the chamber, above which an increase in wind speed does not continue to affect canopy-air heat exchange.