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

of the external quantities of S would be impossible
in the context of isolated subsystems .
What is needed is some translation of sub -
system external variables into system externa l
variables. Clymer and Bledsoe (1970) have
addressed a similar question in consideratio n
of "interfacing" two subsystems at the sam e
level and use the term "slave model" to designate
the set of equations for changing resolution.
This term seems inappropriate in the
context of composition of subsystem proper -
ties into properties of the whole ; such a
process is more master than slave . I have
chosen the term "Ghost System i2 and notationally
designated So as the integrating sub -
system which derives the properties of th e
whole from the properties of the parts . In its
simplest form, So is a resolution expander of
inputs and reducer of outputs . Whether it will
need to assume a more complex form is yet t o
be seen .
In this manner we have, by statements (1 )
and (2), defined the ecosystem at two organizational
levels, and may elaborate lower level s
by the device of decomposing subsystems i n
the same manner . That is, if S may be defined
at two levels, so may the elements of {Si}, an d
the subsystems so defined, to any desire d
degree of fineness . Ultimately, it is necessary
to end with a behavioral model for all sub -
systems, because it is this model that yield s
explicit form to the relational expressions .
Several points are of interest here . First, i t
is not clear that a hierarchical decompositio n
is always desirable . That is, a finer resolutio n
may not best take the form of a decomposition
of next most coarse form of interest .
Hierarchical forms, however, have man y
advantages and will be used wherever possible .
Second, it is quite clear that a uniform resolution
is unnecessary . That is, a very coarse
resolution model of one subsystem may b e
coupled with a fine resolution model of
another, and such an arrangement will have
many advantages . Not the least of these is th e
advantage of manageability . If we want to
take a closeup view of some parts of the system,
there is no reason to, and many reason s
not to, look at the rest of the system at th e
2 After Koestler's "The Ghost in the Machine ."
same degree of magnification . A hierarchical
subsystem structure will be most conducive t o
a variable resolution, but again, it is no t
necessary .
Application of the
General Theory to the
Forest Ecosystem
With this introduction to the theoretica l
background, we can now take a look at th e
coarser ecosystem structures as we now perceive
them . A diagrammatic convention i s
established which will be recognized as simila r
to Forrester's, but which differs in some
important respects . Let boxes designate compartments
(to be conceptualized as storage
containers), let ovals or circles designate systems
(or subsystems), and let diamonds designate
a set of variables . Solid lines designate
coupled flow variables and dotted lines represent
control variables. As a convention ,
diamonds are eliminated from the flow couplings,
as these variables are apparent fro m
the context, but identification of contro l
variables is important . Occasionally these are
also eliminated from a figure to reduc e
clutter, and in the illustrations given here ,
most control paths are eliminated for th e
same reason .
In figure 1 is depicted a representation of a
system as a whole, with inputs and outputs . At
this level, the entire system (the forest ecosystem)
is defined according to (1) . In figure
2, the same system is elaborated by identification
of major subsystems, the terrestrial biota ,
the aquatic biota and the hydrologic system .
This involves definition of the system according
to (2) and it should be emphasized at thi s
point that there are very many ways in whic h
this could be done .
Implementation of this form (fig. 2) requires
decisions regarding boundaries an d
specifications of the nature of the coupling s
between subsystems . An example will illustrate
both points. Consider the uptake of
water by higher plants . Question : Do we wish
to consider this water as having transferred
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