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

It is obvious that in essence equation 4 i s
identical to equation 3, where PmL = a/b an d
K = 1/b . Both models describe the light dependence
of photosynthesis equally well ; on e
model is not more complex than the other .
Yet, we would choose equation 4 because th e
parameters have more physical meaning . Th e
curve could also be described by :
P=00 + a 1L+ 32 L 2 + 03 L3 . . . (5)
which has even less appeal because it is nearl y
impossible to determine the physical meanin g
of the parameters ((3 i).
It could be argued that equation 4 is littl e
more valid than equation 3. Equation 4 is
derived from the Michaelis-Menton equatio n
that describes the rate of a single enzymati c
reaction as a function of a substrate concentration
. Photosynthesis is not a true Michaelis-
Menton case because it is an integration o f
photochemistry and a chain of enzymati c
reactions . The reductionist might argue that
we should model photosynthesis as a functio n
of the photochemical and the enzymatic reaction
rates . Further, it is obvious that temperature,
substrate availability, and several othe r
factors are important in photosynthesis. Also ,
since equation 4 considers only light an d
(indirectly) CO 2 effects, it falls short of ou r
first criterion, that the model be a system
model insofar as possible. These points brin g
up our third criterion : resolution level .
Resolution
According to Klir (1969) every quantity w e
observe must be determined in space . That is ,
CO 2 concentration, incident radiant energy ,
and temperature may be specified at som e
point in space . This specification may be irrelevent
for some studies (e .g., the location of
a laboratory may be trivial with respect to th e
study) but when concerned with a weathe r
forecast, it would be important to identif y
the spot on earth where temperature is measured.
We may also specify the interval and
accuracy of our measurements, giving the
space-time resolution level (Klir 1969) .
It may be convenient to think of the spatial
resolution level as the hierarchical level of
resolution (von Bertalanffy 1969) . Biological
systems can be placed in hierarchies corresponding
with levels of organization (table 1) .
0 . ?mo
Table 1 .-Levels of organization
1. Subatomi c
2. Atomi c
3. Molecular
4. Subcellular
5. Cellular
6. Organ or tissue
Thus a system can be thought of as a component
in a larger system, and composed of sub -
systems (von Bertalanffy 1969) . Each level of
organization seems to have certain properties
unique to that level, making prediction o f
system behavior from the viewpoint of th e
lower systems somewhat hazardous . This concept
is implicit in the idea of nonsummativit y
of systems. The reductionist might argu e
against this view, but the limits of reductionism
are obvious . It is impossible to predict
all the behavior of a tree from the molecular
level . If we hope to discover general ecosystem
principles comparable to the fundamental
principles of physics, probably we wil l
have to emphasize study from the ecosyste m
level. The ideal gas law, for example, was
derived from observations of the behavior o f
10 23 particles acting in concert. In ecosyste m
study, we are among the particles; a holistic
view is needed .
There are three levels of resolution under
consideration for the photosynthesis model :
the leaf, tree, and stand levels . Each level will
require a different model and approach ; we
cannot extrapolate the leaf level model to a
tree by multiplying the results by the numbe r
of leaves in the crown . The temporal resolution
of the models will be somewhat flexibl e
at the leaf level, but will probably be on a
daily basis for the stand model .
Practical Considerations
7. Organism
8. Community
9. Global
10. Solar Syste m
11. Galactic
12. Universe
13. ? o
There are several practical consideration s
230