Remote Sensing of Canopy Cover and IPAR
canopy assimilation rate is given by
pmol C02
An'cpy " 20 m 2 (leaf hemi-surface area) s
m
2
(leaf hemi-surface area)
x 3
m 2 (ground area)
pmol C02
S 60 m 2 (ground area) s '
This is 54 percent larger than the sunlithhaded method in Example
15.2. This value of 60 pmol C02 m-2(ground area) s-' is approximately the canopy photosynthetic rate that would occur if the canopy
were illuminated with entirely diffuse irradiance at 2000 pmol photons m-2(ground area) s-'. Thus a diffuse irradiance of about 1300
pmol photons m-2(gr~und area) s-' would result in about the same
canopy photosynthetic rate (39 pmol CO2 m-2 (ground area) s-') as
2000 pmol photons m-2(ground area) s-' with 80 percent beam and 20
percent diffuse: This means that diffuse irradiance is more efficient for
photosynthesis than beam irradiance.
Light assimilation responses are not always as nonlinear as the 10°C
curve in Fig. 14.6; for example, the 30°C curve in Fig. 14.6. Comparing
the canopy assimilation prediction from the sunlithhaded method with
the average-APAR method results in 39 pmol C02 mV2 (ground area)
s-' for both methods; this occurs because of the linearity of the 30°C
curve. Considering the greater leaf assimilation rate at 30°C from Fig.
14.6, it may be surprising to find the canopy assimilation rates for 10 and
30°C are nearly equal. This occurs because leaves at 30°C have higher
photosynthetic rates on sunlit leaves and lower rates on shaded leaves, because of the larger dark respiration. Essentially the higher maximum leaf
photosynthetic rate comes at a higher dark respiration cost. Furthermore,
the canopy architecture limits the fraction of leaves that can be sunlit.
Leaf stomatal conductance can be scaled to a canopy conductance
by using the same method as outlined above for photosynthetic rates
if stomatal conductances for sunlit and shaded leaves are known. Using
stomatal conductances appropriate for the leaf assimilation rates plotted in
Fig. 14.6 under humid atmospheric conditions, the canopy conductance
for Example 15.2 can be estimated from an equation like Eq. (15.24)
to be 0.5(1.32) + 0.2(1.68) = 1.0 mol water m-2(ground area) s-'.
Because sunlit LAI approaches a maximum as LA1 increases (LT has
a maximum of about 1.5 for high LA1 canopies with @ = 40") and
the mean shaded stomatal conductance decreases as LAI increases, this
sunlitlshaded approach clearly shows why canopy conductances tend to
reach maximum values that might be expected to be related to sunlit leaf
area index.
canopy assimilation rate is given by
pmol C02
An'cpy " 20 m 2 (leaf hemi-surface area) s
m
2
(leaf hemi-surface area)
x 3
m 2 (ground area)
pmol C02
S 60 m 2 (ground area) s '
This is 54 percent larger than the sunlithhaded method in Example
15.2. This value of 60 pmol C02 m-2(ground area) s-' is approximately the canopy photosynthetic rate that would occur if the canopy
were illuminated with entirely diffuse irradiance at 2000 pmol photons m-2(ground area) s-'. Thus a diffuse irradiance of about 1300
pmol photons m-2(gr~und area) s-' would result in about the same
canopy photosynthetic rate (39 pmol CO2 m-2 (ground area) s-') as
2000 pmol photons m-2(ground area) s-' with 80 percent beam and 20
percent diffuse: This means that diffuse irradiance is more efficient for
photosynthesis than beam irradiance.
Light assimilation responses are not always as nonlinear as the 10°C
curve in Fig. 14.6; for example, the 30°C curve in Fig. 14.6. Comparing
the canopy assimilation prediction from the sunlithhaded method with
the average-APAR method results in 39 pmol C02 mV2 (ground area)
s-' for both methods; this occurs because of the linearity of the 30°C
curve. Considering the greater leaf assimilation rate at 30°C from Fig.
14.6, it may be surprising to find the canopy assimilation rates for 10 and
30°C are nearly equal. This occurs because leaves at 30°C have higher
photosynthetic rates on sunlit leaves and lower rates on shaded leaves, because of the larger dark respiration. Essentially the higher maximum leaf
photosynthetic rate comes at a higher dark respiration cost. Furthermore,
the canopy architecture limits the fraction of leaves that can be sunlit.
Leaf stomatal conductance can be scaled to a canopy conductance
by using the same method as outlined above for photosynthetic rates
if stomatal conductances for sunlit and shaded leaves are known. Using
stomatal conductances appropriate for the leaf assimilation rates plotted in
Fig. 14.6 under humid atmospheric conditions, the canopy conductance
for Example 15.2 can be estimated from an equation like Eq. (15.24)
to be 0.5(1.32) + 0.2(1.68) = 1.0 mol water m-2(ground area) s-'.
Because sunlit LAI approaches a maximum as LA1 increases (LT has
a maximum of about 1.5 for high LA1 canopies with @ = 40") and
the mean shaded stomatal conductance decreases as LAI increases, this
sunlitlshaded approach clearly shows why canopy conductances tend to
reach maximum values that might be expected to be related to sunlit leaf
area index.