Chapter 9 - Silviculture
Contents - Class Homepage
Introduction - Kinds of Treatments - Regeneration Methods - Site Preparation - Sources of Regeneration - Intermediate Operations

Upon successfully completing this section you will be able to:

1. Define silviculture and describe what factors must be considered when making a silvicultural decision.

2. Describe the major types of even-aged and uneven-aged regeneration methods.

3. Describe when site preparation is necessary and be able to describe the tools used in site preparation.

4. Define and describe the common intermediate treatments.

Natural resource managers, such as foresters, manage forests using sound biological and ecological principles. In fact, forest managers rely heavily on and apply the silvics of tree species in making management decisions. Silvics can be defined as the life history and general biological characteristics of trees with a particular emphasis on environmental factors that influence these characteristics. Foresters utilize knowledge of a trees silvical characteristics when they practice silviculture. Silvicuture is often defined as the art and science of controlling the establishment, composition, structure and growth of a forest. More simply put, silviculture is applied forest ecology.

Why would silviculture be called an art? Well first, because there are so many unique situations in forest management. Forests are highly dynamic and complex. Forest management situations can be so complicated that each situation often requires a unique solution. Second, even for a specific management problem different silviculturists may see the situation quite differently. The more experience a silviculturist has the more information they will bring to the table. Similar to art, we all see things a bit differently. Silvicuture is simply not an exact science. As a result, no manual of operations exists for silviculture.

The complex forested ecosystem in the background illustrates several unique solutions.

In this chapter you will be introduced some of the most common silvicultural practices. The majority of what will be discussed is practices that are used throughout the southeastern United States. You will not become an expert in silviculture but you will be able to recognize some of the treatments when you see them in the field, you will understand why they are being done, and you will be able to talk intelligently with foresters.

INTRODUCTION - Landowner Objectives
Sound forest resource management begins with an understanding of the owner objectives. What does the owner of the resource desire? This is not always an easy thing to discern since many times owners do not know what they want. They may only have a vague idea or they may want many different things several of which conflict with each other. Also objectives vary widely depending on whether the land and forest is owned by private individuals, federal agencies or industry.

Some timberlands may be managed for aesthetics and water quality.

This bottomland hardwood plantation is very productive and could rapidly produce high quality timber, but the landowner may enjoy the aesthetic qualities of large trees.

INTRODUCTION - What to Consider
Once the owner’s objectives are determined silvicultural decision making can begin. When making decisions biological, social and economic factors must be considered. The perfect silvicultural decision would be the best biologically, very cheap and socially acceptable. Sometimes very cheap and biologically good tools such as fire may not be socially acceptable. Another good example of this is the use of herbicides. Herbicides are often the most economical way to control weeds (unwanted vegetation), biologically they work very well but they may be socially unacceptable. Other times the owner may want things that economically they cannot afford. It is the job of the forester to balance these three factors when developing plans to meet specific objectives.

Forested wetlands are areas that require special attention to balancing the three factors discussed above.

These pinelands are managed for red cockaded woodpecker habitat.

INTRODUCTION - What to Consider
Many of the practices involved in silviculture are analogous to gardening. For example stages in a silvicultural plan may involve site preparation (tillage), weeding, thinning and final harvesting all steps that would occur in just about anyone’s garden. We will examine each of these practices in the order they would occur in stand development.

In many ways, this activity can be compared to harvesting corn. These trees will be used for a diversity of products (hundreds) such as wood for homes, paper products, toothpaste, cosmetics, film and food products.

Silvicultural prescriptions are applied at the stand level. A stand is a contiguous group of trees of a uniform age distribution, species composition and growth rate so that common practices can be applied to the entire area. A stand is typically the smallest definable management unit. A pure stand is composed of essentially one species (90 %); in contrast a mixed stand would contain several different species.

The large mixed hardwood stand in the bottom part of this infrared aerial photograph was probably the old woodlot for Smithfield Plantation on the Virginia Tech campus (which is visible in the upper right hand corner). The overstory is primarily large, old white and black oaks.

Stands are also classified as being either even-aged or uneven-aged. In even-aged stands the trees are of uniform age and started together as a unit after the previous stand was cut or a field is reforested. Uneven-aged stands are composed of trees of various ages (at least three) and have complicated development patterns. They could have resulted from long term natural processes where small groups (or even single trees) die and are replaced by a new cohort or they could be the result of past repeated entries into the stand where individuals trees or small groups have been removed. Stands with just two age classes are not regarded as uneven-aged but are usually termed two-aged stands. The simplest type of stand development would be a pure even-aged stand such as a pine plantation. The most complicated stand development (and the most complicated to manage) is a mixed, uneven-aged stand.

INTRODUCTION - Stand Dynamics
In all stand types there are many changes with time as the trees grow. First and most obvious the trees increase in height and diameter. Typically in a stand, tree height is much more uniform in distribution than is diameter. Trees are designed so that they sacrifice diameter growth before height growth. This makes sense since without light (which is from above) the will not survive no matter how big around they are. In fact in very dense stands, trees will be still quite tall but they will be very spindly in diameter. A less obvious change is that when tree size increases they begin to compete intensely for light, water and nutrients. As a result of this competition trees begin to die and their numbers begin to decrease. Smaller trees that slip below the canopy generally die first. A young Virginia pine stand may have thousands of stems per acre when young, but when they are 60 years old and 12 inches in diameter it is common for only 100 to 200 trees to remain in a stand.

The crowded 10-year-old Virginia pine stand on the left will eventually self-thin to resemble the 30-year-old stand on the right.

Silvicultural treatments fall into one of two categories:

  1. Reproduction methods deal with the establishment of new stands and occur at or near the end of the previous rotation.
  2. Intermediate operations occur during the life of a stand and are aimed at guiding the stand in the direction that meets the owners objectives. In all of these operations the cutting or killing of trees is involved. Most often the public views the cutting of trees as destructive. In a silvicultural context the cutting of trees should be viewed in a constructive manner. One cannot practice silviculture without cutting trees any more than one can garden without removing the crop. The harvesting of a stand by a clearcut is actually the regeneration method for a new stand not the mining or destruction of a resource. What replaces the harvested trees is more important to the silviculturist than what was cut.

Regeneration methods can be divided into two broad classes: those that produce even-aged stands and those that produce uneven-aged stands.

REGENERATION METHODS - Even-aged Systems - Clearcut
A clearcut can be described as the removal of an entire stand of trees in one cutting. Reproduction is obtained either naturally or artificially. Natural reproduction can come from either exiting seedlings, seed existing in the litter layer, seed from neighboring uncut stands or stump sprouts from the trees cut. A good silvicultural clearcut does not leave any residual trees from the previous stand. Unfortunately good practices do not always occur, and many stands are partially cut taking only the high quality trees and leaving trees of poor form or undesirable species. This type of cutting is more properly termed ‘high-grading’ and is an abusive practice that does not fall into good silvicultural practice.

This clearcut has a good supply of seed from neighboring uncut stands, has all trees removed and is (relatively) visually appealing.

REGENERATION METHODS - Even-aged Systems - Clearcut
No other silvicutural practice has been as controversial with the public as a clearcut. Images of barren hillsides, stumps and eroding stream-banks from a poorly planned clearcut are hard to overlook. However a properly executed clearcut has far less erosion than an agricultural field and quickly re-greens in just a few short years.

REGENERATION METHODS - Even-aged Systems - Clearcut
This is a well done clearcut on the Virginia Piedmont. Notice there are very few residual stems, little exposed mineral soil, irregular edges, and clean enough to plant with little further site preparation. In twenty years the area will again look like the uncut stand in the photo.

REGENERATION METHODS - Even-aged Systems - Clearcut
Biologically clearcuts are an obvious choice for species that require large amounts of sunlight to grow. These types of species are known as shade intolerant species and naturally regenerate following large disturbances such as fire, hurricanes or large insect outbreaks. Clearcuts simulate these types of disturbances. Economically clearcutting is a very inexpensive regeneration technique. Clearly the controversy is a great example of a social issue. The perception of the public is that clearcuts are a bad thing, as a result the public generally disapproves of their use.

These two harvests were very poorly done, with puddled soil, ruts, exposed mineral soil and standing residual trees. This can occur in any type of regeneration method not just clearcutting if proper care is not taken.

When done properly, clearcuts can quickly revegetate. The growth apparent in this picture is one year old, primarily from oak stump sprouts.

REGENERATION METHODS - Even-aged Systems - Seed-tree
A seed-tree removes the entire old stand in one cutting but leaves a small number of trees usually left singly and scattered uniformly throughout to serve as a natural seed source. The number of trees left varies but usually is between 5 to 10 per acre. The actual number left varies with how much seed is produced and the distance seeds can travel. This method works best with light seeded species that have seeds disseminated by the wind (e.g. pine). Additionally it should be reserved for species that require open conditions to regenerate. Trees left must be sufficiently strong to survive the increased wind and exposure of the now very open site. At times seed trees may be left in strips in small groups but this is not as common.

A loblolly pine seed tree regeneration cut.

REGENERATION METHODS - Even-aged Systems - Seed-tree
Once the new stand is established (left), the old overstory seed trees must be removed (below).

REGENERATION METHODS - Even-aged Systems - Shelterwood
In contrast to the clearcut and seed tree method a shelterwood does not remove the majority of the trees in a single cut. However it does result in the formation of a new even-aged stand. At a minimum a shelterwood involves two cuts. The first cut is called the seed cut. In this cut about one half of the overstory trees are removed. Trees removed are uniformly spread throughout the stand. The remaining trees provide seeds for the new stand, and shade and shelter for the developing new trees. Sometime after the new trees are established a removal cut releases the new trees and removes the remaining overstory. More complicated shelterwood cutting systems may have three or more cuts. The first preparatory cut is designed to improve the quality or health of the remaining stand. For example it may appear as a light thinning to build the crowns of the future seed trees. It may also be used to speed the decomposition of the litter layer by increasing light and thereby increasing soil surface temperatures. In this system the preparatory cut would be followed with a heavier seed cut and one or more removal cuts.

A shelterwood cut leaves an intact but very thin canopy, providing light shade and shelter for developing seedlings.

REGENERATION METHODS - Even-aged Systems - Shelterwood
Questions always arise about damage to the new stand when the removal cut takes place. It is inevitable that damage will occur when large trees with wide crowns are felled into openings filled with a dense crop of new trees. However, the damage can be minimized with careful logging and doing it while the new growth is still flexible enough to rebound back. At other times some damage may actually help the new stand by thinning out overly dense stands of new growth.

REGENERATION METHODS - Even-aged Systems - Shelterwood
This type of regeneration cut is best used for species of trees that can grow in partial shade. Eastern white pine is a good example of a species that can be regenerated using a shelterwood. Eastern white pine is considered to be more shade tolerant than many other pines. That means they can survive and grow in some shade. A shelterwood provides just that sort of environment. Species that require full sunlight to grow will not seed into the understory of a shelterwood, as a result nearly pure stands of white pine can occur.

These white pine seedlings (right) developed in abundance following the shetlerwood cut (left).

Both of these pictures show the growth after one year in shelterwood cuts.

REGENERATION METHODS - Uneven-aged Systems - Selection
The selection method is any type of system designed to maintain uneven-aged stands. In this method at no time is the entire stand removed. This method has become increasingly popular to the public since it is perceived as an alternative to clearcutting. There are two main subcategories of the selection method - single tree selection and group selection.

This conceptual uneven-aged stand has four age classes evenly distributed over the landscape in approximately equal area.

REGENERATION METHODS - Uneven-aged Systems - Selection
In single tree selection, individual large, mature trees are removed uniformly across the stand. In theory the holes left from these trees are large enough to allow small groups of new seedling to develop. The basic idea is that it mimics when one old tree dies and falls over in the forest many new young ones begin to grow in the newly available space. With time these new trees grow and the resulting competition between them results in one large tree again in the same space.

This gap created when this large tulip-poplar fell over will quickly fill in with young seedlings. Single tree selection is similar but would remove the tree for forest products.

REGENERATION METHODS - Uneven-aged Systems - Selection
In group selection, small groups of uniformly spaced mature trees are removed from the stand. Since larger openings are created, trees that are moderately tolerant to shade such as oaks can also be regenerated.

A group slection cut in the Appalachian mountains (above) resulted in abundant white oak reproduction (right).

REGENERATION METHODS - Uneven-aged Systems - Selection
In both single tree and group selection the entire stand is never removed at once and equal spaced numbers of different ages of trees occur. When one cohort of trees reaches the mature age or size only those trees are removed. This results in openings for new regeneration to occur. In another set period of time (say 15 years) a new set of trees has reached maturity and is ready to harvest. The trees that started to grow after the previous harvest are now 15 years old. An example stand might include 5 age classes, 15, 30, 45, 60 and 75 year-old trees each occupying equal area. Every 15 years the trees that reach 75 years old are harvested and a new set of trees begin to grow. In this example, no more than 1/5th of the stand is ever cut at a time. This is very appealing to a public that generally dislikes clearcutting.

REGENERATION METHODS - Uneven-aged Systems - Selection
Selection systems however require a much more extensive road system in the stand since it is entered every 15 years instead of once every 75 years (for our example). A more extensive inventory system is also needed to be sure the stand is on track with the right amount of trees in each age class. Also the system does not work well for species that are very shade intolerant (e.g. loblolly pine).

Often a large part of regeneration practices is site preparation. Site preparation can be categorized into three broad groups ---1) the treatment of the logging slash, 2) the treatment of mineral soil and 3) the treatment of competing vegetation. In some regeneration methods a new forest can be established without any purposeful site preparation, at other times site preparation is critical for the establishment of a new stand of trees.

SITE PREPARATION - Treatment of Logging Slash
Often following a harvest, large amounts of residual material (slash) remains on the site. This can be composed of limbs, bark and undesirable trees felled but left on the site. At times several cords of wood can be found on the site. There are many reasons to dispose of this material. Simply the offensive appearance of this slash may be reason enough to dispose of it. However, more often than not the material is removed since it represents a future fire hazard and threat to the new crop of trees. Slash can be so heavy that the site can not be replanted. If machine planting is used, the site needs to very clean for the equipment to get across the site. Even with hand planting slash can make it impossible to move uniformly across the site and irregular and inadequate stocking may occur. The slash also may result in heavy shade and cause mechanical injury to the new crop.

The slash found on this site is fairly typical of a large harvest.

SITE PREPARATION - Treatment of Logging Slash
One of the best and perhaps the most common methods for the disposal of slash is a controlled fire. If the slash is uniformly spread across the site a broadcast burn can be used. If it is patchy, most often the slash is first piled and then burned. Piling of the slash is usually done by using a root rake attached to a large bulldozer.

This bulldozer is piling slash using a root rake.

SITE PREPARATION - Treatment of Logging Slash
When piling slash the amount of mineral soil pushed along with the debris must be minimized. Soil lost in these piles results in an overall reduction of site productivity similar to erosion. Slash can be simply piled or pushed into rows and left unburned. The piles and rows however can not generally be planted and this removes some of the land from production. Slash in piles or rows is referred to as a windrow.

Large amounts of soil that was once spread across the site can be seen in this burned windrow.

Slash on these two sites was scattered, so it was piled and burned.

SITE PREPARATION - Treatment of Logging Slash
During controlled burning, extreme care must also be taken so that the threat of the fire escaping is minimized. Also burning must be timed so that the weather conditions allow adequate smoke dispersal.

The fire below has very poor smoke dispersal and will cause problems with any communities or highways downwind. The fire on the right has excellent smoke dispersal.

SITE PREPARATION - Treatment of Logging Slash
If done properly little or no harm occurs to the site as a result of the burn. If the fire is too hot nearly all organic matter can be removed resulting in serious erosion problems and unnecessary nitrogen loss from the site. Nitrogen is found in the slash and when burned it volatizes and is lost to the atmosphere.

The fire on this site sufficiently reduced slash and left organic matter on the site.

This fire on this site was too intense and nearly all of the organic matter was burned off.

SITE PREPARATION - Treatment of Logging Slash
Some forest scientists argue that slash should always remain on the site. The woody debris represents a long-term supply of organic matter and nutrients. As the slash decomposes it slowly releases nitrogen to the developing new crop of trees. Perhaps it is better to think about the slash as an opportunity not an obstacle. Slash can be considered an opportunity to better manage the sites resources and ensure long term sustainability of the sites productivity. Unless the slash is very heavy attempts should be made to leave it on the site.

This logging slash is not much of an obstacle and should be left on the site

SITE PREPARATION - Treatment of the Mineral Soil
Many times following a harvest there are limitations with the actual mineral soil that need to be addressed in order for the site to regenerate properly. One of the most common problems is compaction along skid trails and at the logging deck. Disking can ameliorate this.

This skid trail is highly compacted, limiting tree growth.

SITE PREPARATION - Treatment of the Mineral Soil
Disk harrowing of sites is very similar to the agronomic application with the exception that the equipment is larger and more heavily constructed. To some extent disking will also incorporate organic matter into the mineral soil. For very heavily compacted sites ripping or some sort of subsoiling may be needed first to properly repair the soil physical characteristics.

SITE PREPARATION - Treatment of the Mineral Soil
Bedding is the mounding of mineral soil with the objective of raising the newly planted seedlings above the winter water table. Bedding of sites is also a common mineral soil treatment on wet coastal plain sites in the southeastern U.S. In wet sites the water table will rise following logging and during the winter months. Under these circumstances young seedlings may be submerged.

Wet coastal flats are commonly ditched and bedded to increase rooting volume

SITE PREPARATION - Treatment of the Mineral Soil
To form beds a bedding harrow is most often used followed by an “hourglass” shaped drum to shape the beds. Seedlings are then planted directly on top of these beds. Bedding can also repair damage due to improper harvesting such as rutting and compaction.

SITE PREPARATION - Treatment of the Mineral Soil
Fertilization is commonly done as a site preparation treatment. Nitrogen and phosphorus are the two most commonly applied fertilizers at the time of planting. In the case of nitrogen it is best if the fertilizer is banded close to the seedlings. Nitrogen is quickly leached from the soil, so getting it close to the small root systems where it can be taken up by the plant is essential. Phosphorus is very immobile and stays in the soil for a long time as a result it can be broadcast without worrying about it leaching from the site.

The effects of adding Phosphorus to forests soils can be dramatic. Both of these slash pine stands are 23 years old. The stand on the right was fertilized with Phosphorus at establishment.

SITE PREPARATION - Treatment of Competing Vegetation
Competing vegetation is a major problem on most recently regenerated forest sites. If the objective is a genetically improved pine plantation, invading hardwood seedlings and sprouts are not desirable. In hardwood plantations competing natural pine seedlings may be the problem. At other times herbaceous grasses or ferns may be so intense that they overtop the newly planted trees. Site resources (light, water, minerals) are limited and best growth will occur only if the resources are directed to the crop trees and not to the competition. Methods to control competing vegetation can be grouped into two broad categories—mechanical treatments and chemical treatments (herbicides).

Weeds can have a tremendous impact on stand growth. The stand to the right of the road received no herbicide treatment.

SITE PREPARATION - Treatment of Competing Vegetation
Mechanical treatments typically involve driving across the site with a piece of heavy equipment pulling one or more large tools used to cut or crush the competing vegetation. If large residual standing trees are left on the site a shear blade is used. This sharp, heavy blade is attached to the front of a bulldozer and can knock down very large stems. For large trees the “stinger “ is first thrust through the tree splitting it. The shear blade can then easily knock the tree down. Shearing is often is the first step before other site preparation treatments can occur.

A stinger mounted on a bulldozer and its aftermath.

SITE PREPARATION - Treatment of Competing Vegetation
For smaller stems (under 3 inches), a drum chopper can be used. This equipment is pulled behind a bulldozer and crushes and breaks down smaller competing vegetation. Often two drum choppers set at slightly offsetting angles are pulled in tandem. This technique does a better job since it imparts a twisting, ripping action. Very often sites are chopped, allowed to dry out and then broadcast burned. Burning reduces the slash and also reduces sprouting from the remaining hardwood stumps and broken stems.

SITE PREPARATION - Treatment of Competing Vegetation
A V-blade is a tool that is often used in front of a bedding plow, disk, or machine planter. A V-blade will push slash to either side and out of the way of either the plow or planter. This is a useful tool on site with a moderate amount of slash and where mineral soil contact is needed for a follow up treatment.

This bulldozer is pushing a V-blade and pulling a disk.

SITE PREPARATION - Treatment of Competing Vegetation
Herbicides are also used in site preparation. When done by a helicopter or hand crews, herbicides have the advantage of not requiring a heavy bulldozer being driven over the site. As a result no site compaction or further disturbance occurs. At times herbicides are used in combination with a fire similar to chopping and burning. The major disadvantage of herbicides is the public’s perception of harmful chemicals being sprayed on a forest. However, most forestry herbicides are very non-toxic (except, of course, to trees).

Helicopters are especially effective when herbicides need to be applied to large areas.

Herbicides can be incredibly effectivie in controlling weeds. In this photo you can clearly see which areas were treated.

Weeds can severely limit growth of planted seedlings. What did foresters do prior to herbicides and mechanical treatments?

In natural regeneration, the existing seed in the forest floor, seed from remaining trees or stump sprouting supplies the new seedlings.

Many hardwoods and some conifers produce stump sprouts.

Foresters often rely on artificial regeneration to produce the next stand. In artificial regeneration planted seedlings or direct seeding of a stand is used. In direct seeding purchased seed are scattered over the entire area (much like a new lawn) or it is applied in spots and rows. Direct seeding is not very reliable and it is difficult to control spacing. As a result, direct seeding is not often utilized in forestry. However in instances such as hydroseeding of strip mines it is used.

It is quite common to use seedlings to artificially regenerate a stand. Following clearcutting, genetically improved seedlings are often planted. This is especially true for plantations. Planting of seedlings offers the best opportunity to improve the genetic composition of the stand and it is very reliable. The seedlings are produced from seeds collected in seed orchards. These seed orchards contain trees that have been selected for superior traits such as straightness or disease resistance. In the near future genetically improved seedlings may all result from tissue culturing techniques (cloning). Planting of seedlings also allows foresters to precisely control the spacing and density of the future stand.

With most conifers (e.g. loblolly pine, Douglas-fir) survival of planted seedlings (except in the driest years) is very high (over 75%). It is a bit trickier with the planting of hardwood seedlings and foresters have had much less success. As a result there is far less planting of hardwood seedlings. However this may be changing since many companies are now investing considerable resources towards intensive management of hardwood plantations. Two hardwood species being grown intensively are hybrid poplars and sweetgum.

These sweetgum seedlings (left) are being grown for use in an intensively managed hardwood stand such as the 5-year-old stand in Franklin, Virginia (right).

Once a new stand is established there are many common practices that fall into the broad category of intermediate operations. By definitions these procedures are any practice that occurs between two regeneration periods. Their general goal is to guide the stand toward the intended objective. Intermediate operations are never intended to replace the existing stand.

Release Operations are some of the earliest intermediate treatments that occur in a stand. By definition release operations occur in stands not past the sapling stage (generally under 5.5 inches in diameter) that are intended to free the stand from competition that is overtopping or threatening to overtop the stand. When the trees removed (cut or killed) are the same age as the crop trees they are called cleanings. When the trees removed are older than the crop trees the treatment is called liberation. A third type of treatment called a weeding is reserved for a release so intense that all vegetation except the crop is removed. Foresters generally refer to all these treatments as release operations.

A common release scenario involves herbaceous weeds over-topping young seedlings. Another very common scenario in the southeastern U.S. involves hardwood stump sprouts (which grow very rapidly). Hardwood sprouts may need to be removed to release a pine crop. Herbicides are most often used in these situations.

All of these stands are three years old. The top stand received no herbicide weed control, the upper right stand had one year of herbicide weed control and the lower right stand benefitted from two years of weed control.

In liberation cutting the older overtopping trees are only actually removed if there is some commercial value in them. Often they are simply killed by injection of herbicides or by girdling. Liberation cutting is not nearly as common as cleaning. Sometimes a good stand of new trees has developed under poor quality trees that were not previously harvested (usually because they were of no value). It would have been better if the older trees had been cut or sheared at site preparation.

Improvement cuttings occur in stands past the sapling stage. As with other intermediate operations, the objective is not regeneration but in this case to improve the quality and composition of the stand. Typically improvement cutting (also known in North America as Timber Stand Improvement, TSI) is done to correct past mismanagement. Perhaps an improvement cut is needed because a cleaning did not occur when the stand was young. Past high grading may leave hardwood stand composition and quality in bad shape. High grading is an abusive practice where the stand is entered and only the best trees are removed resulting in an overall degradation of stand quality.

Removal of trees that have been injured by weather or desease is considered stand improvement.

In an improvement cut trees removed are generally of poor stem quality, undesirable species, diseased or injured. What becomes an undesirable species depends on the owner’s objectives for the stand. When examining a stand for an improvement cut not only should poor trees be selected but also better quality trees should be sought out for release. In fact a critical step is to determine if enough quality stems remain to make the stand viable for future production. At times the stand is in such poor shape that the only alternative is to regenerate.

As in a liberation cut the trees are often killed and left standing in place when doing an improvement cut. However, if there is some market for the wood then they should be removed. In hardwoods firewood production may be justification to remove the trees.

These young eastern white pine would respond well to a liberation from the poor quality hardwood overstory.

Thinning is also intermediate operations that occur in stands past the sapling stage. Their primary goal is to improve growth and quality by decreasing the density of the stand. The distinction between thinning and improvement cutting often is blurry. This is especially the case in mixed stands. In both instances trees past the sapling stage are removed and both are partial cuts that reduce stand density. However the primary goal of an improvement cut is to enhance stem quality and stand species composition. In a thinning, maintaining the growth of the remaining trees is the primary objective. At times both treatments are occurring and foresters will call the operation a thinning/improvement cut.

This loblolly pine stand will need to be thinned to maintain the health and growth of the stand.

Thinning is conducted when the product objective of a particular stand requires a larger tree. For example, if the product is pulpwood to make paper, large trees are not required. If the product objective is sawtimber a thinning may be necessary. Although thinning increases individual growth, thinning does not increase the overall growth of the stand. So if the product objective only requires small stems than a thinning may not be necessary. Many small stems will produce about the same total growth per acre. If the size of individual trees is important than a thinning may be required to redirect the same total growth per acre on to fewer larger stems.

Thinning is often classed into two groups depending on their profitability. Pre-commercial thinning involves the removal of trees that are too small to be utilized. In commercial thinning all or a portion of the wood removed is utilized.

The trees in this stand were too small to yield any profit. Consequently, this was classified as a precommercial thin.

Thinning is also classified by the size of the trees removed. In a low thinning (also called thinning from below) the trees removed are the smaller diameter trees and the larger trees remain. This is a very common method of thinning utilized in the southern pines. It also is considered the most intuitive type of thinning. The trees being removed are exactly those that are likely to die in the near future since they are smaller and being overtopped. High thinning (also known as thinning from above) is almost the opposite of low thinning. High thinning involves the removal of larger trees. The goal of high thinning is to free up space for similar sized but better quality (e.g. straighter) stems. High thinning is not as common as low thinning. One specific application of high thinning would be the removal of large trees damaged by biotic (e.g. insects) or abiotic (e.g. ice) agents. In this case a high thinning could be done which removes these damaged trees and leaves similar sized but undamaged trees.

A third type of thinning that is fairly common is called a row thinning (also called a mechanical thinning). A row thinning is often used in plantations where as a first operation every third row is removed. This releases each tree on one side. Later in the stands development every other row is removed releasing the remaining tree on all sides.

Commonly in the southeast we see a combination of row and low thinning. A mechanized harvester does all the work first cutting out a row of trees forming a corridor for the machine to travel down. As the machine moves down the corridor it reaches in between the rows grabbing smaller trees (low thinning), freeing up growing space for the internal (between row) trees.

This 13-year-old loblolly pine stand was row thinned with a mechanized harvester and low thinned by reaching in between the corridors.

INTERMEDIATE OPERATIONS - Other Intermediate Treatments
Nitrogen fertilization is a common intermediate stand treatment. Close to 1.5 million acres were fertilized in the southeastern U.S. in 1999. As an intermediate treatment nitrogen is most often applied to stands when crown closure occurs.

Crown closure indicates that the crop trees are fully utilizing the site. Fertilizing the stand at this point insures that he crop, not weeds, gets the nitrogen. With the site fully occupied by the trees nitrogen is quickly taken up and little is leached out of the system.

INTERMEDIATE OPERATIONS - Other Intermediate Treatments
In some specific instances sludge or effluent from various sources may be applied to a stand. This is an efficient way to dispose of societal waste products and the forests receive nutrients as well. Care must be taken not to over apply these products. If rates exceed the stand’s ability to take up nutrients they can begin to leach into ground water. However, as an alternative to depositing this waste into landfills it is very appealing.

Sediment from drnking water treatment in Newport News, Virginia is applied to loblolly pine plantations instead of being sent to landfills.

INTERMEDIATE OPERATIONS - Other Intermediate Treatments
Fire is often utilized as an intermediate treatment. In stands of thick barked species it can be used efficiently to remove unwanted understory species or to reduce the litter layer in preparation of a future regeneration cut. Fire can at times be used to thin out a very dense stand. Younger stems with thinner bark are killed, leaving the larger stems—a very efficient low thinning.

This burn reduced the litter layer and fuel load, decreasing the chances of a catastrophic fire.

Understory burns such as this are often used to improve wildlife habitat. The fire killed the smaller understory trees allowing lush herbaceous regrowth which attracts wildlife (e.g. turkeys, deer, songbirds).

Helicopters can be very efficient tools when treating large areas. Here a helitorch ignites an area for site preparation.


In summary silviculture addresses a wide range of landowner objectives. These may include such diverse activities as 1) replanting mix stands to restore habitat for songbirds on former deforested agricultural land for the Audobon Society (right), 2) maintaining stream temperature, water quality, and riparian habitats (lower right), and 3) maintaining historical settings in civil war battlefield National Parks (below).