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

Systems
A system, as defined by Klir (1969), is imposed
upon an object-a segment of nature
(the earth, a community, a tree, an automobile,
a computer)-by the observer from a distinct
point of view. Everything that does not
belong to the object is the environment . The
boundary between the object and its environment
cannot be clearly defined ; thus th e
delimitation of the object is somewhat arbitrary
and reflects the personal perspectives o f
the modeler . Because we usually cannot stud y
an object in its entirety (because of its complexity),
we observe or measure values of certain
quantities . The choice of quantities to
measure depends on what we consider to b e
of interest or important to the given purpos e
(Klir 1969) .
Most scientists are familiar with the concept
of a system, especially as it is applied i n
thermodynamics. For example, the laws of
thermodynamics were derived by studyin g
idealized closed systems in equilibrium . In
dealing with such closed systems it is possible
to study independent state variables such as
pressure, temperature, and volume, which de -
fine the state of the system. A closed thermodynamic
system can exchange heat and work
but not matter with its environment (Daniels
and Alberty 1967) .
As Ludvig von Bertalanffy (1969) pointe d
out, biological systems are open system s
whose structure is maintained by energy, in -
formation, and matter flow through the systems.
The behavior of the system is the resul t
of the interactions of the elements of th e
system and the flow through the system . Because
the thermodynamic system is closed
and idealized, and because the state variable s
are independent (i .e., one can be varied with -
out affecting another), it is convenient and
correct to describe the system in terms o f
total differentials; for example, the relation
of internal free energy, E, of a system to
temperature and volume, T and V, can be expressed
(Daniels and Alberty 1967) :
E = f(T, V)
dE _
(3T
) V dT + () T dv (1)
Equation 1 implies that the variables E, T,
and V are independent ; one can be varied
while the others are held constant, but mor e
importantly, equation 1 implies summativity
in the system where the behavior of a summa -
tive system is the physical sum of the behavior
of the parts . Because this approach is useful
in thermodynamics and other special
areas, biologists often attempt to explai n
observed phenomena by assuming that the
elements of all systems are independent (vo n
Bertalanffy 1969) . We in plant physiology
have accepted as a standard procedure th e
isolation of a plant in a growth chamber, an d
the study of the response of the plant to on e
factor while holding the others constant . This
approach is epitomized by Cleary (1970) who
derived the following model of photosynthesis :
the n
P = f(M,T,L,N,Pr)
dP= (3P) dM y {('r,) dT
am T,L,N P dT M .L.N.P
+ .
. + dpr~ dP r
1, 7'. L,N
(2 )
where P = photosynthesis, M,T,L,N, and Pr
are moisture, temperature, light, nutrition an d
preconditioning effect, respectively . This
model assumes a summative system analogou s
to a closed thermodynamic system .
This assumption is not totally valid in a
biological system like photosynthesis . Light ,
for example, has an effect not only on the
photochemical reactions within the leaf, but
also is converted to heat, which influences
leaf temperature. Likewise, leaf temperature
is affected by transpiration rates, which is in
turn affected by temperature, moisture status
of the leaf and air, and so on, but equation 2
does not account for these interactions. While
this model is inadequate in that respect ,
Cleary (1970) does recognize that photo -
synthesis is not a simple light-and-temperatur e
related phenomenon but represents a complex
interaction of diverse factors, all of whic h
should be taken into account in order to full y
understand the process . Some of the factor s
listed by Cleary (1970) probably are independent,
namely, Pr and N.
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