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

ful in regions with relatively uniform climate .
Probably the most helpful classifications
from a standpoint of predicting the nature o f
forest ecosystems, however, have been thos e
related to environmental gradients (Warming
1909, Sukachev 1928, Pogrebnjak 1929 ,
Bakuzis 1961, Ellenberg 1950, 1956, Row e
1956, Whittaker 1956, 1960, Loucks 1962 ,
Waring and Major 1964) . Unfortunately, al
previously defined environmental gradient s
cannot be directly applied outside the particular
region where they were developed . We feel
environmental gradients can be more widel y
utilized only when the environment i s
coupled to basic processes controlling plan t
growth and composition . We believe a key t o
expanding the gradient analysis approach is t o
focus more closely upon the basic physiological
behavior of plants .
Fortunately a foundation for a process -
oriented approach has already been laid .
Mason and Langenheim (1957) defined th e
idea of an "operational environment" as on e
which directly affects an organism. Implicit i n
their concept of environment is the recognition
of specific plant responses to environmental
stimuli. In fact, these authors felt tha t
"environment" could not exist independently
in an ecological sense . It must have an effec t
upon an organism . Although they did not de -
fine measurable environmental stimuli, thei r
very definition of an "operational environment"
focuses attention upon the influenc e
rather than on the origin of the stimulus .
Thus, in an interpretive ecological sense, i t
is more important to assess the availability o f
water to plant roots than the origin of tha t
water. This distinction greatly reduces the
number of factors requiring consideration, for
although altitude, slope, and other physiographic
features are correlated with vegetation,
such indirectly operating factors may be
ignored if the mode of action can be identified
and measured . Ellenberg (1956), Bakuzis
(1961), Waring and Major (1964), and other s
have suggested that vegetation responds t o
changes in water, temperature, light, and
chemical and mechanical factors. Although
these factors do not operate independently ,
they cannot completely substitute for one
another .
General Approac h
We still have to translate the concept of an
"operational environment" into a design adequate
for research . As a first step we can diagram
the flow of material through a plant into
various compartments and identify the controls
on the rate of flow imposed by the
environment, the organism, or the amount o f
material in a given compartment. Figure 1
Figure 1 . General model of primary production .
Material inputs of H 2 0, CO 2 . and energy flow int o
the system to form carbohydrates . The rate of incorporation
(photosynthesis) is a function of
various plant responses. These responses are
triggered by a host of environmental stimuli :
temperature, light, humidity, soil water potential ,
soil fertility, and mechanical stress. The plant
responses interact to provide two functions: (1) a
control on carbohydrate storage and (2) a contro l
on growth. Losses from the system are throug h
respiration (R), death of roots and other organ s
and in litter (L), and consumption by animals (C) .
presents such a diagramatic model . Th e
rectangles are the compartments, and the flo w
of materials is indicated by solid lines, whil e
dotted lines indicate the transfer of information
through valves controlling the rate of
material flow from one compartment to an -
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