The
amount of light environments (classes) is highly variable within each
retention unit (Fig 4). There is a highly signifcant difference
in DIFN amoung retention levels (F(2,130)=138.6, p<0.001). When
no harvesting takes place (100% rentention), the amount of light
illuminating the crowns of the regenerating understory can reach up 60%
of full sunlight. The opposite holds true when a substantial
amount of the basal area (25%
retention) is harvested. Overall, the composition
of light environments is that increasing the amount of retained
trees, decreases the amount of high light environments. In
other words, percent of full sunlight is lowest in 100%
retention levels (p<0.001) followed by 50% retention level
(p<0.001) and 25% rention level.
dHt:
In the 25% retention level there was a significant species
height reponse (F(2,3)=10.16, p= 0.0461). Hw had a greater height
increment than both Fd and Cw (p=0.0425). There was suggestive evidence
that Fd obtains a higher height increment than Cw
(p=0.1248). This corresponds with traditional definitions of shade
tolerance, where growth preformance is lower (realtively) in higher
light environments. In the 50% retention level there was a
significant species height reponse to the treatment (F(2,3)=63.56,
p= 0.0035). Hw's height remained the highest relative to both Fd
and Cw (p=0.0054). Cw was not significantly larger than Fd
(p=0.6848). As the amount of light reaching the canopy declines,
the growth reponse of Fd becomes incresingly similar to Cw's. In
the 100% retention level Hw was larger than Cw
(p=0.0425)(Fig. 5).
Volume:There
was a significant difference in volume increment among retention levels
(F(2,4)=71.72, p=0.007). There is strong evidence that
a retention level of 25% has a greater increase in
volume than a 100% retention level (p=0.0008), but only
sugegstive evidence that its volume increase is larger than a 50%
retention level (p= 0.0842). The 50% retention level has a greater
volume increase than a 100% retention level (p=0.0028). In the 25%
retention level there is no difference among increase in
heights (F(2,3)=1.64, p= 0.33). In the heavy treatment Fd and Cw
are not significantly different (p= 0.0856). Hw is significantly
larger than Fd (p=0.0104) and Cw (p= 0.0035). In the control treatment
Hw is not significantly larger than Cw (F(1,4)=4.14, p=0.1115). Fd is
not found in the control treatment. As the amount of trees retained
increases the volumes also decrese for both Cw and Fd. Fd decreases at
a faster rate than Cw when 100% of the trees are retained. This
suggests that Cw is more shade tolerant than Fd (Fig 5).
When
species volume increments are compared on a percent of full sunlight
basis, their is no differnence amoung species in their preformance
after 40% of fulll sunlight(p=0.1664;(F(2,30)=1.34, p=0.6778). This
suggests these species have reached their light saturation points. In
light class one(0-20) and two (20-40), Cw does signifcantly worse
in volume increment than the other two species (p=0.0109, p= 0.03,
repectively)
It seems Douglas-fir and Western hemlock would be
the predominate species in the future composition of the stand. The
survival of Douglas fir in low light environments come as a
surprise, as its occurance in the control (100%) treatment was
nill (Fig 7). Though the occurance of Cw was higher and its growth
was lower, it is safer to say that hemlock and cedar will dominate the
furture stand with a smaller component of Fd scattered through out the
stand. This scattering occurance will not be based on light but some
other factor.
The
response of species to shading shows a logarithmic pattern, where
volume increase is proportional to the log increase in LAI. All species
respond reasonably well to the variable natural log of LAI, as all
regressions were significant (p<0.03). Hw and Cw respond similarily
to shadding, though Hw is consistently achieving more volume in all
levels of shadding. As LAI increases, there seems to be a slight
increase in the rate at which Hw increases its volume (Eq.4).
Conversely, Cw's volume increment has a flater slope (Eq.5). This
suggests Cw and Hw have similar tolerance to shade. Fd has the steepest
slope and its increment in volume is most affected by an increase in
LAI (Eq. 3). Cw plateau's it volume increment around an LAI of 4.
In this environment, there is a slight trend that Cw surpasses the
volume gain of Fd (Fig 6).
[3](Fd) Vol=
-0.1541ln(LAI)+0.3377
R2 = 0.6106
[4](Hw) Vol = -0.144ln(LAI) +
0.4119 R2
= 0.3622
[5](Cw) Vol = -0.0805ln(LAI)
+0.2236 R2
= 0.4378
In conclusion, seedlings will increase their volume and height increment with increasing removal of the basal area.
Light
is a poor indicator of growth preformance. Light microsites
created by retention prescriptions vary drastically. Douglas-fir
responds to these microsites in a smilar way as western hemlock, in
terms of growth preformance. Though the prescence of douglas-fir in the
unharvested (100%) retained stand is nill. This suggests that light
isn't an important factor in determining the species survival, just its
preformance.
Typically, planted trees follow a negative
logorthimic curve in response to increasing LAI. Shade tolerant species
have larger increases of volume in lower light levels than higher
higher light levels, realtive to shade intolerant species.
Specifically, at an LAI of approximately 4 the volume increment changes
from (Hw >= Fd >> Cw) to (Hw > Cw >= Fd).
Finally,
light environments have a similar affect on volume increment among
species; as species start to reach their light saturation points after
40% of full sunlight. The growth response is not indicative of
classical shade tolerance rankings. Where Hw does significanty better
than the other two species in all light environments. Classical shade
tolerance ranking (Cw>Hw>>Fd) does estimate the survival and
composition of future species, though Fd did survive in increasingly
low light environments.
The results obtained in this study
should be compared to other silvicultural systems (shelterwood), to
fully understand the ecology of forest operations.
