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

coast. Without the benefit of good photo -
synthetic data, we used laboratory studie s
conducted by our colleague, D . P. Lavender .
Lavender found that the dry weight increas e
of Douglas-fir seedlings was closely related to
both air and soil temperatures . With these
data a temperature-plant index was develope d
by summing the potential growth possible for
each day during the growing season (Cleary
and Waring 1969) . Other variables were
assumed nonlimiting. In the field, the Temperature
Growth Index ranged from 30 near
timberline to nearly 100 on oak and pin e
forests. Douglas-fir was not found where th e
index was below 40 .
To visualize this information more effectively,
the distributions of selected conifer s
are presented in relation to these two rathe r
simple plant response indices (fig. 4). The distributional
patterns closely reflect the adapta -
I . I . I I
10 20 30
PLANT MOISTURE STRESS, ATM .
Figure 4 . Distribution of natural regeneration in relation
to gradients of moisture and temperature de -
fined by plant response indices (Waring 1970) .
Validation stands, symbolized by 0, had vegetation
predicted by the intercept of their plant
response indices .
tion of the various conifers . In a local region ,
the variation in distribution of different
species provides a means of predicting the environment
through association with measurements
taken on reference plants . The plan t
response indices were correctly predicte d
from knowledge of plant distributions fo r
three validation stands, indicated as divide d
circles in figure 4 . It is significant that suc h
predictions are possible without physiological
observations on species other than the reference
plants and without special attention t o
events controlling establishment . Once such
relationships are established, the vegetatio n
can provide understanding of the operational
environment without requiring additional
measurements of any kind . We shall expand
this idea later .
Measurement and Interpretation of
Stomatal Respons e
Conifer stomata are most difficult to observe,
usually being sunken and occluded b y
wax . The resistance which they offer to th e
transfer of water vapor from the interior of
the needles can be assessed by determinin g
the rate of water vapor movement with a
diffusion porometer (Waggoner and Turne r
1971). The aperture of stomata may be estimated
by observing the pressure necessary t o
force a 50-percent ethanol solution through
the pores (Fry and Walker 1967). Th e
diffusion resistance is then determined by
calibrating these pressures with reduction i n
transpiration under known vapor pressure
gradients (Reed 1971) . The latter procedure
was followed in our past fieldwork . We found
that stomatal resistance increased as the soi l
moisture became less available during th e
growing season . Further increase in stomatal
resistance was possible during the day i f
evaporative stress was high . These relationships
were quantified and developed into a
simulation model by Reed (1971) . Wit h
knowledge of temperature, humidity, and
nocturnal plant water stress, the model predicted
on a daily basis, both potential transpiration
(PT) and transpiration (T) wit h
stomatal control . These values were summed
for the entire season and confirmed the obser -
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