Jake Hendee, Consulting Forester
Why Timber Stand Improvement (TSI)?
Timber stand improvement encourages faster growth and better forest health, an improved mix of species, and optimal wildlife habitat. Timber stand improvement is an action that landowners can take to improve their woodlands immediately. Four common methods of timber stand improvement include crop tree release, low thinning, cull tree removal, and invasive species removal.
A general rule of thumb for timber stand improvement is that removing or deadening a tree should only be done if it will benefit a better tree or allow for regeneration of a more desirable species.
Three techniques are acceptable for removing undesirable trees. You can 1) double girdle (Figure 1) at ground level (no herbicide needed); 2) single girdle (Figure 2) at any level and apply an approved herbicide such as 20% glyphosate (Roundup) to the cut; or 3) fell the tree and leave a stump cut to near ground level. Any technique that does not utilize herbicide should be applied near ground level to ensure that any sprouts grow into well-formed trees. Girdling techniques are preferred over felling because they allow trees to slowly break up and not damage crop trees on their way down. Girdling larger trees (greater than 6" in diameter) and cutting down smaller trees (less than 6" in diameter) is generally the quickest way to move through the woods.
Figure 1. Double girdle method
Figure 2. Single girdle method with herbicide
The cut should be at least 3/4" deep in order to sever the cambium all the way around the tree. Trees will grow over girdle cuts like the black walnut tree below (Figure 3). A deep cut ensures the best kill rate. Late summer through early spring is the period when girdling will most effectively kill the tree.
Figure 3. Grown over girdle cut
Crop Tree Release
Crop tree release is the most common form of timber stand improvement. Crop tree release focuses on trees whose stem is 4" to 12" in diameter at breast height and trees with crowns in the canopy or just below the canopy. During crop tree release, the goal is to pick the best trees for timber and/or wildlife values and remove trees of less desirable form and/or species from around them. A general rule of thumb is to release between one or two sides of the canopy. In central Illinois, black walnut trees (in bottomland areas) and oaks (in upland areas) are very often good crop tree choices. Species such as hedge and locust will never be crop trees. A wide variety of species (hickory, sugar maple, black cherry in the uplands and sycamore, cottonwood, ash and hackberry in the bottomlands) can be crop trees depending on surrounding trees. Tree species diversity should also be considered in selecting crop and remove trees. A few examples are illustrated to the left and below. In Figure 4, the crop trees with circles are the white oak, hickory, and sugar maple (from left to right) because they are well formed and maintain a vigorous crown in the canopy. On the other hand, the sugar maple trees of poor form (two on left) will be removed to decrease the competition for light. The northern red oak (far right X) will also be removed because its small crown will not likely succeed in supplying sugars for vigorous growth.
Figure 4. Upland Crop Tree Release
The photos below exhibit an upland crop tree release (Figure 5) and a bottomland crop tree release (Figure 6). Trees with circles are again crop trees and trees with Xs are remove trees. In the upland example, three black oaks in the 8"-10" diameter range are selected as crop trees. To open these trees' canopies to light and allow them to grow faster to maturity, less well-formed black oaks and a hickory are selected to be removed. Much the same way, a sugar maple, boxelder, and sugar maple (from left to right) are removed to favor a black walnut crop tree in Figure 6.
Figure 5. Upland Crop Tree Release
Figure 6. Bottomland Crop Tree Release
Figure 7. Bottomland Crop Tree Release
In the above picture, adjacent trees are both selected as crop trees because of good form and spacing. The small diameter and short height of surrounding trees means that none in the foreground of the picture need to be removed.
Low Thinning (or Site Preparation for Natural Regeneration)
Shade tolerant trees not typical of historical Illinois forests are increasingly taking over native ecosystems and compromising the future of the forests we have known for centuries. These maples and elms were historically killed by frequent fires, but the removal of fire from the landscape has allowed shade tolerant trees to invade the understory and midstory of many forests and eventually replace oaks in the canopy of the forest. After timber harvests or disturbance, the shade tolerant trees in the midstory become the next stand. Removing the midstory weed trees allows light to reach the forest floor, allowing less shade tolerant and more desirable trees to germinate and proliferate, thus continuing to dominate for hundreds more years. In the illustration below, sugar maple trees about 2"-6" diameter with Xs are removed in order to prepare for oak regeneration. The best time to complete low thinning is generally just prior to or just after an overstory removal (commercial harvest, crop tree release, or storm), at the time of a major seed crop.
Figure 8. Low Thinning for Oak Regeneration
Cull Tree Removal
Cull tree removal is timber stand improvement that kills large undesirable trees by girdling or felling. Trees that are not commercially valuable nor ecologically valuable should be removed in order to maintain a good stand of trees. Oftentimes, a history of destructive timber harvesting has left the largest, least valuable trees in the stand and have removed the most valuable and vigorous growing stock. In the first case below, a rotten maple has grown up along a white oak. Killing this sugar maple tree will minimize mechanical damage to the oak and allow more desirable trees to use that canopy space.
Figure 9. Cull Tree Removal
The picture below shows a stand that has been taken over by bushy hedge trees. Killing these trees will allow the next generation to thrive in their space and prevent further takeover by the undesirable hedge in the future.
Figure 10. Cull Tree Removal
Invasive Species Removal
Invasive, exotic species pose a major threat to Illinois forests because of their ability to suppress native plants and regeneration of important tree species. Three woody shrub species, bush honeysuckle, multi-flora rose and autumn olive, are especially of concern in central Illinois. Invasive species removal is a long-term process, requiring hard work to remove and a sustained commitment to monitoring regrowth. Three techniques are possible to eradicate these woody invasive species.
- Cut stem, apply 20% glyphosate (Roundup) to fresh cut.
- Foliar spray, apply between 1% and 5% (depending upon species) glyphosate to foliage of invasive plant.
- Mechanical removal with large equipment, followed by foliar application of common Roundup or generic to sprouts.
Figure 11. One of the many threats: 100% bush honeysuckle cover.
(Note that it is first to leaf out and last to drop leaves in the forest.)
In all forms of timber stand improvement, all herbicides should be applied according to their labels to maximize kill and minimize negative and unintended impacts.
Good places to continue learning about timber stand improvement include the Indiana Forest Improvement Handbook (Indiana) and Timber Stand Improvement: A Guide for Improving Your Woodlot by Cutting Firewood (Missouri Department of Conservation).