Geoscience Reference
In-Depth Information
100 W m
-
2
50 W m
-
2
-0.4 mg m
-
2
s
-
1
-0.15 mg m
-
2
s
-
1
7 W m
-
2
7 W m
-
2
1 ppmv
3 K
0.5 hPa
Foliage
density
Temperature
Humidity
CO
2
Figure 6.24
Counter-gradient transport within a canopy: proiles (lines) and luxes
(arrows) in a pine forest with a canopy height of 20 m (dashed line indicates can-
opy top): temperature and sensible heat lux (left), vapour pressure and latent heat
lux (center) and CO
2
concentration and CO
2
lux (right). (Data from Denmead and
Bradley,
1987
.)
forms the surface temperature that determines the upwelling longwave radiation and
sensible heat lux. To determine the temperature of a leaf, we need to consider its
energy balance:
HQLE
leaf
= −
*
(6.40)
leaf
vleaf
The net radiation is determined by the radiative luxes leaving and entering the leaf
(see
Section 6.6.1
) and the leaf evapotranspiration depends on the stomatal opening,
leaf temperature and the ambient humidity. If we use a resistance law for the sensible
heat lux (
H
), the leaf temperature can be expressed as:
=−
ρ
pa
c
(
T
−
T
) /
r
b,h
leaf
TT
r
(
)
b,h
=+
QLE
*
−
(6.41)
leaf
a
ρ
c
leaf
vleaf
p
where
r
b,h
is the laminar boundary-layer resistance for heat (similar, but not equal
to the boundary-layer resistances for water vapour; see
Figure 6.12
). Though Eq.
(
6.41
) clearly shows the factors that determined the leaf temperature, in principle
it is an implicit expression for the leaf temperature: net radiation and transpiration
themselves depend on the leaf temperature through the emitted longwave radiation
and the saturated water vapour concentration in the stomata, respectively. Equation
(
6.41
) shows that the degree to which the leaf temperature is coupled to the air tem-
perature depends on the boundary-layer resistance and the energy input to the leaf.
r
b,h
depends on the thickness of the laminar boundary layer: for thin boundary layers, the
resistance is low. The boundary-layer thickness in turn depends on wind speed (the
higher the wind speed, the thinner the boundary layer) and the distance over which
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