Thinning and 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. Click on the image to the right to watch a short video about thinning.
The purposes of this type of intermediate cutting include:
- to stimulate growth of the residual stand
- to increase the total yield of merchantable 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.
Many landowners plant pines with the intention of harvesting them at some point in the future. When pulpwood markets are favorable, a complete stand harvest within 15 to 20 years is possible and may bring an acceptable return. However, longer rotations can bring higher financial returns on larger diameter trees if the landowner is willing to begin thinning their pine stand when trees are 10 to 15 years old. Pine sawtimber, poles and/or plylogs are most often the forest products with the highest value and, if economic returns are a priority, the desired products out of a timber stand. Thinning is a partial tree harvest in an immature stand to maintain or accelerate diameter growth of the remaining trees and, if done properly, can bring substantially higher revenues when trees are harvested at 25 to 40 or more years of age. Waiting to thin your pines after 16 or 17 years of age can result in less than favorable results as the pines take longer to respond.
Thinning is cutting in an immature stand to:
- stimulate diameter growth of residual trees and
- increase the total value of merchantable wood, and
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 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)
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:
- crown extends above the canopy level
- full sunlight from above and the sides
- well-developed, large crown
- largest diameters and exceptional tree vigor
- codominant crowns form the main canopy layer
- sunlight from above but restricted at the sides
- medium-sized crowns
- crown reaches only to the lower part of the main canopy
- full sunlight only partially from above if at all
- small, crowded crown
- crown entirely below the main canopy
- no direct sunlight
- usually the smallest trees with poorly-developed crowns
- very low vigor
Forest health is the focus of forest management and the purpose of thinning. The primary purpose of thinning is to remove poor performing trees and leave a healthy, vigorous stand. A healthy forest produces more tons of valuable timber per acre resulting in more tons of higher quality wood available to sell. The various insects and diseases that affect pine stands in the South have evolved to exploit unhealthy, stagnated or damaged trees that are stressed. Healthy pine stands resist insect, disease and wind related damage. If done early in the pines development thinning is an important tool in preventing problems with insects, diseases or other stresses such as wildlfire or strong winds.
Fusiform rust is a native, fungus-caused disease which deforms and kills pines. Since the late 1950s, it has increased to epidemic proportions in slash and loblolly pine plantations throughout the South. This disease was first reported in the early 1900s and was neither widespread nor prevalent at that time. The spread of fusiform rust increased as the acreage of young, intensively managed pines increased across the South. The fungus causing fusiform rust is greatly favored in young, rapidly growing, pine plantations of slash and loblolly pines, especially when established in high rust hazard areas and in close proximity to oaks, especially water oak, which are alternate hosts for the fungus. Oak abundance generally increases in areas where fire is absent. Most stems infected with fusiform rust disease should be removed in a thinning. Larger diameter stems with minor disease on branches can continue to have good growth rates and withstand high winds after thinning. If the stem infection rate of a stand exceeds 50%, the best option may be to clearcut and regenerate with genetically improved, rust-resistant pines. However, if there are at least 150-200 healthy, well-formed trees per acre, removing the diseased trees and retaining the healthy ones is usually the best option. If there is an abundance of red oak species, especially water oak, in surrounding stands, they should be reduced if possible. A professional forester can help you make appropriate management decisions to minimize or deal with problems associated with fusiform rust. More information about this disease ...
Southern Pine Beetle
Southern pine beetles (SPB) are native, aggressive insects that live predominantly in the inner bark of pine trees. Trees attacked by SPB often have hundreds of light-colored, dime-size resin masses (i.e., pitch tubes) on the outer tree bark. SPB feed on living bark tissues where they construct winding S-shaped galleries on the inside of the bark, which can effectively girdle and kill a tree. In addition, SPB carry and introduce blue-stain fungi into trees. These fungi colonize the water-conducting tissue and can block water flow within the tree. Once SPB have successfully colonized a tree, the tree generally will not survive, regardless of control measures. An important way to prevent SPB infestations in pine stands is to maintain high tree vigor. This can be achieved by thinning dense stands to a basal area of 80 sq. ft. per acre or less to reinvigorate tree growth. More information about SPB and its control can be found at http://edis.ifas.ufl.edu/IN333.
Cost-share assistance for thinning pine stands, prescribed fire and other treatments is available through the Florida Forest Service’s Southern Pine Beetle Assistance and Prevention Program.
Annosum Root Rot
Loblolly and slash pine are particularly susceptible to this disease which may be scattered through a stand or occur in pockets of dying or dead trees. Trees generally yellow and lose needles as they die from this disease, although they may just turn red in a short period of time. Dead trees gradually fall over from a loss of root support. Wind-blown fungus spores from nearby infection centers generally enter a stand by landing on freshly cut stumps or wounds during the colder months of the year. The stump and subsequent root infections spread to adjacent trees through root contact. The disease is most prevalent on well-drained sandy soils with higher pH such as those found on old agricultural fields. Prevention measures include prescribed burning during winter months prior to thinning to eliminate the spore-producing conks, thinning in high hazard areas during summer, and treating freshly cut stumps with borax immediately after thinning. More information about this disease ...
When and How Much to Thin?
The first thinning should take place shortly after the crowns of the trees start to close (tree branches of neighboring trees begin to touch each other). 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 branches with green needles. In southern pines, optimum growth and vigor are maintained when the live crown makes up at least 40% of tree height (a live crown ratio of 40% or higher). Thinning is most beneficial for stand growth before the average live crown ratio falls below 40%.
Another factor that influences thinning decisions is the marketability of the removed trees. The first commercial thinning should remove pulpwood-size, and perhaps some chip-and-saw-size, trees if they are poorly formed or diseased. Pulpwood logs must be at least 10.5 feet long and 2-3 inches in diameter at the small end; some local markets require larger log sizes. To meet these minimum specifications, trees must be about 16 feet tall and have an average DBH of at least 5 inches before they are cut. It may be necessary to thin smaller trees if the average live crown ratio of the stand is below 40% and trees do not grow at least 5% per year in diameter. With the demand for woody biomass on the rise in some regions for energy production, these trees may have a market. Otherwise, “precommercially” thinned trees are usually left on the ground to decompose. 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. See http://edis.ifas.ufl.edu/fr243 for information on precommercial thinning loblolly pine.
The number of trees to remove depends on the initial stand density, site quality and management objectives. For timber objectives, a thinning should reduce stand density to a level that maximizes individual tree growth without sacrificing full utility of the site. Density and stocking should be approached from the quality of the residual stand first; and second, the density of the residual stand. Depending on the site, the density and quality of the trees in the stand you are working with, and your management objectives, the residual basal area after the first thinning will ususally fall between 45 to 85 square feet per acre of the very best trees capable of producing a higher value product. These will be the healthiest, best formed trees in the dominant and codominant crown classes. A suggested rule of thumb is to use basal area as a result, not a target. Basal area does not take into account the age of the stand, site productivity, and tree health and quality. Focus growth on the best trees in the stand and the basal area will follow.
Thinning, especially when followed by prescribed fire, can be great for wildlife habitat. Thinning allows more sunlight to reach the forest floor, encouraging the growth of herbaceous plants and shrubs, which provide food and cover for many upland wildlife species in the southeast. Subsequent thinnings and a prescribed fire regime during the rotation will promote an open tree canopy, diverse groundcover, and productive wildlife habitat. . See http://edis.ifas.ufl.edu/uw132 for more information on the effects of fire on wildlife habitat.
How to Thin
A combination of thinning methods is usually used to reach economic and/or wildlife habitat objectives. No matter which thinning method is used, thinning during times of drought or extreme wet weather should be avoided to avoid damage to the site. Likewise, damaging residual trees during logging should be avoided. When trees are damaged, such as “bumper” or “turning” trees at the end of thinned rows, they should be removed at the end of the logging operation. Landowners are encouraged to consult with or hire a professional forester to assist with thinning and other forest management activities. See http://edis.ifas.ufl.edu/fr125 for tips on selecting a consulting forester.
Combined Row and Selection Thinning
Although most discussions about thinning southern pines are about which rows to thin, the focus should be on what comes out of the remaining rows. Modern equipment, though large, is capable of taking out trees in the rows between cut rows, as in a 5th or 7th row thinning. Generally, the further apart the cut rows, the better. Think of the cut rows as access for the harvester to cut selected trees out of the remaining rows. It is best to remove trees based on selection thinning from fewer cut rows rather than taking out every 3rd or 4th row. The first thinning is the most important thinning and sets the growth rate for the rest of the rotation. Properly executed thinnings consistently produce higher valued products thus more revenue. In addition to revenue goals other management objectives are maximized such as wildlife habitat improvement.
Removing every 3rd or 4th row is essentially clearcutting 33% or 25% of the stand without regard to quality, and leaves only 66% or 75% of the stand to select from. Unless there is excessive disease or extreme variabity in density (see fusiform rust guidelines above), this should be avoided. Leaving the trees distributed over a larger portion of the stand can be much more profitable in the long-term because you can select your best trees to grow into larger, more valuable products.
The premise for thinning is simply to take out the poor trees and leave the healthy crop trees for potential future harvest. Trees that are diseased, crooked, forked, suppressed or otherwise of poor quality or health should be removed in the first thinning. For best results, hire a professional forester to mark every thinning. If marking is not feasible for some reason, each thinning should be closely supervised against contractual guidelines, especially the first thinning. Do not assume the logger or harvester operator will leave the trees appropriate for the long-term health and productivity of the stand.
Thinning is an important silvicultural practice, which redistributes the growth potential of the site to the best trees. Diameter growth rates are maintained or increased on residual trees after thinning, which increases the return on investment from higher value trees. Biologically, thinning accelerates stand development by favoring the tallest, best-formed trees over those that are diseased, overtopped, crooked, forked, or otherwise undesirable and likely to die on their own if left in the stand long enough. In addition thinning provides periodic income, improves access for equipment, recreation and hunting, and creates a generally healthier stand. Thinning is also beneficial for wildlife, especially when combined with prescribed fire or herbicide use to control competing vegetation. By allowing more light to reach the forest floor, thinning promotes growth of plants important as food and/or cover for wildlife species. Landowners are encouraged to consult with or hire a professional forester to assist with thinning and other forest management activities.
University of Florida Extension Service Publications
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.
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 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.
Timber Sales & Marketing
Marketing timber involves selling forest products in a competitive market to get the best return on your investment or to meet other objectives. This process requires some planning and pre-sale preparation before you advertise or talk to prospective buyers. Timber sales should be approached in a business-like manner to ensure that both the seller and buyer are satisfied with the results. It is highly recommended that landowners get the help of a professional forester to sell timber. See these publications or pages for all the steps of the process and how to get help:
University of Florida Extension Publications
University of Florida Extension Service Publications