HGT = -285.5 + 17.8SI50 + 111.4log(TPA): r2 = 0.56
We normally think of stand density having a large influence on diameter. The
first equation does show this; the log of trees per acre has a strong influence
on stem diameter, larger than the influence of site index. When only the 100
largest trees per acre are examined, however, the effect of stand density is
not significant. The mean diameter of these larger trees does not appear to
be affected by the density of its neighbors. This suggests that alder does a
better job of canopy stratification than expected. Site index does account for
some of the variation among the 100 largest trees.
Height is usually thought of as being relatively independent of stand density.
The first height equation seems to contradict this rule of thumb: stand density
does explain a lot of the variation in height. The coefficient for log (TPA),
however, is positive, indicating that height increased with stand density. For
the largest 100 trees per acre, stand density is still an important explanatory
factor, even more important than for the whole population of plots. Site index
also continues to be an important explanatory variable.
Figure 2 shows an interesting interaction between density and site index. The
effect of density is quite strong at low site index. At the highest site index,
the effect has weakened considerable; the points plotted above site index of
34 seem to show only a slight density effect on mean height for all trees.
Studies of density effects in young stands of red alder and Douglas-fir elsewhere
have shown a maximum height growth at intermediate densities (Knowe and Hibbs
1996, Scott et. al. 1992). We see the maximum height at the greatest densities.
These two statements may not be at odds; for the size of our trees, 1200 TPA
may be an intermediate density. Alternatively, our study design may just not
be appropriate to pick up the phenomena. For example, our density intervals
are large so our study design may not be sensitive to this phenomena.
This second set of analyses examined the relationships between tree quality
characteristics (deviation, forks, lean, and damage) and stand density. We used
individual tree characteristics (not plot means) for 1) all the trees on a plot
and 2) the 100 trees per acre with the largest diameter.
Deviation is a measure of the sweep in the lower stem. It is measured in inches
as the distance between the trunk and a 9 foot pole placed against a trunk with
its lower end 1 foot above the ground. Forks are forks in the trunk. Lean is
the angle in degrees of a line connecting the stump to the tip of the tree.
Damage is evidence of any of several kinds: weather, animal, human, insect,
disease, and other.
Table 2 shows that only the incidence of forking is affected by density, with
lower incidence of forking occurring at high densities. A surprise is that there
is no difference in incidence of any characteristic between the total population
of stems and the 100 largest per acre. Our initial expectation had been that
the larger trees would have scored better than the average tree on most of these
characteristics. This result suggest that thinning regimes that simply favor
the larger trees will not improve stem quality in terms of these 4 characteristics.
| Treatment |
Deviation |
Forks |
Lean |
Damage |
|
All Trees |
| 1 (100 tpa) |
22 |
22 |
14 |
15 |
| 2 (230 tpa) |
23 |
20 |
13 |
16 |
| 3 (525 tpa) |
24 |
14 |
11 |
14 |
| 4 (1200 tpa) |
31 |
13 |
13 |
22 |
|
Largest 100 TPA |
| 1 (100 tpa) |
23 |
22 |
15 |
15 |
| 2 (230 tpa) |
27 |
20 |
11 |
15 |
| 3 (525 tpa) |
22 |
12 |
11 |
15 |
| 4 (1200 tpa) |
25 |
12 |
11 |
19 |
