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Figure 12.13 The influence of shelter belts on wind-velocity distributions (expressed as percentages of the velocity in the open). (A)
The effects of one shelter belt of three different densities, and of two back-coupled medium-dense shelter belts. (B) The detailed effects
of one half-solid shelter belt.
Sources : (A) After Nägeli, and Geiger (1965). (B) After Bates and Stoeckeler, and Kittredge (1948).
compares diurnal energy flows during July for a pine
forest in eastern England and a fir forest in British
Columbia. In the former case, only 0.33 R n is used for
LE due to the high resistance of the pines to tran-
spiration, whereas 0.66 R n is similarly employed in
the British Columbia fir forest, especially during the
afternoon. Like short green crops, only a very small
proportion of R n is ultimately used for tree growth, an
average figure being about 1.3 W m -2 , some 60 per cent
of which produces wood tissue and 40 per cent forest
litter.
During daylight, leaves transpire water through open
pores, or stomata . This loss is controlled by the length
of day, the leaf temperature (modified by evaporational
cooling), surface area, the tree species and its age,
as well as by the meteorological factors of available
radiant energy, atmospheric vapour pressure and wind
speed. Total evaporation figures are therefore extremely
varied. The evaporation of water intercepted by the
c Modification of the humidity environment
The humidity conditions within forest stands contrast
strikingly with those in the open. Evaporation from
the forest floor is usually much less because of the
decreased direct sunlight, lower wind velocity, lower
maximum temperature, and generally higher forest
air humidity. Evaporation from the bare floor of pine
forests is 70 per cent of that in the open for Arizona in
summer and only 42 per cent for the Mediterranean
region.
Unlike many cultivated crops, forest trees exhibit a
wide range of physiological resistance to transpiration
processes and, hence, the proportions of forest energy
flows involved in evapotranspiration ( LE ) and sensible
heat exchange ( H ) vary. In the Amazonian tropical
broad-leaved forest, estimates suggest that after rain up
to 80 per cent of the net solar radiation ( R n ) is involved
in evapotranspiration ( LE ) (Figure 12.14). Figure 12.15
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