BIOLOGICAL AND OPERATIONAL CONSIDERATIONS IN
IMPLEMENTING SILVICULTURAL REGIMES
Constance A. Harrington
USDA Forest Service
Pacific Northwest Research Station
Olympia, Washington 98512
and
Walter H. Knapp
A.G. Crook Company
Beaverton, Oregon 97006
ABSTRACT
Selection of the silvicultural practices appropriate for carrying out a High Quality Forestry
regime depends on site and stand conditions and forest management objectives. Site
conditions include site productivity, soil factors, physiographic factors such as slope and
aspect, and the context of the site in the broader landscape. Stand conditions include life
histories of tree species, specific susceptibility to tree damage, rooting patterns, economic
premiums, species composition, and stand density. Management objectives include (but are
not limited to) the production of high-quality wood. Stand development under long rotations
can take several paths. There is much uncertainty about specific silvicultural methodology for
long rotations because stands have not yet been managed for long periods. Caution about
imposing a single silvicultural regime is justified.
Keywords: long rotations, Pacific Northwest, silvicultural regimes, site conditions, stand
conditions.
INTRODUCTION
This workshop was convened to discuss High Quality Forestry (HQF), a silvicultural
regime proposed by M. J. "Gus" Kuehne for National Forests in western Washington
(see Appendix to this volume). In this paper, we discuss the biological and operational
considerations involved in implementing that regime. Because such implementation
has not yet occurred, our comments are fairly general and may be thought of as a
"checklist" of items to consider. Although HQF has been specifically proposed for
western Washington, some of our comments are illustrated with examples from east
of the Cascades--appropriately so because regimes similar to HQF may be considered
for other geographic areas.
About this file: This file was created by scanning the printed publication.
software have been corrected; however, some mistakes may remain.
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Misscans identified by the
Harrington, C.A.; Knapp, W.H. 1994. Biological and operational considerations in implementing silvicultural regimes. In: Proceedings of the High
Quality Forestry Workshop: The Idea of Long Rotations; 1993 May 10-13; Silver Falls State Park, Oregon.
Seattle: College of Forest Resources, University of Washington: 35-42.
The first topic we cover is site conditions: that is, what site conditions should be
considered prior to implementing a silvicultural regime that calls for repeated entries
and an extended rotation (1 50 to 200 years)? In some locations, existing site
conditions may require alterations of the proposed regime; in other locations, the
regime may not be feasible. The second topic covered is stand conditions. The HQF
regime has been presented as starting at age 0, that is, when the stand has just been
regenerated or is about to be. However, some managers may consider imposing this
regime on stands somewhere between age 0 and final harvest age under HQF. Our
discussion, therefore, focuses on how existing stand conditions may affect
implementation of HQF. Finally, we consider the role of management objectives and
mention possible constraints on implementation of innovative silvicultural regimes.
SITE CONDITIONS
There are many site-related conditions to consider when selecting and implementing
silvicultural regimes. Failure to consider them often results in stands that do not meet
management objectives, increased costs for planned activities, or the need for site
restoration. The key site conditions that should be evaluated are site productivity, soil
factors, physiographic position, exposure to wind and other climatic factors, and
position within the landscape.
Site productivity is important because it controls potential growth rates of the stand
and will thus determine the expected yields from and timing of intermediate
treatments. Before selecting a silvicultural regime, silviculturists should determine
whether the management objective for the stand is compatible with its growth
potential. They should also be concerned with maintaining the long-term productive
capacity of the site; thus, soil or site factors that predispose an area to compaction,
erosion, or excessive loss of organic matter need to be considered. In addition, the
factors limiting tree growth should be examined. For example, if a site is nutrientdeficient, care should be taken not to reduce the nutrient capital during silvicultural
operations. In some stands, fertilization may be needed in conjunction with thinning
if stand development is to proceed as desired.
On some sites, soil factors may be important for reasons other than their direct effect
on site productivity. A site with compacted glacial till close to the soil surface, shallow
soils over bedrock, skeletal soils, poorly drained soils, or those with other limitations
to rooting will be more subject to windthrow than sites where rooting depths are less
restricted. Sites with this limitation can sustain only a restricted range of silvicultural
regimes. Other soil factors that should be considered are those, such as susceptibility
to mass movement, that limit equipment usage.
Physiographic factors, such as steepness and length of slope, also limit equipment
usage and the design and location of road systems. Because operating equipment on
steep slopes is usually more expensive, changes may be necessary in timing and
intensity of intermediate treatments (e.g., it may be necessary to remove more volume
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per entry but reduce the number of entries).
As the 1993 Inaugural Day storm in Washington State reminded many of us, wind
exposure is an important factor in selecting the appropriate regime. Stand location
should be considered in relation to prevailing winds and topographic features that
might block them. In addition, the shape, size, and structure of a stand will affect its
susceptibility to damage. How wind exposure has affected or may affect silvicultural
treatments should also be examined. Stands not yet scheduled for long rotations with
multiple entries may have high height/diameter ratios that indicate increased risk of
wind damage. Fertilization often increases crown density, which may not be desirable
on highly exposed sites. Pruning can increase or decrease susceptibility to wind
damage depending on tree and stand conditions. Consideration of the possible
damaging effects of wind is important in all silvicultural regimes but warrants additional
emphasis when long rotations are planned.
Climatic factors other than wind that should be weighed are frost pockets (and how
they affect regeneration) and the level of hazard associated with heavy snows and ice
storms. Hazardous sites may need altered prescriptions to ensure that management
objectives are met.
A final site factor to consider is the condition of the landscape within which the
silviculturist is operating. How adjacent stands are managed can influence the timing
of silvicultural activities as well as the degree of fire, wind, and insect hazards. Such
complex interactions are often best analyzed using a geographic information system (
GIS).
STAND CONDITIONS
The structure, composition, and condition of the stand directly affect the silviculturist's
ability to produce high-quality wood and other outputs from HQF. The species
composition of the stand will determine what options are realistically available to the
manager. The following characteristics of all species suited to the site should be
evaluated:
Longevity. Species that survive and grow well for extended periods are suited to
long rotations. Examples include Douglas-fir, western hemlock, and ponderosa pine.
Conversely, short-lived trees such as red alder must be managed on different
regimes. If the existing stand is not at rotation age, a decision must be made as to
how long it will be held prior to harvest and the implementation of another regime.
Susceptibility to wounding and pathogens. Thin-barked species are especially
vulnerable to damage during harvest. Logging damage may ultimately lead to decay
and loss of value. Thus, species such as western hemlock and the true firs are
probably best managed under regimes that minimize the number of entries. In
addition, some species are more susceptible to specific pathogens than others and
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may not be suited to sites where those pathogens are present. For example, sites
where inoculum levels of laminated root rot are high are best managed for resistant
species such as western redcedar or one of the pines, or for immune species (e.g.,
hardwoods). Susceptibility to pathogens is important to stand management under
any silvicultural regime but becomes more critical with extended rotations.
Rooting pattern. Shallow-rooted species such as western hemlock are less likely to
withstand winds than deeply rooted species such as Douglas-fir. Wind-firmness is
essential to long rotations with multiple entries.
Economic premium. One could favor species expected to command a premium price
for high-quality wood. However, those dealing with long rotations would probably
be more prudent to favor the species with the best growth potential on the site, even
if it is not the most valuable in current markets.
Species mixture. The silvical attributes of the various species in mixed stands affect
how such stands develop as well as the wood quality and vigor of all their members.
For example, a shade-tolerant species such as western hemlock could affect the
branching characteristics of a less tolerant species such as Douglas-fir by
encouraging it to self-prune, a result that would increase its economic value in the
long term. Conversely, on many sites in eastern Washington and Oregon, ponderosa
pine often loses vigor when growing in competition with white or grand fir. In many
cases, these potentially high-value pines will not survive to produce merchantable
products if not favored in early stand entries.
Stand density affects the health and vigor of individual trees and ultimately of the
stand itself. In overly dense stands, the lower branches of trees are shaded out. As
a result, crowns are shorter and narrower, and diameter growth is slowed. Vigor
declines, and, if an excessively dense stand is thinned, the residual trees are often
more susceptible to windthrow and snow or ice damage. Gross volume growth in
dense, untreated stands may be high, but merchantable production may be low. Older
stands that are excessively dense are often not able to respond to manipulations as
readily as younger stands or those with lower densities.
High stand densities not only have a detrimental effect on tree health, but also increase
certain biological hazards. For example, in eastern Washington and Oregon,
susceptibility to outbreaks of mountain or western pine beetles in pine stands increases
markedly at high stand densities. West-side stands of Douglas-fir increase in
susceptibility to attacks by the Douglas-fir bark beetle as the number of recently dead
stems greater than 1 1 inches in diameter increases; the risk rises regardless of whether
the dead stems are standing or on the ground, and whether they died naturally or from
human intervention (e.g., disease, harvesting or the creation of snags).
Stands that have too few trees to fully occupy the site may be less vulnerable to
environmental damage such as windthrow or snowbreak, but their production of wood
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may be reduced. Nevertheless, a surprisingly small number of well-distributed trees
can occupy a site and provide a high rate of production. For example, a 45-year-old
stand of Douglas-fir at Black Rock State Forest in western Oregon was thinned to
about 50 trees per acre and underplanted with western hemlock. The stand is now
about 85 years old, and is growing as well as or better than other comparable stands
with heavier stocking. The Douglas-fir trees in this stand are very large, and have
relatively clear boles without pruning, i.e., they are producing high-quality wood.
Because of the low number of trees per acre, however, it would not be reasonable to
plan multiple thinnings prior to final harvest.
Tree condition also affects future management options. For example, if snowbreak
occurred in a stand many years ago, many of the stems could be forked or have decay
columns (which would reduce value) or be structurally weakened (which would
increase long-term susceptibility to windthrow). Managers must also consider how to
handle future damage--that is, if a stand is badly damaged, will it be salvaged and
regenerated or be retained until the end of the planned rotation?
Stand structure probably affects the development of the future stand as much as any
other characteristic. Yet many of the young stands now being managed in western
Oregon and Washington have developed from plantations and are of relatively simple
structure. Much of the research on young stand development was done in such
stands; consequently, our understanding of structurally complex stands is limited.
Stand structure and composition determine not only stand development and thus the
potential for management, but also the risk of damage or loss. The risk of wildfire is
associated with the amount and arrangement of fuels, the topography, climate, and
ignition sources and probabilities. Of these factors, the arrangement of fuels most
directly affects fire hazard. For example, the "ladder" fuels associated with stands of
multiple size classes often result in crown-fires, which can become stand-replacement
fires.
Stand structure and composition also affect risk to insects and diseases. As
mentioned above, species differ in their susceptibility to root rots and other
pathogens. In many plant communities east of the Cascade Range, conversion of pinedominated stands to grand fir and Douglas-fir has resulted in prolonged epidemics of
western spruce budworm. When insect or disease problems become serious,
management options for developing long-rotation regimes emphasizing quality are
diminished.
Other biological conditions should be considered when developing management
prescriptions. These include the effects of other vegetation--shrubs, grasses, forbs,
etc.--and of animals on the stand. Although these are standard considerations in
developing any silvicultural prescription, they take on special importance for long
rotations. For example, in some areas bear damage in plantations of Douglas-fir has
recently become a major problem. With short rotations, there are more frequent
opportunities to replace damaged stands. With long rotations, however, there are
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fewer opportunities to replace the stand, and the reduction in growing stock--often the
best trees in the stand--may reduce the quality and quantity of wood produced. On
the other hand, if site and stand conditions make regeneration costly or difficult, long
rotations will reduce the number of times regeneration is necessary.
These disturbances--insects, diseases, wind, fire, other vegetation, and animal
damage--can operate independently or can interact over time, often resulting in the
destruction of entire stands. In some cases, they also affect adjacent stands or entire
landscapes (as in stand-replacement fires). Many types of risk increase over time, and
some risk factors increase in a non-linear fashion. Silvicultural practices can play a
major role in reducing many of these risks. However, the practices needed will vary
with current stand conditions and may involve much more than thinning entries on a
predefined schedule.
MANAGEMENT OBJECTIVES
Management objectives also influence the choice of silvicultural regime. In this paper,
we assume that production of high-quality wood is one of the major management
objectives. However, even with that assumption, there are several other
considerations that may be involved in selecting and implementing a silvicultural
regime. First, are there objectives for specific wood products? That is, is the product
of highest value likely to be poles, veneer, or sawn timber? Can we anticipate how
changes in industry standards or technology are likely to affect the relative price
premiums? If all trees are not pruned, how will pruned trees be tracked and future
values determined? Although many of these questions cannot be answered with
surety, any assumptions made about them should be acknowledged in forest
management planning.
Many forest lands, especially those in public ownership, will not be managed solely for
timber production. If development or maintenance of specific stand structures is
needed to meet specific wildlife objectives, then the appropriate silvicultural regimes
are likely to be different from those selected primarily to maximize production of highquality wood. For example, if structures and patterns associated with mature and
old-growth forests are desired in order to attract species like the northern spotted owl,
then stands will contain more snags, more large-diameter trees with cavities and large
branches, and more hardwood and conifer trees in the midstory. Furthermore, it is
likely that more trees would be left unpruned (to provide perches for animals or
surfaces for lichens or other plants), less salvage would occur, and utilization standards
would be lowered to provide continuing inputs of large woody debris. Although highquality wood can be produced under these conditions, yields would be substantially
reduced.
If the management objective is to provide stands with high aesthetic appeal, then
regimes may need to be altered to encourage growth, appearance (e.g., flowering), or
shape of understory and midstory plants such as rhododendron, dogwood, vine maple,
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or western redcedar. Concern for appearances would also affect the size, shape, and
location of management units and might influence the timing and frequency of some
activities. The effects of landscape-level objectives on a site-specific regime would
vary, but the perspective of landscape-design appears to be compatible with the
concept of long-rotation forestry.
The public has also expressed a desire to see large-diameter trees and stands of older
age classes in the National Forests. Long-rotation regimes that reduce stand densities
at early ages may be an excellent way to meet some of these objectives and still
provide a flow of goods and services from public lands. However, management for
multiple objectives may require some level of compromise in meeting each individual
objective.
CONSTRAINTS
Site-specific implementation of silvicultural regimes may require modification,
depending on the presence of various legal or social constraints. For example, there
are limits on the size of a harvest unit and on how close it can be to other, recently
regenerated units. Similarly, spatial limits on crown cover may need to be maintained (
for example, for dispersal habitat required for the northern spotted owl). There may
also be constraints on the timing or implementation of silvicultural practices because
of concerns for reducing nesting success of birds, fire hazards along roads, or the need
to maintain shade in riparian corridors. As mentioned earlier, silvicultural practices
must be implemented in ways that do not impair long-term productivity. Furthermore,
there may be practical constraints on activities because of local markets or the
unavailability of labor and equipment. For example, equipment may not be locally
available to prune trees to the desired height (HQF calls for pruning to 44 feet).
CONCLUSIONS
Selection of a general silvicultural regime (such as HQF) for a stand should be
dependent on current site and stand conditions and on management objectives.
Implementation of the regime will be limited by these factors and by existing and
anticipated constraints on activities. In addition, managers must plan for unanticipated
events, such as windthrow, ice breakage, or bear damage. It should also be
recognized that several alternative pathways of stand development would probably
achieve the same long-term objectives.
HQF is biologically feasible for long-lived species on suitable sites. We do not,
however, yet have silvicultural data from any stands managed under that regime for
an extended period (i.e., through multiple thinnings as the regime calls for); thus, we
are unable to quantify the costs and benefits of HQF in relation to alternative regimes.
Several regimes that would be alternatives to HQF have recently been proposed or
implemented. Recent changes in attitudes toward forestry practices have highlighted
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the folly of adopting a single regime over wide acreages. Thus, we hope that today's
forest managers will select a range of regimes and thereby provide their future
counterparts with a diversity of options.
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