Abstract:
In western Oregon, hardwood trees occupy 20% of the
timberland but account for less than 1% of the timber
harvest. Information about how to manage them effectively
is limited.
The objective of this study was to examine: 1)
effect of thinning on tree growth, plant moisture stress,
and crown cover expansion and 2) the inter-relationships
of these three factors.
The study site for red alder was on the western slope
of the central Oregon Coast Range. The trees were about
20 years old when thinned in 1986. The red alder
experiment was designed as a completely randomized design
and each treatment was replicated three times. The
average stem densities per ha were 1466(100%), 535(36%),
and 412(28%).
The Pacific madrone study site was about 8 km west of
Central Point in southwestern Oregon. Tree ages ranged
from 40 to 45 years when thinning was done in 1984.
Measurements continued for 5 years. The study was
designed as a randomized block design and composed of
three blocks. The average stem densities per ha were
2290(100%), 486(21%), and 272(12%).
The data were analyzed for two groups of sample
trees, average plot trees and crop trees. The average
plot trees were random samples of each plot. The crop
trees were dominant trees that would be left to harvest
at the end of the rotation.
The effect of thinning on soil moisture
availability was evaluated by measuring predawn plant
moisture stress (PMS) with a pressure chamber once in a
summer.
To determine the effect of thinning on light
availability, five fisheye photographs of crown cover
were taken per each plot and analyzed using an automated
fisheye photograph analysis system.
For tree basal area, red alder crop trees in the
heavily thinned plot increased 66% (13.1 cm²) more than
those in the control stand but average plot trees did not
grow more. Tree growth of red alder was negatively
correlated with stem density, but was not significantly
correlated with plant moisture stress. For Pacific
madrone, average plot trees in the heavily thinned stand
increased 286% more than those in the control stand, while crop trees increased 589% (13.2 cm²) more. Tree
growth of Pacific madrone was significantly correlated
not only with stem density but also with plant moisture
stress. However, it was not significantly correlated with
crown cover. Plant moisture stress might be a reason for
marked effects of thinning on Pacific madrone tree
growth.
Total height and merchantable height growth were not
significantly different among the treatments for both
species. For red alder, however, height growth of
the heavily thinned plot was significantly less than
that of the control plot. Total height and merchantable
height growth were not significantly correlated with stem
density or plant moisture stress.
There were no significant differences in plant
moisture stress among the treatments for red alder. For
Pacific madrone, however, plant moisture stresses of the
thinned plots were 37-42% lower than those of the control
plot in the third and fifth year after thinning.
The increases of stand basal area and volume were
not significantly different among the treatments for both
species. If the management objective is to increase
growth of crop trees without losing stand productivity,
it is recommended that the stands should be managed at a
low density range (stand density index 300-360 for red
alder, 200-290 for Pacific madrone).
