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

several models together to form the model for
the watershed as a whole, is the reverse of the
decomposition process previously discussed.
Now we are faced with the question of defining
appropriate external variables of th e
watershed as an object after having initiall y
defined the watershed as a collection of sub -
objects .
These questions are obviously appropriate
to consideration of a general model for, say ,
the entire H . J. Andrews Experimental Forest ,
or for the Coniferous Forest Biome, sob that
we consider them central to our overall
objectives. By concentrating on the proble m
of stratifying a watershed, and on the problems
of modeling the strata and the collective ,
we are attempting to devise strategies which
will be useful in later extension of the modeling
process .
It turns out that if we view the construction
of strata in essentially the same perspective
as the construction of subsystems in th e
U-C structure, the same general criteria hold .
Generally speaking, subsystems (strata )
should be constructed in such a manner tha n
couplings are minimized (or simplified), an d
such that important processes are containe d
within the subsystems . This criterion is no t
always compatible with the second one, tha t
strata should be environmentally homogenous,
within, and occasional conflicts arise .
In any event, we are attempting hierarchica l
stratification of watersheds, with topographic
features defining the primary strata and vegetation
types the secondary, the two levels giving
uncoupling and homogeneity, respectively .
Summary
Some of the ways in which the Universe-
Coupling structure is being used in building a
hierarchical, modular system of models for
the Coniferous Biome have been discussed .
These models are hierarchical by virtue of th e
identification of systems at one level o f
organization as subsystems of the system s
defined at the next higher level. They ar e
modular by virtue of the uncoupled nature ;
identification of the coupling variables at an y
level allows complete flexibility of subsystem
representation, provided that the integrity of
the couplings is maintained .
A general criterion for constructing sub -
systems is provision of simple coupling s
among subsystems with tight relations contained
within . This principle is violated quite
badly when trophic levels are used for sub -
system definition . An alternate structure for
consumers is being investigated in our program,
as reported in the paper by Strand and
Nagel .
In applying the U-C structure to spatial
units, another criterion is employed . Here it i s
desirable to provide homogeneity of environment
and process within strata, with variatio n
among strata of no concern . This criterion i s
sometimes in conflict with the first one o f
minimal couplings, so that compromises are
necessary .
The paper has dealt primarily with aspect s
of structure and criteria for application of th e
structural form . Emphasis has been on th e
first two of the three tasks which the U- C
structure defines, to wit : (1) specification of
the coupling variables, (2) elaboration of
internal structure of subsystems . The third
task, analysis and synthesis of behavior of the
subsystems as objects, is given little attention ,
but this probably is the most important, fro m
the point of view of ecosystem theory . As
mentioned earlier, the conceptualization of
holistic system properties and behavior i s
poorly advanced. The U-C structure provides
an excellent basis for the attempt to develop
this concept .
One last point is made regarding the differences
between the Universe-Couplin g
approach and the State Variable approach .
Since the State Variable model can als o
assume hierarchical form (Goguen 1970), an d
under common explicit models of subsyste m
behavior the U-C model will reduce to a stat e
variable model, one might question the practical
validity of the distinction . Essentially, th e
differences are that one (U-C) is structur e
oriented and the other variable oriented an d
that one (U-C) is oriented simultaneously to
holistic and mechanistic representation an d
the other solely to mechanism . In the application
of ecosystem models so far produced, th e
state variable approach seems adequate ,
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