Forest Stand Improvement

Practices Used in FSI
Tree Selection for Enhancement
Forest Regeneration Methods

Forest Stand Improvement (FSI) or Timber Stand Improvement (TSI) is the manipulation of species composition, stand structure and stocking by cutting or killing selected trees and understory vegetation. The purpose of a FSI is to increase the quantity and quality of forest for wildlife and/or timber production by manipulating stand density and structure. FSI is also used to reduce wildfire hazards, improve forest health, restore natural plant communities, achieve or maintain a desired native understory plant community for wildlife, grazing, and/or browsing.

Various practices used and tree species selected should fit the chosen emphasis for the woodland. The number of trees to keep depends on species, type of site, management goals and size of the trees. Foresters are available to help determine a woodland's potentials and limitations and to help develop and carry out a suitable management plan.

Types of trees usually removed are:

  • Trees inferior because of their species.
  • Trees interfering with the growth and development of selected desirable trees.
  • Damaged trees (broken off, bent over, fire scarred, seriously barked, etc., but expected to live at least one cutting cycle). Cavity trees are important for wildlife see Forest Management for Wildlife section.
  • Seriously diseased trees or trees serving as a breeding ground for undesirable insects.
  • Trees with multiple stems.

It is important to remove or kill these types of trees as soon as possible after an area has been logged to properly release younger trees. Variety is important in a woodland environment, so those trees necessary for den trees, aesthetics or special foods should be selected and retained in the stand when beginning a program of FSI.

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Practices Used in FSI

Site preparation for natural reproduction in understocked stands means preparing the site to allow natural seeding or resprouting of desirable species, or underplanting seedling stock to fully use the available growing space. This practice is used in poorly stocked stands to fill large openings and increase stand density, or to improve the species composition.

Thinning is cutting trees from an immature stand to increase rate of growth and improve the form of the remaining trees. Proper space varies depending on species, purpose of management and quality of the location (site). Table 1 gives a range of spacing for trees of various diameters.

Table 1. FSI spacing recommendation guide for cutting of overstocked trees.

Tree diameter

Spacing range

Tree diameter

Spacing range

2 in

4 to 7 ft

9 in

14 to 19 ft

3 in

6 to 8 ft

10 in

15 to 21 ft

4 in

7 to 10 ft

11 in

17 to 22 ft

5 in

9 to 12 ft

12 in

18 to 24 ft

6 in

10 to 14 ft

13 in

19 to 26 ft

7 in

11 to 15 ft

14 in

21 to 27 ft

8 in

13 to 17 ft

15 in

22 to 29 ft

Certain species or management purposes may require other spacings. In any thinning, the tallest desirable trees are usually favored.

Release is removing or killing undesirable older overtopping trees to encourage fast growth and better quality of vigorous young desirable trees. Killing can be the use of herbicides basal area treatment, hack and squirt, or girdling. Removal of sawlog quality trees can be done using even-aged or uneven-aged management techniques. Trees that are not sawlog quality can be cut and left lay on the forest floor as down woody debris.

  • Basal Area Treatment involves the application of a herbicide to the lower 12 to 18 inches of the trunk or stem from early spring to mid-fall. Some species can be treated during the winter. The selected herbicide is mixed with diesel fuel, kerosene, or other suitable carrier and applied until the bark is saturated. The low volatile ester formulations are the only oil soluble products registered for this use. This method is effective on trees of all sizes, but is most commonly used on small brush.

  • Hack and Squirt is best suited to trees at least 4 to 5 inches in diameter. Bark on larger trees is often too thick for most water soluble sprays to penetrate, so it is necessary to provide a direct pathway for herbicide entry into the plant's vascular system.Use a hatchet to make a series of downward cuts in the bark around the entire circumference of the tree trunk. For most species, it takes about one cut for every 2 inches of trunk diameter. Frill cuts are overlapping cuts in the tree bark around the stem. Immediately apply the selected herbicide into the cuts. Avoid application during heavy upward sap flow in the spring, when sap flowing out of the wound will prevent good absorption. Apply herbicides registered for this purpose undiluted or in dilution ratios from one-half to one-quarter strength. Read the product label to determine the appropriate dilution. Amine formulations of Garlon, Grazon, and 2,4-D are generally more effective than esters. Roundup undiluted or half-strength is excellent for hack and squirt applications.

  • Girdling involves cutting a groove or notch into the trunk of a tree to interrupt the flow of sap between the roots and crown of the tree. The groove must completely encircle the trunk and should penetrate into the wood to a depth of at least 1/2 inch on small trees, and 1 to 1-1/2 inches on larger trees. Girdling can be done with an ax, hatchet, or chain saw. When done with an ax or hatchet, the girdle is made by striking from above and below along a line around the trunk so that a notch of wood and bark is removed. The width of the notch varies with the size of the tree. Effective girdles may be as narrow as 1 or 2 inches on small-diameter trees, and as wide as 6 or 8 inches on very large-diameter trees. When a chain saw is used to girdle, two horizontal cuts between 2 and 4 vertical inches apart are usually made completely around the tree when no herbicide is used (Figure 2) and one horizontal cut is made completely around the tree when herbicide is used.

Pruning is removing limbs to produce the maximum clear lumber or veneer in the butt log. Prune only selected hardwood trees where high-value species are grown on good sites. This is recommended primarily in managing black walnut. In pruning lower limbs of young trees, don't remove too much of the food-producing leaf surface of the tree. At least one-half of the living crown of the tree should be left intact. In general, trees should be pruned before they reach 8 inches in diameter. Limbs to be removed should be pruned before they reach 2 inches in diameter to reduce the wound size, to ensure proper closing and to lessen the impact of entry by insects or disease organisms.

Vine removal
from a wildlife standpoint only invasive vines like kudzo and Japanese honeysuckle should be removed. Other vines such as trumpet, native honeysuckles vines, and wild grapes provide important habitat and food. Vines not retained because of wildlife food value, fall color, etc., should be killed at the same time other stand improvement work is done. Remove them by cutting the vine as low to the ground as possible and immediately treating the stump with a herbicide such as glyphosate.

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Forest Regeneration Methods

Thinning of commercial quality trees can be done using two main methods, even-aged and uneven-aged management. Even-aged management is a type of forest regeneration management practice that creates stands consisting of shade intolerant trees of the same age class. Methods of even-aged management include: clearcutting, seed tree, and shelterwood techniques. Uneven-aged management is a type of forest regeneration management practice that creates stands consisting mostly of shade tolerant trees of the varying age and size classes. Uneven-aged management includes: group selection and single tree selection.

Even-aged Management

Clearcut is the most common method of regeneration among the even-aged management practices. This technique involves one cut, which may remove the entire stand. Clearcutting is for landowners whose goals require a large amount of new growth seedlings, and young shade intolerant trees. These cuts will provide the highest level of forage, shade intolerant tree mast, and woody stem density, and will attract ruffed grouse, rabbits, deer, and edge-loving songbirds.

Clearcutting provides the best results in northern states where aspen regernation responds the best to these types of cuttings by an explosive root sprouting shoots that can number 5,000 to 70,000 stems per acre. In southern states clearcutting has less of a benefit for ruffed grouse which rely more heavily on mast production for winter food as opposed to aspen buds in the north.

This technique benefits edge-loving wildlife the most when the cuttings are from two to 10 acres in size and a different portion is cut every 10 to 20 years. Cuts of 20 acres or more will result in large proportions of shade intolerant trees such as aspen, pin cherry, black cherry, and red oak. Cutting in patches or narrow strips will produce more intermediately tolerant and tolerant trees. Best regeneration occurs when cuts are made in a north/south orientation to receive full amounts of sunlight.

Landowners that subscribe to this technique should consider leaving a buffer zone of trees of at least 100 feet around wet areas, and saving valuable snags and mast producing trees at the rate of one to five individuals per acre. Leaving small clumps oaks, hickories, and pine in clearcuts larger than 5 acres is also encouraged to maintain diversity of vegetation and wildlife. It is suggested, in any forest management plan, to leave 1/4 to 1/3 of an acre uncut per 10 to 15 acres of timber harvested area to maintain diversity.

Seed Tree Cut a technique used in even-aged timber management that involves removing most of the stand in on cut, but retaining shade intolerant species, to provide seed for regeneration. These seedtrees can be left either alone or in small groupss. These trees do not provide enough cover to have any significant sheltering effect on the regeneration. The seed trees are then harvested after regeneration is established. This technique is most often used for pines.

Shelterwood Cut is a technique used in even-aged timber management that is the most complicated. It is used to provide protection and shade for the regeneration area. This technique results in two to three even-aged classes of trees, and is used to regenerate trees that thrive in partial shade. It involves a series of two or more cuts over 15 to 30 years, in which the first cut removes 50 to 70 percent of the canopy. The rest of the stand, called the shelterwood, is left to provide a partial canopy that protects the regenerating stand. In the first cut, thickets of saplings or poles that are extensive enough to form a stand are left. After 5 to 10 years, when the new growth is well established, a second cut can either remove all or half of the shelterwood stand. If only half of the stand is removed on the second cut, then a third cut is used 10 to 20 years later to remove the last half. The final cut may leave trees that are long survivors such as sugar maple, oaks, white pine, shortleaf pine and hemlock.

There are three ways to implement the shelterwood technique. The "uniform" method harvests trees that are evenly scattered throughout the stand. The "group" method removes groups of trees at each cut. The "strip" method uses an alternating or progressing pattern that moves through a portion of the stand at each cut.

The shelterwood technique is used to regenerate moderately shade tolerant species. It is especially successful in regenerating oak. Oak rebounds in forests that allow some sunlight to enter, while maintaining some shade and shelter for seedlings to become established. By creating space for large oak trees, acorn production increases and oak regeneration from seed is successful.

Uneven-aged Management

Uneven-aged management is preferred for landowners who wish to maintain a small amount of edge, and manage a relatively mature, diverse forest with little amounts of disturbance. It is also a good technique to use when a long-term supply of quality sawlogs is an objective.

This technique promotes regeneration of shade tolerant trees, such as sugar maple, basswood, beech, and ash. If trees selected for harvest are in groups more than 0.5 acres in size, then oaks, hickories, red maple, and other intermediately shade tolerant species will grow.

Uneven-aged management employs small cuts that remove 10 to 30 percent of the trees of all sizes at each cut. Trees are selected based on species, quality, biodiversity, and size. Selection sites should be areas that are too dense for optimum growth. The goal is to provide proper spacing to encourage rapid growth and reproduction. Thin lightly every 10 years or so to prevent severe disturbance and to encourage continuous rapid growth. This technique creates a more diverse canopy and allows sunlight to reach the forest floor which encourages growth of a herbaceous layer. It also creates a multiple layer forest. This creates nesting and feeding sites for a variety of wildlife species.

Single tree selections choose individual trees for cutting, and are used in stands dominated by shade tolerant trees such as beech and sugar maple. This method is good for wildlife that do not require openings or shade intolerant mast producing trees as it maintains a relatively continuous forest canopy. Single tree selection is also often used to obtain firewood. Group tree selections choose groups of trees for cutting, and are used to provide wildlife with shade intolerant mast producing trees and shrubs as it permits more sunlight. Another way of providing these trees and shrubs is to plant them along the forest edge, or along logging roads or trails.

Tree Selection for Enhancement

Trees to be enhanced through FSI must be existing or future canopy trees. Crown level is where the most intense competition for sunlight is, so trees that are competing in the forest canopy should be examined for possible thinning. The first time a forest is treated to improve growth, the trees cut should include undesired species, trees of poor form or health, trees of low vigor, or trees at risk for damage due to unstable branching patterns. Understory trees typically are not removed, unless their removal will benefit your goals for the forest stand. A forest's understory provides structural diversity and food for wildlife. In terms of numbers of trees, generally a spacing of 20-25 feet between crop trees is optimal, with no substantial competition between them at crown level. Trees should have growing room at crown level on at least three sides so their crowns can develop more fully.

Oaks, ashes, and yellow poplar are tree species that are often managed for in FSI operations. These species are valuable for both wildlife and timber production. When FSI thinning is done, some individuals of other tree species are cut down or killed in place to release healthy and more valuable specimens. Species that are often culled out to some extent in thinning operations because of their competitiveness are maples and beech. The reason these species become dominant is that they are shade tolerant; that is, they grow in the shady understory, awaiting harvests or other disturbances to remove the historically dominant canopy trees (oaks, ashes and yellow poplar). The shade tolerant species' growth as understory trees gives them a decided height advantage over new seedlings that may spring up when the canopy is finally removed. At that time, the shade tolerant trees begin to form a new canopy, blocking the sunlight that the shade intolerant oak and ash seedlings need to grow. Without a thinning operation in such situations, desirable seedlings may never make it to fruiting age. However, even in stands where non-crop species are overabundant, they should never be completely eliminated from a stand. Also, healthy and well-formed specimens of any species should normally be left in the stand.

There are limitations on the effectiveness of thinning. Usually, trees must be relatively young to benefit from thinning. Trees that are 10-30 years old will often grow taller, produce greater crowns, and yield more fruit as a result of being released from competition, but trees that are 50 or more years old may not respond to a thinning. Also, the site and soil types in an area will largely dictate the growth potential of a forest stand.

Landowners interested in enhancing their woodland or forest for wildlife through FSI should contact a wildlife biologist or certificated forester to help with planning your FSI. Also see our section on Forest Management for Wildlife.

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Video provided by Missouri Department of Conservation.

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