Resistance above canopy

We now estimate the resistance of the turbulent air immediately above a plant canopy. We will let Kj average 1.0 m2 s 1 for the first 30 m above the plants, a typical value in a moderate wind during the daytime. By Equation 9.4, the resistance over this 30-m interval then is... 

JcOz above a plant canopy can be -20 pmol m 2s 1 at midday. For comparison, the flux density of CO2 into an exposed leaf of a mesophyte at a moderate light level can be 11 pmol mT2 s-1 (Chapter 8, Section 8.4E). Using Equation 9.4, an analogy with Ohm s law (AE = IRy namely, Acf = Jjrf), and a resistance of 30 s m-1 for the lower 30 m of the turbulent air we calculate that the decrease in CO2 concentration across this... 

Figure 9-8. C02 resistances, flux densities,and concentrations within and above a corn crop at night. Height in the turbulent air above the plant canopy is not to scale. Air temperature is 20°C, and the atmospheric air pressure is 0.1 MPa (see Table 8-2 for the conversion factor between mmol m-3 and (xmol mol-1).

---