BLENDING SILVICULTURE AND GENETIC IMPROVEMENT TO ENHANCE FOREST
HEALTH AND PRODUCTIVITY
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Dean S. DeBell
USDA Forest Service
Pacific Northwest Research Station
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INTRODUCTION
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Most social and economic trends in North America indicate a need to enhance the
health and productivity of our forest land. Whether wood production is the primary or a
secondary objective of forest management, much information is needed. It is needed to
develop sound alternative management systems, to determine probable outputs of
wood and other values obtainable with each, to assess various trade-offs, and to come
to responsible, sustainable d cisions.
Knowledge and experience in silviculture and genetics must contribute in a major way
to future strategies and decisions. The contributions of silviculturists and geneticists to
modern forest management have been substantial, but additional silvicultural and
genetic questions have repeatedly been identified as major issues by forest managers
and scientists. Most of the issues involve interrelationships between the two
disciplines. They commonly have ties to other specialties such as wood technology or
wildlife ecology. Progress will require thorough integration of existing information as
well as integration of future research and development activities.
In this paper I will discuss eight such issues that involve the integration of silviculture
and genetics - in both research and practice. These issues include concerns as well
as opportunities. Most of them are related to some degree to management of all
forests, but four seem particularly important in forests managed for multiple purposes or
objectives. The other four probably are most important in forests dedicated primarily to
wood production.
SILVICULTURE- GENETICS INTERRELATIONSHIPS
Multi-purpose Forestry Examples
The influence of partial cutting on genetic composition is an important issue in some·
multi-purpose forestry approaches. On public lands and some other ownerships, the
predominant harvesting method is shifting from complete clearcutting to some form of
partial cutting or to clearcutting with small numbers of reserved trees. There are
concerns about possible dysgenic selection and inbreeding depression, particularly if
the stand is regenerated by natural seeding after logging. Such concerns diminish
greatly if the site is planted with improved and well-adapted stock. The shift to partial
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cutting, however, has been associated with a trend toward greater use of natural
regeneration, particularly on some public forests.
Are such concerns valid? Millions of acres of mixed hardwood forests in the East have
been degraded at the species level by selective removal of preferred species and size
classes. Presumably similar degradation may occur within species if trees retained are
few and they are ·selected for reasons unrelated to their genetic worth. Or
worse- if they were selected for retention because they were poor quality, low value
trees; i.e., they were selected to minimize economic losses associated with a mandate
for tree retention. On the brighter side, there is evidence suggesting that some genetic
traits of a stand can be improved by thinning; thus, the opportunity exists to enhance
the genetic make-up of a stand through partial cutting.
..,..
Work is needed to assess patterns of genetic variation associated with alternative
harvesting and regeneration practices. These assessments should involve not only the
amount of genetic diversity, t:>ut also its quality as reflected in the health and vigor of
offspring. With such information, foresters can adjust marking guidelines and develop
regeneration strategies to avoid potential problems and also capture whatever
improvements may be possible.
·
There are also questions about the performance of genetically-selected stock in
partially cut stands. Tree improvement programs have evaluated progeny (and thus
selected parent trees) based on early growth performance in clearcut environments. In
such environments, competition has been reduced among trees being evaluated and
with other vegetation. In partially-cut areas, however, planted seedlings will encounter
higher levels of shade. And if site preparation is less intensive, competition for moisture
and nutrients will be increased. We do not· know whether the performance of improved
stock relative to unselected stock in these partial cuts will be better or worse than that
observed in clearcut testing environments.. Millions of dollars have been invested to
date in tree improvement programs. The potential gains in stand production that may
be obtained are substantial, and they will affect future harvest levels. Thus, we need to
evaluate performance of selected stock under the range of environments likely to be
encountered in alternative silvicultural systems. Such information may determine what
genotypes are planted in various systems and should also influence what systems are
selected for use.
·
The re-establishment and management of Western White Pine provides an excellent
example of what may be accomplished when silvicultural and genetic improvement
activities are integrated. Western white pine was a much more important component of
many western forests, including those of the Douglas-fir region, prior to importation of· blister rust. Various Ribes species serve as the alternate host for this disease. Attempts to control the problem by removing Ribes or by spraying antibiotic fungicides on young trees were ultimately unsuccessful. But fortunately, selection and screening programs produced rust-resistant strains; these have made re-establishment possible. Alternative silvicultural practices may further increase the likelihood of trees surviving to I
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maturity and harvest. For example, reductions in broadcast burning and increased
shade associated with partial cutting may hinder the establishment and growth of Ribes
seedlings. Pruning has been done in many young plantations to reduce infection sites
and create a less favorable microenvironment for the rust. And, of course, it will
increase the amount of clear wood produced.
Questions related to deployment of selected and unselected planting stock have been
important issues in re-establishment of commercial forests for some time. Recent
developments in ideology and practice have also raised questions about planting
versus use of natural regeneration. Thus, there is a need for documented comparisons
of the advantages and disadvantages of natural versus planted stands and how these
may vary at different sites or with' different management objectives. Similar evaluations
are needed for pure vs. mixed species stands, regardless of establishment method, and
for pure versus mixed families or clones in plantations.
Interest has increased in management of other species and in mixed species stands for
many reasons-economic and ecological. Even when they are only 50 or 60 years old,
mixed species stands may provide habitat niches and other values that sometimes are
assumed to occur only in much older or late successional stands.
Production Forestry Examples
One of the most important issues in production forestry involves the yields to be
expected when stands established with genetically improved stock are harvested. Most
estimates of gains due to tree improvement are based on results 'of progeny tests. Most
of these tests are less than 20 years old, families were planted in single tree or small
row plots, and few traits were assessed. Density effects and competition among
genotypes usually were not evaluated. Although measurements show promise of
increased tree growth in selected families, the projection of such results in terms of
increases in usable volume per acre at harvest is no simple task. But doing so is very
important.
Designed experiments are underway to provide stand-level comparisons of yield for
selected and unselected stock; experiments are also on-going that will provide better
insights on competitive effects and how to account for them in yield projections.
Results, though, will not be available for several years. In the meantime, opportunities
exist for silviculturists, geneticists, and modelers to collaborate in the analysis and
interpretation of existing data from genetic tests and silvicultural experiments in order to
make realistic estimates of long-term gains in stand yield.
One fundamental component of the previous issue involves assumed limits to gains in
stand productivity obtainable through genetic improvement. Productivity of a stand is a
function of tree growth rate and number of trees per unit area. Silvicultural and genetic
efforts to improve productivity have focussed on enhancing tree growth rate and, to
some extent, on increasing harvest index and product quality. It has generally been
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assumed that opportunities to improve productivity by increasing the number of trees
that can be grown to a given size were limited. This assumption is commonly stated in
terms of self-thinning trajectories or limiting diameter-density lines for individual
species. Some scientists, including geneticists, have assumed that productivity
enhancement is largely limited to decreasing the time it takes the stand to grow to the
limiting size-density line.
Evidence is beginning to accumulate to the contrary. We now know that there are
differences among loblolly pine seed sources, and there appears to be differences
among hybrid poplar clones in the maximum size or current growth that can be attained
at given levels of stand density. These differences may result in yield gains of 30
percent or more. Families of western conifers should be examined for such differences
in tolerance .to crowding or competition. If found, they could provide a basis for
substantial productivity increases in advanced generations of improvement.
Even if differences in size:.ciensity relationships do not occur among families, this does
not mean that genetic gains ne
· ed be limited to early stages of stand development. It
will mean that thinning will be needed to keep the stand below the limiting threshold,
whatever its level. Thus, trends toward increased thinning and longer rotations have
the potential to extend the period and amount of growth benefits achievable from
genetic selection and thus returns from investments in tree improvement programs.
Work is also needed to determine fundamental physiological and morphological traits
related to tree performance, particularly to the quantity and quality of wood products.
Simple measures like size of terminal leaves may be closely related to growth and
vigor; branching patterns to stem form; and branch retention may be indicative of
tolerance of competition. Identification of easy-to-assess indicators can lead rapid
improvement in silvicultural and genetic practices.
During the past decade, most such work in forestry has been focussed on hardwoods
as part of the Department of Energy's biomass programs. The results have been
impressive and have been implemented in the form of clonal hybrid plantations. Similar
progress might be made with conifers; we could superimpose such research in existing
progeny tests and take advantage of the wealth of data and experience already
available for selected families in our tree improvement programs.
The opportunities afforded by fiber farming or short-rotation.intensive culture of
hardwoods, particularly Populus and Eucalyptus, deserve specific mention. The
combined work of geneticists, silviculturists, physiologists, and others led to
development of rapidly growing hybrid clones and successful methods for establishing
and tending young stands. Some Eucalyptus plantings in Hawaii and Brazil have
produced the highest rates of usable wood increment measured to date in any natural
or managed forest. Intensively cultured poplar plantings in the Northwest may average
60 feet in height and 6 to 7 inches in diameter at six years, producing nearly 500 fe per
acre per year. Some reductions in certain risks and some increases in uniformity of
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stem size and wood properties have also been achieved. But benefits are not confined
to wood production alone.
The poplar plantings along the lower Columbia River have provided nutritious forage
and effective cover for the long-endangered Columbian white-tailed deer. These
benefits have contributed to major increases in deer populations, and the Fish and
Wildlife Service is now considering either "delisting" the deer or "downlisting" its status
from "endangered" to "threatened." It is interesting that this improved situation is
occurring in the face of- if not as a result of- private investments in the most
sophisticated, intensive wood production system in North America.
Because the fiber farming approa·ch has been very productive with hardwoods - with
production rates equivalent to three to five times that of conventional management
regimes, there is budding interest in using it to grow conifer fiber. This might entail
selection and management for rapid early growth as well as specific wood
characteristics within the so-called "juvenile core." Such approaches to rapid
production of small conifer roundwood need not be restricted to short rotation and
single-species schemes; they might be combined in two-species, two-stage systems in
which a large .portion of a dense initial stand is harvested for pulp logs in early thinnings
and the remainder of the stand is managed for larger trees on a much longer rotation.
Mixed plantings of hemlock and Douglas-fir - even clones thereof - might be managed
in this way.
SUMMARY AND CONCLUSIONS
Much recent ideology and policy- especially that associated with U.S. Federal forests­
- has given short shrift to intentional manipulation (both silvicultural and genetic) of
forest vegetation, particularly when objectives involve multiple values and resources
other than wood. This will change with time. Sound knowledge and appropriate
practices will be needed to provide the range of values and products desired from forest
lands. And this is so whether the forests are intentionally managed for multiple
objectives or for one primary objective such as wood production. The examples I've
discussed represent but a few of the important issues that silviculturists and geneticists
must face jointly - and with the collaboration of other forest managers and scientists.
We can resolve the concerns and capture the opportunities through research and
through well-designed monitoring of management activities. And by doing so, we will
be able to improve the capacity of our forest lands to meet the increasing and often
conflicting demands that society places upon them.
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