Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 241

length and radiation in the near infrared of 1.0 μ m. Assume the extinction coefi-
cients for both wavelengths to be 0.7 and 0.3, respectively. Compute the change in
spectral composition of the radiation at the same two levels as in question a) (where
the spectral composition can be expressed as the ratio of the lux densities of radi-
ation of 0.5 and 1.0 μ m; above the canopies lux densities for both wavelengths are
assumed identical).
Longwave Radiation
The effect of the leaves on longwave radiation inside the canopy is located mainly
just below the top of the canopy. The top leaves receive incoming longwave
radiation from the atmosphere (which has a relatively low temperature and emis-
sivity). Most leaves below the top do not 'see' the sky and receive their incoming
longwave radiation from the leaves above, which have a relatively high tempera-
ture and an emissivity close to one. Hence downward longwave radiation will on
average be higher within the canopy than above it. Just as in the case of shortave
radiation, the horizontal variation of downward longwave radiation can be large,
depending on the fraction of sky the speciic point in the canopy is exposed to (see
Figure 6.23 ).
6.6.2 Air Temperature
The air temperature in a canopy will depend both on the temperature of the air above
the canopy, the exchange of radiation (both the amount of radiation and the vertical
distribution) and the exchange of heat with the soil. In Figure 6.20 a typical proile is
shown. The highest temperature is located at the height of maximum foliage density,
and hence maximum radiation absorption. Below that level, the temperature shows a
stable stratiication (temperature increases with height).
During nighttime, the reverse will happen. Maximum cooling will occur at the top
of the vegetation, whereas some heat input from the soil will enter the air in the trunk
space. As a result, the air below the foliage will be unstably stratiied at night, giving
rise to buoyancy-induced convection and strong mixing (Jacobs et al., 1995 , 1996 ;
Dupont and Patton, 2012 ).
6.6.3 Wind Speed
Within the canopy, the wind speed decreases strongly at the top of the canopy and
stays rather constant with height below that. In vegetations that have a relatively open
structure at the bottom (such as forests) the wind speed close to the ground may
show a secondary maximum due to the fact that at that level the air encounters fewer
obstacles.