422
W. Beyschlag et al.
20.5.1 Step 1: The Case of Single Plants
The simplest case of a nonhomogeneous canopy is a single plant growing on
a horizontal surface such as an isolated tussock of a desert bunchgrass. Ryel
et al. (1993) recently developed a photosynthesis model for this situation,
where the procedure used for calculating whole tussock-photosynthesis is
analogous to that for uniform canopies (see Fig. 20.1.) except in three
dimensions. The tussock is defined as cylindrical in shape with concentric
subcylinders defining regions of different foliage density and character (Fig.
20.8). These subcylinders are further subdivided into layers to allow additional subdivisions of foliage and physiological characteristics in the tussock.
Instead of using a single vertical array of points representing horizontal
canopy layers, light interception and photosynthesis are calculated for sunlit
and shaded foliage in a three-dimensional array of points in the cylinder
subsections. The points are uniformly arranged in equally spaced horizontal
planes. The array is set with the same number of points (n) in three
dimensions. Minimum values for n can be determined by sensitivity analysis.
Incident light and net photosynthetic rate are calculated separately for each
point. Mean net photosynthesis for each subsection is then calculated from
all points within the subsection, weighted by the size of the tussock subsection and fraction of sunlit and shaded foliage, and summed over all subsections to obtain whole-tussock photosynthesis.
This three-dimensional canopy model and similar developments of
Norman and Welles (1983), Grace et al. (1987a), and Wang and Jarvis
(1990) are substantial departures from the homogeneous canopy models and
they represent a significant advantage for use with plants which are in
clumped or discontinuous distributions.
20.5.1.1 Case Study: Do Tussock Grasses Sacrifice Carbon Gain
from Having Steep Leaf Angles?
Compact, isolated tussock grasses often possess steeply inclined foliage, but
the implications of foliage orientation for this growth form have not been
thoroughly pursued. The tussock growth form is widely distributed in grass
steppe and desert regions, but the architectural advantages of these plants
are not always so obvious (Caldwell et al. 1983). Much of the daily
irradiation is intercepted by the lateral surfaces of the tussocks, and there
can still be considerable self-shading within the tussock (Caldwell et al.
1983). Steeply inclined foliage may have benefits for reduced potential for
photoinhibition and greater water-use efficiency, but there may also be
disadvantages in the effective harvesting of shortwave radiation at certain
times of the day (Werk and Ehleringer 1984).
After parameterization and thorough validation, Ryel et al. (1993) used
the three-dimensional model to examine the effects of foliage orientation on
W. Beyschlag et al.
20.5.1 Step 1: The Case of Single Plants
The simplest case of a nonhomogeneous canopy is a single plant growing on
a horizontal surface such as an isolated tussock of a desert bunchgrass. Ryel
et al. (1993) recently developed a photosynthesis model for this situation,
where the procedure used for calculating whole tussock-photosynthesis is
analogous to that for uniform canopies (see Fig. 20.1.) except in three
dimensions. The tussock is defined as cylindrical in shape with concentric
subcylinders defining regions of different foliage density and character (Fig.
20.8). These subcylinders are further subdivided into layers to allow additional subdivisions of foliage and physiological characteristics in the tussock.
Instead of using a single vertical array of points representing horizontal
canopy layers, light interception and photosynthesis are calculated for sunlit
and shaded foliage in a three-dimensional array of points in the cylinder
subsections. The points are uniformly arranged in equally spaced horizontal
planes. The array is set with the same number of points (n) in three
dimensions. Minimum values for n can be determined by sensitivity analysis.
Incident light and net photosynthetic rate are calculated separately for each
point. Mean net photosynthesis for each subsection is then calculated from
all points within the subsection, weighted by the size of the tussock subsection and fraction of sunlit and shaded foliage, and summed over all subsections to obtain whole-tussock photosynthesis.
This three-dimensional canopy model and similar developments of
Norman and Welles (1983), Grace et al. (1987a), and Wang and Jarvis
(1990) are substantial departures from the homogeneous canopy models and
they represent a significant advantage for use with plants which are in
clumped or discontinuous distributions.
20.5.1.1 Case Study: Do Tussock Grasses Sacrifice Carbon Gain
from Having Steep Leaf Angles?
Compact, isolated tussock grasses often possess steeply inclined foliage, but
the implications of foliage orientation for this growth form have not been
thoroughly pursued. The tussock growth form is widely distributed in grass
steppe and desert regions, but the architectural advantages of these plants
are not always so obvious (Caldwell et al. 1983). Much of the daily
irradiation is intercepted by the lateral surfaces of the tussocks, and there
can still be considerable self-shading within the tussock (Caldwell et al.
1983). Steeply inclined foliage may have benefits for reduced potential for
photoinhibition and greater water-use efficiency, but there may also be
disadvantages in the effective harvesting of shortwave radiation at certain
times of the day (Werk and Ehleringer 1984).
After parameterization and thorough validation, Ryel et al. (1993) used
the three-dimensional model to examine the effects of foliage orientation on