Shaping Stand Development
Through Silvicultural Practices
Dean s. DeBen Roberto. curtis, Constance A. 'Harrington,
and john C. Tappeiner
Social Expectations and Silvicultural Opportunities
Silvicultural Practices 143
Early Density Control 143
Thinning in Older Stands 143
Nutrient Management 145
Pruning 146
Managing Dead Wood 146
Implementation 147
Literature Cited 147
Regeneration systems and harvest cutting patterns,
including retention of live and dead trees, determine
the nature of a forest after harvest and influence early
(see Tappeiner et al., Chapter 9, and Franklin
et al., Chapter 7). Subsequent silvicultural treatments
are the tools for channeling stand development to
specific or multiple objectives, which may
over the lifetime of the stand.
During the past half century, silvicultural practices
as spacing control, fertilization, and pruning
been applied extensively throughout the world
142
wherever soils, climate, social policies, and markets
combine to favor and sustain forestry enterprises. Re­
search, development, and implementation of these
practices were justified primarily on the basis of
quantity or quality of wood that could be grown or
harvested. The basic knowledge and experience ac­
quired, however, can be used to meet additional ob­
jectives. Expansion of social desires and concerns for
forest values and products may provide the justifica­
tion for further silvicultural investments.
In this chapter, we first provide some background
141
142
Section II. Silvicultural Systems and Management Concerns
on changing societal expectations and their influence
on silvicultural opportunities. Next, we describe sev­
eral silvicultural practices, how conventional applica­
tion influences stand development, <!-nd how they
might be applied or modified to meet other or addi­
tional objectives. Finally, we discuss considerations
related to implementation throughout broader forest
landscapes.
Social Expectations and
Silvicultural Opportunities
Public expectations and support for the multiple ben­
efits of forests have been kindled for decades through
forestry advertisements by both public and private
organizations. Increased attention to such values is
now demanded by various segments of society and
required by public law.
Although managed forest landscapes appear more
natural and more diverse than landscapes associated
with most human endeavors, extensive areas of
young managed plantations do contrast markedly
with natural mature and old-growth forests. Many of
the features associated with older natural forests
(Franklin et al. 1981) are minimized with conven­
tional management. Such features may include large
trees, snags, down woody debris, ragged edges,
within-stand structural complexity, and diversity in
age, size, shape, and distribution stand patterns
across the landscape (Hansen et al. 1991). Many of
these features contribute to wildlife habitat (Thomas
1979, Hunter 1990) or are otherwise considered re­
lated or essential to other forest values and general
forest health. The desire to retain some of these fea­
tures in managed forests has stimulated interest in
various modifications of conventional silvicultural
practice (Franklin et al. 1986, DeBell 1989).
Existing young natural stands (or plantations) will
not necessarily develop into stands comparable to
present old growth in the absence of human inter­
vention. Our present younger natural stands were
established after severe fires (or Jqgging and fire) in
the 19th and early 20th centuries. They were estab­
lished and are developing under considerably differ­
ent and milder climatic conditions than did present
old growth that originated in the more severe climate
of the Little Ice Age (Henderson and Brubaker 1986).
The general nature of silvicultural practices needed
to foster features similar to those of natural old
growth is fairly apparent-although the specifics of
implementation are not. It may be obvious, for exam­
ple, that a given feature provides important habitat
for one or mor species. But little is known about the
response of the forest ecosystem in general-or even
of specific species-to various levels or distribution
patterns of that feature. Even less is known about
other biological or economic costs associated with
such management. The answers will vary with forest
type, landscape, and specific stand and site condi­
tions. These uncertainties should not be used to jus­
tify reluctance or failure to modify conventional prac­
tices, however. But they should make us wary of
widespread implementation or legislation requiring
specific untested modifications, and they should be a
stimulus, for experimenting with a range of ap­
proaches in a designed and controlled manner.
The greatest near-term opportunity to develop
such management knowledge and experience and to
provide more diverse habitat lies in millions of acres
of existing forest plantations, which range in age
from 1 year to 40 or 50 years. Plantations dominate
the landscape on industrial lands, and scattered
smaller plantations are a major feature of pu lic
lands. Most in the Pacific Northwest were planted as
pure Douglas-fir. There are, however, some mixed­
species plantings-and many plantations acquire a
considerable mixture of other conifers, hardwoods,
and shrubs through natural seeding or sprouting.
Plantations are sometimes disparaged as uniform
monocultures of minimal or even negative value for
purposes other than wood production. This superfi­
cial and shortsighted view arises, in part from the
that many plantations in the Pacific Northwest are
now in the early stem exclusion stage. In this stage,
stands are relatively uniform, and dense canopies
have shaded out understory vegetation. Even with­
out further management, this uniformity is ,,o,u -J
modified by natural processes and by agents such
root diseases and snow breakage. With management,
stand differentiation will occur much more
Most of these plantations are now· in a very
stage of development, and there are major
143
B. Shaping Stand Development Through Silvicultural Practices
nities to mold them-individually and in a landscape
context-toward a variety of objectives.
such openings in abundance as stands grow older,
even without intervention.
Thinning in Older Stands
#T;c,cz=:=:'::"J
silvicultural Practices
Objectives for forest lands will vary by ownership, ge­
ographic location, and condition of surrounding
stands and landscapes. Continued pursuit of rapidly
growing uniform stands focused primarily on wood
production will be, a major or sole objective of some
plantations. Accelerated development of habitat for
threatened or endangered species may be a primary
objective for others. Judicious modification of silvi­
cultural practices can produce man ged forest land­
scapes that meet a wide range of objectives. A num­
ber of modifications of conventional silvicultural
practices merit consideration, i:lnd some are now
being applied by several organizations.
Early Density Control
Conventional early (pre-commercial) thinning is
widely applied near the end of the stand initiation
stage to enhance the survival, growth, and value of
residual trees. Thinning specifications usually are
aimed at leaving the most valuable larger trees at rel­
atively even spacing (Reukema 1975). This increases
stand uniformity, but the reduced stand density also
accelerates tree growth and promotes development
of a shrub and herbaceous understory. It frequently
leads to early establishment of tolerant tree species
such as western hemlock and western red cedar.
Modifications could involve selection criteria for
residual trees, spacing distances, and intentional cre­
ation of small openings. Trees to be left after thinning
could be selected to increase size and species di­
versity, thus accelerating stand differentiation and
increasing structural complexity. Spacing could be
widened to enhance development of understo­
ries. Moreover, spacing might be varied in patches
throughout the plantations; small openings or gaps
might be created to retain some components of the
early initial stage and ultimately develop patches
of younger trees. The root rot diseases common
throughout the Douglas-fir region often produce
(
.;:
Thinning in older tands has long been a generally
accepted practice in European countries, and it rests
on over a century of experience and research embod­
ied in an enormous literature. Yet, it has not been
widely applied in the Pacific Northwest until very re­
cently because markets for small material were poor
and large volumes of old timber were available. But
the economic and social context has changed, and
thinning is assuming increasing importance. A con­
siderable body of research on thinning practices
dates back to about 1950 (Worthington and Staebler
1961, Reukema 1972, Reukema and Pienaar 1973,
Reukema and Bruce 1977, Oliver et al. 1986, King
1986, Curtis and Marshall 1986). Thinning combined
with extended rotations can maintain forest cover for
long periods while still producing wood products.
The traditional purposes of thinning are to main­
tain growth rates of residual trees and promote stem
quality and vigor during the stem exclusion stage.
Over time, thinning produces larger trees and visu­
ally more attractive stands (Figures 8.1a and 8.1b).
One recent study in the Pacific Northwest (Marshall
et al. 1992) showed that over a 20-year period, thin­
ning in a young stand produced increases of 33 to 56
percent in diameter growth of the largest 80 stems
per acre compared to the unthinned condition. Thin­
ning also provides income and timber flow during
the intermediate stages of stand development. Thin­
ning may or may not promote vertical stratification,
depending on how it is done. It usually accelerates
understory development and succession and move­
ment df the stand into the understory re-initiation
stage. If begun early, changes can be striking over
comparatively sh@rt time periods (Figures 8.2a and
8.2b)-although on some sites, thinning can also
produce dense shrub layers that inhibit establish­
ment of desired conifers.
Thinning encourages seedling establishment of
conifers (Del Rio and Berg 1979), hardwoods (Fried et
al. 1988), and shrubs (Tappeiner and Zasada 1993,
Huffman et al. 1994, O'Dea etal. 1995), but also re­
sults in vegetative expansion of shrubs by rhizomes
144
Section II. Silvicultural Systems and Management Concerns
( appeiner et al. 1991) and layering. By reducing
overstory density and providing a seedbed, thinning
results in invasion of plants not previously in the
stand and the vegetative spread of those already es­
tablished. This generally produces a dense, diverse
understory of shrubs; herbs, and tree seedlings and
saplings.
Regeneration of understory conifers after thinning
will often enhance the development of stands with
structures similar to old-growth stands. On some
sites, however, dense understories of salal, Oregon
grape, salmonberry, vine maple, and other shrubs
(a)
(a)
(b)
(b)
Figure 8.1 (a) Unthinned portion.of an SO -year-old
Douglas-fir stand in the Black Rock ''Forest Manage­
ment Research Area near Fall City, Oregon. (b) Nearby
portion of the same SO-year-old stand, heavily thinned
at about age 45.
Figure 8.2 (a) Unthinned area in 45-year-old plan­
tation at Iron Creek study of levels of growing stock
near Randle, Washington. (b) Adjacent area in same
45-year-old plantation, with early and repeated thin­
ning.
pi ng Stand Development Through Silvicultural Practices
develop and prevent establishment of a second
of conifers. Conifer seeds will germinate under
shrub layers, but seedlings may not survive.
shrub layers are quite persistent. They produce
aerial stems (ramets) annually that replace older
as they die, maintaining a dense cover (Buff­
et al. 1994). Thus, on some sites (particularly in
Oregon Coast Range), some disturbance of these
layers and possibly underplanting of conifers
be necessary to establish multilayered conifer
mjght include favoring trees of di­
species and sizes to foster croW!). stratification
understory development. Small openings or
could be created, which ultimately will be occu­
by younger trees. Root rots often do this inde­
of human intervention. Additional species
be introduced through underplanting, if seed
of desired species is lacking. An example of
modification is irregular thinning in the Forest
"'""'""r.,"n' Study now being conducted in Fort Lewis,
(Carey 1993). As stands grow older,
trees will die and provide the snags needed for
If these are judged insufficient, additional
can be created by intentionally killing or top­
selected trees.
A less obvious but important effect of repeated
is that trees and stands maintain rapid
to older ages than without thinning. Culmi­
age for repeatedly thinned Douglas-fir in the
Northwest has yet to be determined-but it is
, �;cc:aL•cL than that for dense unthinned stands (War­
and Staebler 1961, Curtis 1994, l995). If ter­
and harvesting methods permit thinning, larger
rr"''" can be grown on longer rotations with no loss
and perhaps with even an increase in wood produc­
tion per unit area. Stem quality and value per unit of
may be increased. Repeated thinnings and
longer rotations may also provide additional oppor­
tunities and flexibility to capture benefits from stands
composed of mixed species with differing tolerances
and growth patterns.
· · ·· .. · ·
Nutrient Management
Nutrient management is an important consideration,
particularly in the Douglas-fir region where soils are
145
young and considerable nitrogen is immobilized in
organic matter. Inadequate supplies of available ni­
trogen limit natural productivity and rates of stand
development on many sites. Relatively small and in­
frequent applicatigns of nitrogen fertilizers have been
used to increase wood production in conventionally
managed forests. These applications often produce
striking responses in tree growth and stand develop­
ment (Chappell et al. 1992a.). The largest and most
long-lasting responses occur when nutrient deficien­
cies are severe (Miller and Tarrant 1983) and when
fertilizer application is combined with thinning
(Chappell.et al. 1992b).
In newly established and very young plantations,
broadcast application of fertilizer results in earlier
canopy closure and greater stand uniformity (DeBell
et al. 1986); in older stands where trees have begun
to differentiate into crown classes, fertilizer applica­
tion accelerates the stand development process of
crown differentiation and competition-related mor­
tality (Miller and Pienaar 1973, Miller and Tarrant
1983). Other properties, including soil organic mat­
ter, may be enhanced, and site resources such as
amount and nutritional value of wildlife forage· (Sul­
livan and Rochelle 1992) may be increased.
A modification of conventional practice that could
be considered is small-scale individual tree or group
application. This could be used to promote develop­
ment of a layered canop_Yt in contrast to broadcast ap­
plications, which simply increase the overall rate of
stand development. It also could be used to enhance
the growth and nutritional value of forage in open­
ings created for that purpose. Individual tree applica­
tion was deemed practicable and profitable more
than 15 years ago by scientists at the Washington De­
partment of Natural Resources (Anderson and Hyatt .
1979). Coupled with selective thinning and pruning,
individual tree applications could also increase pro­
duction of high -quality wood.
Introduction or favoring of nitrogen-fixing plants
also merits consideration. This may provide benefits
similar to those of fertilization on some nitrogen-de­
ficient sites (Miller and Murray 1978, Binkley 1983,
Tarrant et al. 1983). In addition, it may have addi­
tional benefits in terms of economic and ecological
diversity. Selected red alder trees are now favored in
early thinnings of young conifer plantations on nitro­
146
Section II. Silvicultural Systems and Management Concern s
gen-deficient soils of the Siuslaw 'National Forest
(Turpin 1981).
Pruning
Pruning has been advocated and used primarily to
increase the amount of clear wood produced in
young stands. It also has been recommended as part
of a program to control blister rust in western white
and sugar pines (Russell 1988). Mqst pruning for im­
proved wood quality has been done during the stem
exclusion stage. Current practice is to begin pruning
much earlier than was common in the past. ;.
If pruning were begun prior to stand closure, re­
moval of lower branches could increase the amount
of light reaching the forest floor and favor develop­
ment of the woody and herbaceous understory.
Other modifications could include altering the num­
ber of trees pruned and the portion of the crown that
is removed. In regimes with repeated thinnings on
relatively long rotations, removal of some pruned
trees during thinning could increase financial returns
and perhaps offset costs of other silvicultural activi­
ties.
Managing Dead Wood
In stands managed for wood production, dead trees
have been view d primarily as problems to be
avoided or minimized: as such they often are har­
vested before they die or deteriorate. Thus, spacing
guidelines are based on the number of trees that can
be grown to some desired size prior to the onset of
significant competition-related mortality. Trees killed
by insects, diseases, and fire are commonly harvested
immediately if economics and accessibility permit.Yet
dying, dead, and down trees are important compo­
nents of many forest ecosystems, and a variety 'of or­
ganisms are associated with them. These organisms
range from cavity-dwellers like bears, squirrels, and
woodpeckers to amphibians and invertebrates to di­
verse vascular and nonvascular plants and microor­
ganisms. Consideration should therefore be given to
dead wood management in multipurpose forests,
particularly those where conservation of biodiversity
is a primary management objective.
Plans for management of snags and down wood
must consider numbers, sizes, species, and dynamics
of decay and persistence. Current ideas regarding de­
sired snag numbers are speculative, but based on as­
sumptions derived from basic information on wood­
pecker populations and snag use. Forest managers
and wildlife biologists from nearly every region have
suggested that an average of 5 to 10 snags per
hectare (two to four snags per acre) is adequate
(Hunter 1990). Generally, there is a minimum size
(diameter and height) suitable for each species, but
larger is always better because more species will
make greater use of the snag. Biologists seem to pre­
fer slowly decaying species because they will persist
longer. However, both hard and soft snags are
needed. New snags of suitable characteristics must
be recruited periodically to replace those that have
fallen and are providing values associated with down
wood. Lacking information to the contrary, managers
can assume that management efforts that provide
adequate snag populations will also result over time
in adequate amounts of down wood.
Three matters specific to dead wood management
should be considered in silvicultural efforts to shape
stand development and vegetative habitat:
1. Protection or retention of standing dead trees
(snags) and down trees that have developed naturally.
At least some and perhaps all of these should be re­
tained during silvicultural operations where they
consistent with management objectives and safety
siderations.
2. Creation of snags and down trees in stands where
they are scarce or absent due to past human or
disturbances. Trees can be killed by girdling or
with herbicides-yet the biological effectiveness of
methods is uncertain because decay proceeds from
outside in rather than from the inside out. Some
gists and managers have tried othef techniques,
ing blowing off tops with explosives, cutting off
with saws, inoculating trees with fungi, and """ ----··
beetles with sex pheromones (Bull and Partridge
Conner 1983). Operational and safety
may make it more feasible to create and preserve
within small uncut groups of trees, rather than
uted over an area. Because of the importance of snag s
wildlife, and the high economic cost of devoting
trees to such purposes, it is critical that uncertainties
resolved and methods for snag creation be nP,telcw c
Stand Development Through Silvicultural Practices
Creation of cavities. In general, retention of existing
perhaps supplemented with snag creation, will
cavities sufficient to meet wildlife objectives. In
instances, however, installation of nest boxes
for wood ducks and bluebirds) or creation of cavi­
by den-routing (Carey and Sanderson 1981) or by
a hole with a chainsaw and covering it with a
(Carey and Gill 1983) may be appropriate.
approaches currently are being evaluated for ef­
on flying squi;rel populations in young-growth
·forests (Carey 1993).
In general, ·many standard silvicultural practices,
as thinning, fertilizing, and pruning, can be ap­
to favor the growth and development of trees of
characteristics (size, species, form, age) in
locations. With appropriate rotation lengths
. ·"""'
·
Curtis, Chapter 10, and Franklin et al., Chapter
and an understanding of the dynamics of snag
it should be relatively easy to provide
dead
and down wood in amounts and loca­
·'"u" ·
M
consistent with forest management objectives.
..
Selecting and applying silvicultural measures re­
quires managers to identify specific objectives. What
is the desired future stand condition? W hat are the
147
relative values to the landowner and to society of the
forest outputs involved-wood, wildlife (what kinds
of wildlife?), water, aesthetics. And what are the as­
sociated costs?
Decisions also. require that options be considered
in the context of the surrounding landscape, rather
than on a stand-by-stand basis. Will productivity for
any of these values be markedly influenced by treat­
ment of the stand? Can a particular treatment regime
enhance or provide conditions that are needed and
now lacking in the larger unit? Applicable measures
and reasonable objectives will differ with stand type.
It must also be recognized that there is much un­
certainty, both in objectives and current judgments,
of treatment effects on values other than wood pro­
duction. Management objectives of a few years ago
were markedly different from those of today, and
those a few years hence will differ from current ideas.
For example, it is generally recognized that some
snags and down wood are important, but we do not
know how much is necessary or how it should be
distributed in different forest landscapes, and we will
not know without long-term experimentation and
monitoring. Incomplete knowledge and uncertainty
about the future are facts of life.
Despite uncertainties, we have a large body of
knowledge based on research and experience show­
ing that stand characteristics and rates of stand de­
velopment can be markedly altered within relatively
short time periods through silvicultural treatment­
for whatever future objective may be selected.
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