Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 242

Figure 6.23 Penetration of radiation from the sky through the crown of a pine forest
to the forest loor. Photo courtesy of Bert Heusinkveld.
One important feature in the wind speed proile in Figure 6.20 should be noted: the
inlection point just below the canopy top (point where the second derivative of u ()
changes sign). This inlection point is the cause of an instability that in turn causes
large scale turbulent eddies that sweep into the canopy (carrying momentum and
CO 2 ) and cause ejection of air (carrying heat and moisture). These large-scale struc-
tures are eficient in the transport of momentum and scalars, but are hardly related to
the local gradients of those variables. Hence, transport counter to the gradient (rather
than along the gradient, as assumed in standard K- theory; see Chapter 1 ) is an impor-
tant feature of canopy turbulence (see Figure 6.24 and Finnigan, 2000 ).
6.6.4 Leaf Temperature
The temperature of the leaves is of vital importance for the biological processes tak-
ing place in the leaf. Furthermore, seen from above the canopy, the leaf temperature