Thinning & Improvement Cuttings
Depending on the landowner's objectives and planned product outputs at the end of the rotation, it is sometimes necessary to carry out some sort of cutting within the established pine population in the middle of the rotation. The purposes of this type of intermediate cutting include:
- to stimulate growth of the residual stand
- to increase the total yield of merchantable (useful) material;
- to remove dead or damaged trees from the stand;
- to prevent or inhibit the spread of an identified, harmful insect and/or disease;
- to increase sunlight reaching the ground to promote herbacous plant growth for wildlife.
- Thinning
- Stand Density & Crown Position
- When & How Much to Thin?
- Thinning Methods
- Improvement Cuttings
- Additional Resources
Thinning
Many landowners plant pines with the intention of harvesting them sometime in the future, usually within 15 to 20 years if markets for pulpwood are favorable. However, when pulpwood markets are down, longer rotations can bring higher returns on the investment if you are willing to thin your trees 10 to 15 years after planting. This stand improvement can optimize growth on the remaining trees and result in substantially higher revenues when the stand is 20-30 years old.
Thinning is cutting in an immature stand to:
- stimulate diameter growth of residual trees and
- increase the total value of merchantable wood.
Thinning redistributes the growth potential of the site to the best trees so they can get bigger faster. Biologically, thinning influences successional trends by favoring the tallest, best-formed trees over those that are overtopped, crooked, forked, or otherwise undesirable. Individual residual tree response will be increased diameter and volume growth. Therefore, you should especially consider thinning if wish to harvest sawtimber-, pole- or chip-n-saw-sized products at the end of the rotation.
For the landowner, thinning can bring:
- an increased rate of return on the investment from high-value trees,
- periodic income,
- improved access for equipment,
- a healthier stand by salvaging trees that may soon die, and
- enhanced wildlife habitat through increased herbaceous ground cover.
Combined with prescribed fire, thinning is an essential wildlife management tool in pine stands.
Stand Density & Crown Position
Before introducing specific methods of thinning it is first necessary to discuss the underlying concepts of stand density, crown position and forest health. These will dictate if, when, and how to thin.
Stand Density
Stand density describes how much a site is being used and the intensity of competition between trees for the site's resources (i.e., water, light, nutrients, space). At higher densities, the growth rates of individual trees slow down because there are more trees competing for the site's limited resources. Trees are usually thinned to achieve a particular density target.
Measures of Density
Trees per acre. In homogeneous, even-aged stands of known age, site quality, and history, the number of trees per acre is a useful measure of stand density.
Volume per acre. Since many objectives relate to volume, it is often used as a measure of density. Volume is interpreted in relation to some standard, such as volumes represented in a yield table. It is generally expressed as cubic feet (solid wood), board feet, tons or cords per acre. A cord is 128 cubic feet of stacked roundwood (whole or split, with or without bark) containing wood and airspace; an example of a cord is a stacked pile of firewood 4-ft high x 4-ft wide x 8-ft long.
Basal Area. Basal area is a measure of stand density developed by foresters. It is the total cross-sectional area of the trees in a stand, at breast height (4.5 feet above the ground), measured in square feet per acre. Basal area (BA) of a single tree in square feet is calculated using the formula below:
BA = .005454 * d2
Where: d = diameter, in inches, of a tree at breast height (dbh)
Crown Position
The method used to control stand density will depend on the position of the crowns (branches and foliage) of the trees in the stand. Most planted pine stands have an even-aged structure, which means there is little or no difference in the relative ages of the trees. Even-aged stands are characterized by a gradual decrease in the number of trees per acre with time, individual tree growth slowing over time, and trees growing at different rates. This variation in growth results in 4 distinct crown classes:
- Dominant
- crown extends above the canopy level
- full sunlight from above and the sides
- well-developed, large crown
- largest diameters and exceptional tree vigor
- Codominant
- codominant crowns form the main canopy layer
- sunlight from above but restricted at the sides
- medium-sized crowns
- Intermediate
- crown reaches only to the lower part of the main canopy
- full sunlight only partially from above if at all
- small, crowded crown
- Overtopped
- crown entirely below the main canopy
- no direct sunlight
- usually the smallest trees with poorly-developed crowns
- very low vigor
When & How Much to Thin?
Timing
The first thinning should take place when or shortly after the crowns of the trees start to close. This is when diameter growth will begin to decrease due to the trees' limited ability to capture sunlight, which is needed to produce the carbohydrates necessary for diameter and volume growth.
An important indirect measure of a tree's ability to capture sunlight is live crown ratio. Live crown ratio is the percentage of a tree's height occupied by the living crown. In southern pines, optimum growth and vigor is maintained when the crown makes up at least one-third of a tree’s height, a live crown ratio of 33% or higher. Thinning is necessary before the average live crown ratio falls below 33%.
Another factor to consider when deciding when to thin is the marketability of the trees removed. A first thinning will generally remove pulpwood-size trees. For pulpwood, the average diameter of the trees to be cut should be at least 6 inches in diameter, with at least 10.5 feet to a 4-inch top.
It may be necessary to thin trees that are not merchantable if the average live crown ratio of the stand is below 33%. In this case thinning should be regarded as an investment in the quality of the stand for the future, when final harvest returns may justify the operation.
How Much? - Thinning Intensity
The number of trees to remove depends on your objectives. For timber objectives, a thinning should reduce stand density to a level that maximizes individual tree growth without sacrificing net volume per acre per year. For high-value wood products, a good rule of thumb for the first thinning is to cut the stand back to 80-85 square feet of basal area per acre (400-430 6-inch trees per acre). This density should leave enough space to maintain an adequate average live crown ratio without reducing the total yield of the stand.
For wildlife habitat objectives, you may need to thin back to 70-75 square feet per acre (350-380 6-inch trees per acre) or less in order to open up the understory to more sunlight so that desirable wildlife food plants can grow. In most cases, periodic prescribed burning will also be necessary to promote the development and maintenance of understory wildlife habitat.
Thinning Methods
There are four distinct methods of thinning, a combination of which are often used due to the irregularity of the relative crown positions of the trees in most stands. In selecting thinning methods, keep in mind that there are no standardized thinning treatments or residual densities for all places, and there is no single thinning method that satisfies all needs.
Low Thinning
Otherwise known as "thinning from below," low thinning removes trees from the lower crown positions to make room for taller, larger-crowned trees to grow. This type of thinning closely mimics the natural course of stand development as it eliminates the trees least likely to grow into the dominant or codominant crown classes. There are 4 levels, or grades, of low thinning that are defined by the intensity of the cut:
- Grade A: remove overtopped trees only
- Grade B: remove overtopped and intermediate trees
- Grade C: remove overtopped, intermediates, and some codominant
- Grade D: remove overtopped, intermediates, and most codominants
Crown Thinning
Crown thinning, or "thinning from above," removes from middle and upper crown classes to favor development of the most promising trees of the same classes, with adequate regard to spacing. Crown thinning favors the same trees as low thinning but focuses more on removal of competing trees in the dominant and codominant classes. For example, a forked dominant tree might be removed in favor of a straight codominant.
Mechanical Thinning – Row Thinning
A predetermined spacing or pattern dictates the selection of trees to remove in mechanical thinning, with little regard to crown position. This type of thinning is advantageous in dense, young stands that have not yet differentiated into crown classes; and in more mature stands with more than enough dominant trees to provide a fully stocked stand in the final years of the rotation.
A commonly used method of mechanical thinning is the South is row thinning, which removes designated rows of trees. This type of thinning, also known as "road" thinning, is most practical because it facilitates harvesting operations (skidding and trucking), especially for a first thinning in a dense stand. The condition and density of the stand dictates the selection of rows to remove.
In very dense plantations (700 to 1,000 trees per acre) with little or no fusiform rust infection, removal of every third row is favorable because every residual tree is freed on one side and removal of one-third of the stand is a good intensity for a first thinning.
Removal of every fourth or fifth row is advantageous in stands with some diseased trees (20% to 50% infection) because selective removal of infected trees from residual rows will not diminish the stand, and a middle row can be removed in subsequent thinnings if necessary. Stands severely infected or damaged by insects, disease or fire (50 to 75% infected or damaged) should be clearcut and regenerated. The table below summarizes which method is best for different objectives, densities and conditions:
Thinning guidelines for pulpwood-size pine stands:
Situation |
Alternate Row |
Third Row |
Fourth Row |
Fifth Row |
| 1,000 or more trees per acre, disease not a problem |
X |
|||
| 700 to 1,000 trees per acre, disease not a problem |
X |
|||
| 20-50% of stand infected with disease | X |
X |
||
| Wildlife is major objective | X |
(USDA Forest Service)
Row thinning can also be used to control the density of dense, naturally-seeded pine stands by cutting 10- to 12-ft-wide swaths between rows of trees 15-ft wide. This will reduce the density of the stand and provide access for a subsequent free thinning in the remaining rows.
Free Thinning
Also known as crop tree release or selection thinning, crop trees that meet your standards are released by removing competing trees around them, leaving any trees that do not affect the crop trees. This method essentially combines the low thinning and crown thinning methods. It is common to use the cleared strips of a row thinning to access trees marked for a free thinning in remaining rows.
Improvement Cuttings
Improvement cuttings are responses to problems beyond our control or that came about because of neglected actions in the planning period or managment during earlier stages of stand development.
Improvement cuttings are applied to stands past the sapling stage by treating the trees in the main crown canopy - remove poor trees to favor the good ones. Since this type of cutting yields poor-quality trees, it will not return much money per acre. Like thinning, these treatments are an investment in better-value growth and increased long-term returns.
Salvage Cutting
With catastrophic factors, we can sometimes salvage cut to recover economic values and improve future value. Salvage cutting removes trees killed, damaged, or that appear likely to suffer death or damage from a destructive agent. This treatment also serves to recover volume before it is lost.
Sanitation Cutting
Sanitation cuttting eliminates trees attacked or in danger of attack by a destructive agent. This treatment prevents or inhibits the spread of the problem to other trees and is combined with salvage cutting.
Additional Resources
University of Florida Extension Service Publications
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