Photosynthesis of Vascular Plants
415
Smith (1937), with dark respiration represented by an Arrhenius equation
and assumed to be light-independent. Iterative procedures then balance
rates of net CO 2 fixation with rates of CO2 entry into the leaf (Weber et al.
1985).
20.3.2 Model Validation
The value of a model beyond meeting its specified objectives is to generate
time patterns of behavior that do not differ significantly from the real system
(Forrester 1961). Such model "validation" provides a degree of confidence
that the model can adequately perform to meet its objectives. Parameters
for validation are selected based upon the model objectives and the ability
to obtain reliable field measurements. Since these models calculate some
outputs, such as total canopy gas exchange, that are difficult to measure in
the field, a subset of model outputs is compared with more easily measured
parameters to test their correspondence. Figure 20.2 shows the comparison
between measured and simulated values of leaf temperature, leaf conductance, and net photosynthesis of Q. cocci/era at four different times of
the year using the model of Caldwell et al. (1986). Good correlation was
found in all cases for these parameters, suggesting that the model can
reliably calculate these parameter values under these and similar environmental conditions.
The approach of Caldwell et al. (1986) was to construct the model based
on the knowledge of basic processes and to arrive at canopy function as a
direct consequence of the interaction of the component parts. This approach
is very instructive in the investigation of mechanistic interactions of model
components as affected by the processes of light interception, energy balance, and diffusive conductance. One of the objectives of this model, to
calculate parameters that cannot be measured, is illustrated in the following
example.
20.3.3 Case Study: How Do Different Parts of the Canopy Contribute to
Total Canopy Photosynthesis?
Figure 20.3 shows the contribution of different foliage classes to total canopy
photosynthesis of Q. cocci/era throughout the year as calculated with the
model. In these situations, the net 24-h CO2 gain contributed by all the
shaded leaves in the canopy was nearly the same as that from leaves in
the uppermost layer, which comprised only 13% of the total leaf area. The
contribution of the lower 50% of the total canopy foliage is similar to that of
all the shaded foliage during the summer months, but is much lower in the
winter months.
415
Smith (1937), with dark respiration represented by an Arrhenius equation
and assumed to be light-independent. Iterative procedures then balance
rates of net CO 2 fixation with rates of CO2 entry into the leaf (Weber et al.
1985).
20.3.2 Model Validation
The value of a model beyond meeting its specified objectives is to generate
time patterns of behavior that do not differ significantly from the real system
(Forrester 1961). Such model "validation" provides a degree of confidence
that the model can adequately perform to meet its objectives. Parameters
for validation are selected based upon the model objectives and the ability
to obtain reliable field measurements. Since these models calculate some
outputs, such as total canopy gas exchange, that are difficult to measure in
the field, a subset of model outputs is compared with more easily measured
parameters to test their correspondence. Figure 20.2 shows the comparison
between measured and simulated values of leaf temperature, leaf conductance, and net photosynthesis of Q. cocci/era at four different times of
the year using the model of Caldwell et al. (1986). Good correlation was
found in all cases for these parameters, suggesting that the model can
reliably calculate these parameter values under these and similar environmental conditions.
The approach of Caldwell et al. (1986) was to construct the model based
on the knowledge of basic processes and to arrive at canopy function as a
direct consequence of the interaction of the component parts. This approach
is very instructive in the investigation of mechanistic interactions of model
components as affected by the processes of light interception, energy balance, and diffusive conductance. One of the objectives of this model, to
calculate parameters that cannot be measured, is illustrated in the following
example.
20.3.3 Case Study: How Do Different Parts of the Canopy Contribute to
Total Canopy Photosynthesis?
Figure 20.3 shows the contribution of different foliage classes to total canopy
photosynthesis of Q. cocci/era throughout the year as calculated with the
model. In these situations, the net 24-h CO2 gain contributed by all the
shaded leaves in the canopy was nearly the same as that from leaves in
the uppermost layer, which comprised only 13% of the total leaf area. The
contribution of the lower 50% of the total canopy foliage is similar to that of
all the shaded foliage during the summer months, but is much lower in the
winter months.