Applying ecological principles to a forest stand
to enhance growth of desirable species or to promote regeneration is
termed silviculture. Like a gardener tending a garden, removing
weeds, encouraging the best plants, and finally harvesting the
produce, you use silviculture to tend the forest.
Silvicultural treatments usually, but not always, involve felling
trees. They are generally divided into procedures designed to
reproduce forest stands and intermediate treatments that maintain
vigor and desired composition and stand structure.
The descriptions of the silvicultural systems below are representative of ideal conditions. These techniques are often adapted to local conditions. For example, while a clear cut removes all of the trees in a stand, often landowners will leave a few trees in a clump to offer wildlife habitat or vertical structure. The California Forest Practices Act also strictly defines some of these silvicultural systems, usually in terms of how large an area that the treatment can be applied to and the number of trees that must remain after the treatment.
The goal of reproduction cuttings is to get new
trees growing in the stand in an efficient manner. Reproduction may
either be from natural seeding or by planting seedlings to regenerate
the stand.
Reproduction cuttings are divided into even-aged
systems, which produce trees of nearly equal age (but not necessarily
of equal size) and uneven-aged systems designed to produce stands
with a range of ages. The selection of a silvicultural system depends
on the growth characteristics of the desired species, which relate to
the landowners objectives. Outside factors, such as insect
infestation might dictate which system is best to control a
particular problem.
Even-aged Reproduction Cutting:
The goal of even-aged reproduction
cuttings is to create a forest stand with trees that are roughly the
same age. Even-aged stands, of various ages, can be distributed
throughout the property to provide a continuous flow of products.
Clear Cut Method:
With the clear cutting silvicultural system, all trees in a stand are
cut at one time. Seeds from surrounding trees, root sprouts, or
artificial seeding or planting are used to regenerate the stand.
Clear cutting provides an open environment, with
plenty of sunlight for trees to be reproduced in the regenerated
stand. As such, clear cutting is desirable for regenerating
shade-intolerant species. Tree species that require shade to
regenerate successfully would be discouraged by clear cutting. Site
preparation, which is the removal of woody logging debris and weed
competition, is required for successful regeneration of either
naturally seeded or planted trees.
Clear cutting alters wildlife habitat by replacing
mature stands of trees with herbaceous plants, shrubs, and seedling
trees. Different wildlife use these different forest stand
structures. Clear cutting can enhance habitat for a number of other
species. In a small holding, it may be impossible to provide wildlife
habitat for all species that inhabit a particular type. With a
carefully executed schedule of clear cuttings, a variety of habitat
types, including stands of young trees, middle-aged trees, and older
trees, can be established.
The big disadvantage to clear cutting from a small
landowner’s point of view is visual. Clear cutting is a dramatic
change in the character of the the forest. The forest changes
abruptly from a mature stand to a very young one. Logging debris is
clearly evident.
The advantage of clear cutting is efficiency. It is relatively easy to lay out a clear cut block. There is little to no danger of damaging remaining trees.
If you decide that clear cutting is the
appropriate technique, consider keeping cut blocks small. Design cut
blocks with curved rather than straight edges. Leave islands of
mature vegetation to provide hiding cover for wildlife and provide
vertical structure and diversity in the future stand.
Seed Tree: The
seed tree system is an even-aged regeneration method that removes
most of the stand while leaving a few desirable trees to produce
seed. Usually, but not always, the seed trees are removed after
regeneration is established. The quality of the seed trees, their
distribution, and the timing of the harvest are important
considerations when using the seed tree silvicultural system.
The seed tree silvicultural system is also applied generally to shade-intolerant species that regenerate best in full sunlight. Often site preparation is required to remove logging debris and competing plants before the new stand becomes established.
Usually the seed trees are removed after the regeneration is well established. If you wait too long, however, the young trees can be damaged during seed tree removal. Seed tree silviculture has similar advantages and disadvantages to clear cutting. After the seed trees are removed, the new stand will closely resemble a regenerated clear cut block.
Shelter Wood: When the species to be regenerated require some protection from direct sunlight to become established, the shelter wood system is applied. Depending on the initial condition of the stand, two or three cuttings are used to regenerate the stand. The objective of the initial cuttings is to improve the vigor and seed production of the remaining trees and to prepare the site for new seedlings. Shade and shelter from extreme conditions are provided by the remaining trees. Subsequent harvests remove the shelter wood trees and allow the regeneration to develop as an even-aged stand. The species favored by the shelter wood system are the more shade-tolerant species that naturally regenerate in the shade. Because the forest is removed in several stages, there is less direct visual impact until the final harvest.
Uneven-aged Reproduction Cutting:
An uneven-aged forest has trees of all age
and size classes. Frequent periodic cuttings occur to establish and
maintain this structure. The residual stand must be considered when
using this system. The possibility of residual trees being damaged
from repeated harvesting operations makes careful logging
crucial.
A disadvantage to the uneven-aged silvicultural
systems are the number of roads that need to be opened and the
frequency with which they are used. With uneven-aged silviculture,
the entire forest is visited every cutting cycle - typically every 10
or 20 years. The entire road system must be activated for every
harvest. With even- aged methods, only those stands that are actually
operated upon need open roads.
Single Tree Selection: In
single tree selection, a forester evaluates every tree in the stand.
Undesirable trees are removed, overly dense areas are thinned, and
mature trees are harvested. Seedlings of shade-tolerant trees develop
wherever they can find space. Cuttings are more frequent, providing
income on a regular basis but at a lower return per harvest to the
landowner. Single tree selection systems are more difficult to
manage. Damage to the remaining trees is a consideration. Single tree
selection rates high in visual quality.
A key temptation to avoid in single tree selection is removing only the biggest and best trees and leaving the rest. Known as high grading, this misapplication of single tree selection eventually reduces the quality of the trees that are growing on the site. A well-marked single tree selection harvest removes the less desirable trees, those with poor form or those injured by insects or disease along with the crop trees.
Group Selection:
A group
selection silvicultural system requires harvest of small groups
rather than individual trees. The openings resemble clear cuts, but
since they are small (<1 acre to 5 acres), the edge trees still
provide a protected environment for the developing regeneration in
the group opening. This method is similar to single tree selection in
that harvests are frequent. Its advantage is that it is easier for
the logger to avoid damaging the residual stand.
One final note about reproduction cuttings to keep in mind is that some of these silvicultural systems blend into one another. A seed-tree system leaving many trees per acre might be viewed as a shelter wood. Large areas removed in a group selection system might be misconstrued as small clear cuts. It is up to the landowner to work with the forester to ensure that the proper system is applied to meet the landowner's objectives. Environmental considerations, desired wildlife species, financial criteria, local market conditions, and stand conditions are all important aspects in determining the proper silvicultural system.
To reach its full potential, a tree needs room to
grow and access to sunlight, nutrients, and water. The surrounding
vegetation should be free of disease or harmful insects. To meet the
landowner's objectives, the stand should be comprised of desirable
species. You may desire a series of intermediate treatments to
improve the quality of the existing stand. If these treatments
produce revenue from the sale of the trees harvested, then they are
called commercial. Pre-commercial operations occur when the trees
removed cannot be sold. The cost of pre-commercial operations is
justified by improved growth and stand characteristics which
significantly increase the value of the stand at harvest.
Thinning: Thinnings
are prescribed to reduce inter-tree competition and to accelerate
growth on the trees that remain. Thinnings are grouped as commercial
or pre-commercial. They might be termed as high thinnings (or
thinning from above) where dominant and co-dominant trees are
removed or low thinnings (or thinning from below) where smaller trees
with crowns below the dominant trees are removed.
The obvious question is how do you select which trees to take or leave. The answer depends on a lot of factors - species, landowners’ objectives, site conditions among others. Some rules-of-thumb might be helpful. Thinning early in the life of a stand is usually better than later. When tree crowns begin to touch is often used as an indicator that thinning could be considered. There is a balance between letting the trees develop full, long, and healthy crowns and having enough shade to prune lower branches are shed to produce clear wood in the lower portions of the tree. Trees with less than one-third of their total height in live branches rarely respond well to thinning. Trees with poor form or showing evidence of damage, insects, or disease are usually removed.
Improvement Cuttings: Improvement cuttings are made to remove undesirable trees
to favor desirable ones. The choice of species depends on the
landowner's objectives for timber, wildlife, aesthetic appeal, or
recreation. Many trees that have little to no timber value might be
left to favor these other uses.
Crown classes are used to describe these trees.
Dominant trees are the largest trees in the forest stand. They have
crowns over and above the other trees, receiving full sunlight on all
sides of their crown. Co-dominant trees also receive full sunlight on
much but not all of their crowns. They are mixed with other trees.
Intermediate trees get full sunlight only on a small part of their
crown, most of it is shaded. Suppressed trees receive almost no full
sunlight. Mortality represent dead trees. A wolf tree has an
unusually large crown for the stand. Often wolf trees grew up in
relatively open conditions and the rest of the forest stand fills in
around it.
Crown class is one method used to select trees for
silvicultural prescriptions. The type of tree selected depends on the
landowner’s goals. For purely timber production goals, wolf
trees use up too much space. But as they die and decompose, wolf
trees can make excellent snags for wildlife. Some species, like white
fir, can survive as suppressed trees and respond if the surrounding
trees are removed. Others, like ponderosa pine, do not survive well
as suppressed trees and will not respond after a point.
One way to gauge the health of a tree is to look at its crown ratio. The crown ratio is the percentage of the total height of the tree that is in live branches. In the example, the dominant trees have about a 50 percent crown ratio; the co-dominants have a crown ration of about 40 percent; intermediates about 30 percent; the suppressed trees about 20 percent. The wolf tree has a crown ratio of about 80 percent. Larger crown ratios mean that the tree can produce more food from sunlight during a given day. Once a tree has less than about a 33 percent crown ratio, it usually cannot be released from competition. Trees with low crown ratios are more susceptible to insect attacks.
Sanitation cuttings are prescribed to remove trees
infested with insects or disease. These infested or infected trees
may pose a threat to the remaining stand. In general, losses to
insects and disease can be minimized through a regular thinning
program. Proper thinning maintains the trees in the best
physiological condition making them less susceptible to attack.
Natural catastrophes such as fire, windstorms, or ice storms may wreak havoc in a forest. Salvage cuttings are prescribed to utilize damaged trees, thus, reducing economic loss.
The Forest Practices Act defines many of these
silvicutural treatments with specific guidelines as to the amount of
acres involved, the number and size of the trees that should be left,
etc. The specifics vary by region. If you are planning any of these
procedures, you should check with the California Department of
Forestry and Fire Protection and a Registered Professional Forester
about the applicability of these treatments for your property.
Foresters often try to fit the landowner’s goals into one of these specified treatments to reduce the cost of Timber Harvest Plan preparation. Often the actual prescription will be a blend of treatments. Most of these silvicultural treatments can be adapted to the landowner’s specific goals. Having clearly defined goals and objectives will help you create the forest structures that meets your needs.
In accordance with applicable State and Federal laws and University policy, the University of California does not discriminate in any of its policies, procedures, or practices on the basis of race, religion, color, national origin, sex, marital status, sexual orientation, age, veteran status, medical condition, or handicap. Inquiries regarding this policy may be directed to the Affirmative Action Director, University of California, Agriculture and Natural Resources, 300 Lakeside Drive, 6th Floor, Oakland, CA 94612-3560. (510) 987-0096.
Publication of this series was in part funded by the
California Department of Forestry and Fire Protection
under Contract numbers 8CA96027 and 8CA96028