426
W. Beyschlag et al.
other, including situations where they overlap (Fig. 20.11). The structural
parameters needed for the other cylinders are the same as for the plant of
interest. Foliage of the neighboring plants can cause attenuation of direct
beam and sky diffuse shortwave radiation before this radiation reaches the
target plant (Fig. 20.11). This approach has been successfully used to determine that significant competition for light between adjacent tussocks does
not occur unless the plants are less than one diameter apart and that the
uniform placement of tillers (turf growth form) is much more efficient in
light interception and carbon gain than bunching of tillers unless the density
of bunches forms a nearly uniform canopy (Ryel et aI., unpubI.). Recently,
a modified version of the model has been employed to evaluate the consequences of losing different age classes of needles for light interception and
canopy photosynthesis of dense declining stands of red spruce [Picea abies
(L.) Karst.]. Due to increased light levels within the tree crowns, a beneficial
effect on photosynthesis and water use efficiency of the remaining younger
needles results from the loss of older needle age classes. In terms of whole
tree photosynthesis, the reduction of photosynthetically active foliage area
can nearly be compensated by this phenomenon when there is substantial
tree overlap (Beyschlag et aI., unpubI.). Similar analyses have been used in
assessing net photosynthesis in stands of Picea sitchensis (Norman and Jarvis
1975) and Pinus radiata (Grace et ai. 1987b).
20.6 Conclusions
Although measurements of single-leaf photosynthesis can presently be performed in the field with a high degree of accuracy, the photosynthetic rate of
single leaves is often not representative of photosynthetic behavior for the
entire canopy. Simulation models which calculate light interception and net
photosynthesis provide an effective way to overcome this problem. As
discussed in this chapter, models are now available for both homogeneous
or nonhomogeneous, single-species or mixed-species canopies. They have
been developed with modular formats (Reynolds et ai. 1987) which allows
replacement of model components by other routines which may be more
suitable in applications for other species or situations. While this family of
models has proved to be a useful tool to address many ecophysiological
questions, their outputs also provide the carbon gain inputs needed by
allocation and growth models (e.g., Johnson and Thornley 1985; CharlesEdwards et ai. 1986; Reynolds et ai. 1987; Buwalda 1991; Webb 1991).
Precise calculations of canopy net photosynthesis may often be limited by
the quality of input parameter estimates. The time and effort involved in
obtaining model input parameters can be considerable, and there is potential
for cumulative errors to arise in these parameter estimates. However, these
limitations are less of a problem when the models are used to address
W. Beyschlag et al.
other, including situations where they overlap (Fig. 20.11). The structural
parameters needed for the other cylinders are the same as for the plant of
interest. Foliage of the neighboring plants can cause attenuation of direct
beam and sky diffuse shortwave radiation before this radiation reaches the
target plant (Fig. 20.11). This approach has been successfully used to determine that significant competition for light between adjacent tussocks does
not occur unless the plants are less than one diameter apart and that the
uniform placement of tillers (turf growth form) is much more efficient in
light interception and carbon gain than bunching of tillers unless the density
of bunches forms a nearly uniform canopy (Ryel et aI., unpubI.). Recently,
a modified version of the model has been employed to evaluate the consequences of losing different age classes of needles for light interception and
canopy photosynthesis of dense declining stands of red spruce [Picea abies
(L.) Karst.]. Due to increased light levels within the tree crowns, a beneficial
effect on photosynthesis and water use efficiency of the remaining younger
needles results from the loss of older needle age classes. In terms of whole
tree photosynthesis, the reduction of photosynthetically active foliage area
can nearly be compensated by this phenomenon when there is substantial
tree overlap (Beyschlag et aI., unpubI.). Similar analyses have been used in
assessing net photosynthesis in stands of Picea sitchensis (Norman and Jarvis
1975) and Pinus radiata (Grace et ai. 1987b).
20.6 Conclusions
Although measurements of single-leaf photosynthesis can presently be performed in the field with a high degree of accuracy, the photosynthetic rate of
single leaves is often not representative of photosynthetic behavior for the
entire canopy. Simulation models which calculate light interception and net
photosynthesis provide an effective way to overcome this problem. As
discussed in this chapter, models are now available for both homogeneous
or nonhomogeneous, single-species or mixed-species canopies. They have
been developed with modular formats (Reynolds et ai. 1987) which allows
replacement of model components by other routines which may be more
suitable in applications for other species or situations. While this family of
models has proved to be a useful tool to address many ecophysiological
questions, their outputs also provide the carbon gain inputs needed by
allocation and growth models (e.g., Johnson and Thornley 1985; CharlesEdwards et ai. 1986; Reynolds et ai. 1987; Buwalda 1991; Webb 1991).
Precise calculations of canopy net photosynthesis may often be limited by
the quality of input parameter estimates. The time and effort involved in
obtaining model input parameters can be considerable, and there is potential
for cumulative errors to arise in these parameter estimates. However, these
limitations are less of a problem when the models are used to address