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

Cleary and Waring 1969, Reed 1971, Emmingham
1971, Waring and Youngberg 1972) .
In this article we will restrict discussion to
interpretation of results, concentrating upo n
the operational effects of water, temperature ,
and soil fertility . Mechanical stress from sno w
creep or ice storms was important as was win d
(Tranquillini 1970), but such adverse mechanical
forces are closely associated wit h
temperature gradient. Light, too, was important
in understanding succession an d
accounted for more than 75 percent of th e
variation in terminal elongation of Abies
when other environmental factors were
restricted to narrow ranges (Emmingham
1971) . In our stands, which were mature
forests, maximum height of mature trees wa s
used as an index to productivity . In such a
situation, earlier reductions in growth due to
shading or mechanical damage could be ignored
.
Measurement and Interpretation of
Plant Water Stress
During the growing season, plant moistur e
stress measured by the Scholander pressur e
chamber (Scholander et al . 1965, Waring an d
Cleary 1967, Boyer 1967) provided a goo d
estimate of plant water potential (Waggone r
and Turner 1971) . We took measurements a t
monthly intervals, with more frequenc y
during September when the vegetation wa s
under the greatest stress .
Plant moisure stress usually increases fro m
a predawn minimum to some maximum leve l
in the afternoon . Predawn stress represent s
the nearest equilibrium between soil and plant
water potential . Not only the diurnal patter n
but also the seasonal changes in plant wate r
stress are important . The first reflects an imbalance
between transpiration and water
uptake ; the latter can be related to the avail -
ability of soil water over a season . In figure 3 ,
three contrasting forest types are shown t o
have different seasonal water stress curves .
The type dominated by Engelmann spruce
(Picea engelmannii) was restricted to mois t
sites, while oak (Quercus kelloggii) grew
where stress was sufficient to bring about
cessation of cambial activity in the reference
Figure 3. Seasonal changes in minimum plant moisture
stress associated with different forest ecosystems
(Waring 19701 . All data are from 1- t o
2-m-tall Douglas-fir. For this particular set of data ,
the end of season Plant Moisture Stress Inde x
would be 30 for the Oak Type, 18 for the Pin e
Type, and 7 for the Spruce Type .
plants. Where no rainfall occurs during the
summer, the minimum night moisture stres s
at the end of the growing season is an index
to the plant moisture conditions throughout
the entire season . From figure 3, we see the
oak type had a plant moisture stress index o f
30 ; the pine, an index of around 18 ; and the
spruce, 7 atmospheres . Where summer precipitation
is important, a mathematical description
of the seasonal trend is desirable (Ree d
1971) . Winter drought due to cold or frozen
soils should also be similarly evaluated .
Measurement and Interpretatio n
of Temperatur e
At each of the 25 forest stands, air temperature
and soil temperature were recorded
on 30-day thermographs because both root
and shoot temperatures are important . At the
time, we did not have photosyntheti c
response information to help interpret th e
effect of various combinations of temperatur e
and light. Because winter temperatures are
often below freezing, winter photosyntheti c
activity is probably less in the study regio n
than in the milder climate along the Pacifi c
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