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 Figure 1 |
Silvicultural Pathways for Coastal Pacific Northwest Forests
Nearly 80% of low elevation stands in coastal Pacific Northwest
(USA) forests are second-growth stands or plantations dominated
by Douglas-fir, western hemlock, and (less frequently) red alder.
The stands may be either pure single species stands or mixtures
of these species and others (e.g., western redcedar, white pine,
and grand fir). Historically, silvicultural systems were developed
to maximize the wood production of one or a few species. In the
coastal PNW clearcutting was the favored silvicultural system
for the establishment of Douglas-fir because Douglas-fir grows
fastest in a high light environment and has light, wind-dispersed
seeds. However, a wide range of silvicultural treatments can be
applied to coastal PNW conifer stands (Figure 1). The resultant
changes in stand structure over time will tend to follow one of
several silvicultural pathways, although the ability of a given
stand to follow a certain pathway is a function of its current
structure.
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Figure 1 |
NO
ACTION
Stands left to grow without manipulation will remain stable if
they are differentiating well (Figure 2). In differentiating stands
there is wide variation in tree sizes as some trees grow larger
and other trees become suppressed and eventually die. Stands which are
uniform in size tend to stagnate, rather than differentiate, if
left to grow without manipulation. Trees in stagnating stands
grow uniformly tall and generally have little diameter growth.
The trees become increasingly tall and thin and develop increasingly
small live crowns. Eventually, stagnating trees develop high height/diameter
(H/D) ratios and become susceptible to insect attacks, diseases,
and windthrow.
Figure 2
THINNING
Thinning removes trees from a stand to promote growth of the remaining
crop trees. Thinning can accomplish many things:
Figure 3
 Figure
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 Figure
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Figure
6
Figure 7
Figure 8
SELECTION
(Multiple Age Cohorts)
Selection harvesting (i.e., uneven-aged or multiple-cohort management)
is the removal of individual trees or small groups of trees to
promote the establishment of a new cohort of trees or the release
of smaller trees. Selection harvesting was widely practiced in
the US during the 1920-40s; however, it was later de-emphasized
because it did not successfully regenerate stands (Boyce and Oliver
1999). The
basis of this problem stemmed from the mistaken assumption that
smaller trees in forests were younger and would grow vigorously
when released. As a consequence too many older, small trees were
left, inhibiting the establishment and growth of a new cohort
of trees. In the example shown in Figure 9, a Douglas-fir stand
was thinned to 100 trees per acre and had 300 Douglas-fir seedlings
planted in the understory. After 60 years of growth many of the
regenerated trees have died, while those that survived are still
small.
Figure
9 To provide a stand with many age
classes, the stand must be thinned to a low density of residual
overstory trees (Figure 10). Recent studies have shown that an
overstory of 20 vigorous trees per acre will reduce growth of
regenerating Douglas-firs to ~45% of their potential in full sunlight
(Wampler 1996). If the density of residual overstory trees is
>20 TPA, the Douglas-fir regeneration in the understory will
begin to die.
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Conversion of a single-age stand
to a multiple-age stand requires a transition through the savanna
structure. Stands most likely to be converted to multiple-age
stands are those that retain stable overstory trees (Figure 10).
Unstable overstory trees (i.e., small live crowns and high height/diameter
ratios; Figure 11) are more susceptible to breaking and windthrow
and are less likely to respond well to release.
Figure 11As the younger age cohort develops
in the understory of the multiple-age stand (Figure 12), active
silvicultural treatments such as thinning may be needed to reduce
the density of the understory trees in order to maintain their
vigor.
Figure 12
CLEARCUT/SHELTERWOOD/SEED
TREE (Single Age Cohort)
Single age cohort stands (i.e., evenaged stands) can be created
by clearcutting or by retaining a few overstory trees, either
in clumps or distributed throughout the stand (Figure 13).
Figure
13
Early "clearcutting" removed only those trees economically
valuable for timber. The residual trees, containing crooks, forks,
and flat tops were left. Such harvesting practices led to the
establishment of a stratum of slow-growing, deformed trees of
little economic value. Some
species, such as hemlocks, become flat-topped (weak epinasty)
when growing under shade. When released, they retain a crook in
the stem which can still be visible many decades later (Figure
14). Although less valuable for timber, retaining some trees with
such crooks provides wildlife habitat. For example, the trees
in the stand below--which was clearcut 90 years ago--provide marbled
murrelet nesting sites.
Figure 14
Foresters developed the term "silvicultural clearcut" to describe stands where the residual, non-commercial trees were also felled to allow vigorous, straight trees to regrow. Natural regeneration led to stands with irregularly spaced trees which encourages differentiation as the trees grow (Figure 15). In contrast, trees in plantations are usually planted at regular spacings and are more likely to stagnate without timely thinnings (Figure 16).
 Figure 15 |
 Figure 16 |
When trees are grown at narrow spacing, they often slow in diameter before they are large enough to be thinned for profit.The diameter of the tree at the time diameter growth slows is quite predictable. For example, in Douglas-fir trees planted at 8 X 8 foot spacing (680 trees/acre) diameter growth slows dramatically when the average tree is ~8 inches DBH. Douglas-firs planted at 12 X12 foot spacing (300 trees/acre) slow at about 11 inches DBH (Oliver et al. 1986). If an investment is not made to precommercially thin the stand, the trees may grow more in height, but not diameter, which makes the trees increasingly unstable (Figure 17, left). Eventually the stand will stagnate and the trees will bend or blow over in winds or wet snows. Planting trees at wide spacings avoids the costs of precommercial thinning and delays the danger of stagnation, but can create very large branches that devalue the tree for timber (Figure 17, right). Pruning trees can also avoid the large branches; however, pruning many trees at narrow spacings is expensive and does not allow the pruned trees to grow rapidly in diameter (Figure 18, left). Pruning fewer trees at wide spacings costs less and allows the pruned trees to grow clear wood rapidly (Figure 18, right).
 Figure 17 |
 Figure 18 |
Species other than Douglas-fir can be grown productively in pure (single species) stands. For example, western redcedar can be grown in pure stands and will produce timber volumes comparable to Douglas-fir by age 50 on moderate and highly productive sites (Figure 19). Red alder can also be grown in pure stands or mixed with conifers (Figure 20).
 Figure 19 |
 Figure 20 |
MIXED
SPECIES
Mixed species stands have promising features for meeting many
management objectives.
 Figure 21 |
 Figure 22 |
 Figure 23 |
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