Reflection of Light by Plant Canopies
255
a uniform overcast sky (no zenith angle dependence of sky radiance). For
horizontal leaves, Kd = 1, but for a spherical canopy, with L, = 3, Kd
is around 0.7. This is an important point which we return to later.
Note that diffuse radiation, unlike beam radiation fiom the sun, is
distributed relatively uniformly over all leaves with various orientations
for a particular layer in the canopy. Thus the diffuse flux density incident
on a leaf at some depth L in the canopy is the same as the diffuse flux
density estimated on the horizontal at the same depth using Eq. (15.5)
and the diffuse flux density above the canopy.
15.4 Light Scattering in Canopies
The leaves in plant canopies are not black, of course, and do transmit
and reflect radiation. Goudriaan (1977) has shown that the transmission
and reflection of radiation when the leaves are not assumed black can
still be approximated using an exponential model (Eq. (15.1)), but with
a modification to K. If the absorptivity of leaves for radiation is a , then
the total beam radiation (direct and down scattered) transmitted through
the canopy to depth L is
It can be seen that when a = 1 (black leaves) this equation is the same
as Eq. (1 5.1) and when a! is small radiation will be attenuated minimally.
The transmission of light through the leaves therefore gives an additional
amount of radiation under the canopy. Equation (15.6) is an approximation, and Goudriaan (1977) has shown (his Table 5, p. 27) that Eq. (15.6)
works well for a range of sun zenith angles, canopy architectures, and
leaf absorptivity values. For a canopy with a spherical leaf angle distribution, Eq. (15.6) works well for sun zenith angles less than 65". The
transmission of diffuse radiation by the canopy is predicted by a similar
equation, but with Kd as the extinction coefficient. Typical values for a!
are a!, = 0.8 for PAR and a, = 0.2 for NIR radiation. For total solar
radiation, absorptivity is the mean of the values for PAR and NIR, so
cr, = 0.5.
15.5 Reflection of Light by Plant Canopies
For a canopy of randomly located, horizontally oriented leaves with a
LA1 so large that the soil has negligible effect on radiation reflected from
the canopy, the canopy hemispherical reflection coefficient, pgy, is given
by
where a is the leaf absorptivity. This means that for a dense canopy of
horizontal leaves, in the PAR (a = 0.8), p :
,
,
= 0.056; in the NIR
(a = 0.2), p{, , ,
= 0.38; and in the solar (a = 0.5), pgcpy = 0.17.
This canopy reflection coefficient for solar radiation actually is not a
255
a uniform overcast sky (no zenith angle dependence of sky radiance). For
horizontal leaves, Kd = 1, but for a spherical canopy, with L, = 3, Kd
is around 0.7. This is an important point which we return to later.
Note that diffuse radiation, unlike beam radiation fiom the sun, is
distributed relatively uniformly over all leaves with various orientations
for a particular layer in the canopy. Thus the diffuse flux density incident
on a leaf at some depth L in the canopy is the same as the diffuse flux
density estimated on the horizontal at the same depth using Eq. (15.5)
and the diffuse flux density above the canopy.
15.4 Light Scattering in Canopies
The leaves in plant canopies are not black, of course, and do transmit
and reflect radiation. Goudriaan (1977) has shown that the transmission
and reflection of radiation when the leaves are not assumed black can
still be approximated using an exponential model (Eq. (15.1)), but with
a modification to K. If the absorptivity of leaves for radiation is a , then
the total beam radiation (direct and down scattered) transmitted through
the canopy to depth L is
It can be seen that when a = 1 (black leaves) this equation is the same
as Eq. (1 5.1) and when a! is small radiation will be attenuated minimally.
The transmission of light through the leaves therefore gives an additional
amount of radiation under the canopy. Equation (15.6) is an approximation, and Goudriaan (1977) has shown (his Table 5, p. 27) that Eq. (15.6)
works well for a range of sun zenith angles, canopy architectures, and
leaf absorptivity values. For a canopy with a spherical leaf angle distribution, Eq. (15.6) works well for sun zenith angles less than 65". The
transmission of diffuse radiation by the canopy is predicted by a similar
equation, but with Kd as the extinction coefficient. Typical values for a!
are a!, = 0.8 for PAR and a, = 0.2 for NIR radiation. For total solar
radiation, absorptivity is the mean of the values for PAR and NIR, so
cr, = 0.5.
15.5 Reflection of Light by Plant Canopies
For a canopy of randomly located, horizontally oriented leaves with a
LA1 so large that the soil has negligible effect on radiation reflected from
the canopy, the canopy hemispherical reflection coefficient, pgy, is given
by
where a is the leaf absorptivity. This means that for a dense canopy of
horizontal leaves, in the PAR (a = 0.8), p :
,
,
= 0.056; in the NIR
(a = 0.2), p{, , ,
= 0.38; and in the solar (a = 0.5), pgcpy = 0.17.
This canopy reflection coefficient for solar radiation actually is not a