PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

P = F+ R
where F = net assimilation rate
per unit leaf area
R = respiration rate
and by analogy to Gaastra's (1959) Fick's la w
function, he derived :
C-F(ra +rs +rm)-nRrm
F + R = 1
aE C-F(rQ +rs +rm)-nRrm +rx
(20)
The only terms unfamiliar to us are rx, n, aE,
and R . Here R is respiration in light, and n is
the fraction of respiratory flux that is mixe d
in the intercellular spaces (n < 1) . The term
aE represents conversion of light to photosynthate
where E is incident light energy, an d
a is the efficiency of light energy conversion .
The term rx represents resistance to carboxyla -
tion, a parameter which includes biochemical
restraints caused by mineral nutrition, age o f
the leaf, etc .
Chartier's (1970) model differs from tha t
of Lommen et al. (1971) in that the effect of
light is incorporated into the model . Th e
effect of temperature must be included, perhaps
by a multiplicative term (Lommen et al .
1971, Webb 1972) . Chartier's model gives a
quadratic solution for F, as in Lommen et al ,
but many of the terms in both models ar e
difficult if not impossible to measure in a
field study . Further, respiration is represente d
by a single term ; an oversimplification requiring
further work .
Conclusions
In order to be consistent with the systems
viewpoint, photosynthesis must be treated as
a Gestalt, or nonsummative system. The
models described above violate this criterion
to some extent, most often by failing to
incorporate an important factor in the model .
We had hoped to be able to use one of the
models by Lommen et al. (1971) or by
Chartier (1970) as a tool in our field research ,
but two considerations prohibit this : (1) both
models have unmeasurable (in the field )
terms, (2) neither model is complete, i .e . ,
neither expresses photosynthesis as a function
of all the known important factors .
Consequently, it will be necessary to develop
a model of photosynthesis as a function
of light, temperature, leaf resistance and
ambient CO 2 concentration with some simplifications
from the above models which will
increase the utility of the models. The
parameters in this model will be estimate d
from data after the manner of Webb (1972 )
by nonlinear least-squares . The final model
will have much the same utility as a regressio n
model, but will not be linear and th e
parameters where possible will have physical
meaning. Thus, the models will be develope d
with certain specific goals in mind, necessitating
development of different models for th e
tree and stand levels of resolution .
A cknowledgments
The work reported in this paper was
supported by National Science Foundatio n
Grant No. GB-20963 to the Coniferous Fores t
Biome, U .S . Analysis of Ecosystems, International
Biological Program . This is Contributio n
No. 38 to the Coniferous Forest Biome, IBP .
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