Each landowner has their own goals for owning woodlands. Some desire habitat for a variety of wildlife while others desire recreation such as hiking, hunting, and bird watching. Still others want to harvest timber and non-timber products from their woods for home and farm use or to provide periodic income. Most aspire to maintain or improve the health, vigor, and attractiveness of their woodland. For many private woodland owners, all the above uses of their woodlands and more can be enhanced through crop tree management.
Crop trees are trees that provide or have the potential to provide the benefits desired by the landowner. If an improved squirrel habitat is desired, a large-crowned white oak that produces abundant acorns would be a valuable crop tree. If increased economic value is an important ownership objective, a 12-inch-diameter black walnut tree with a straight and relatively defect-free trunk and a relatively small crown would be a valuable crop tree. On the other hand, if fall color was important, a group of blackgum trees, which turn brilliant red in the fall, might be potential crop trees.
In an unmanaged woodland, competition among trees for light, water, and nutrients is often severe and can result in slow growth or even the death of the most desirable trees. In most cases, access to light is the most important factor for increasing tree growth. In a woodland under crop-tree management, crop trees do not experience excessive competition because less-desirable, competing trees are reduced or eliminated. The crop trees that remain are called “released” trees. A competing tree is one that has a canopy touching the canopy of a crop tree. Released crop trees are healthier, more vigorous, more insect and disease resistant, grow faster, and produce additional landowner benefits.
Crop tree management is simple. Once ownership goals and objectives are clearly identified, criteria are developed for selecting crop trees based on those goals and objectives. Next, trees meeting those criteria are located in the woodland, and then some or all those trees are released from competing trees (Figure 1). An important, tangible advantage of crop tree management for most woodland owners is that it focuses management activities on improving the health, vigor, and growth rate of trees that directly satisfy ownership objectives.
This fact sheet introduces the crop-tree management process and provides guidance in its application. Landowners and others seeking a more in-depth written reference to crop tree management are encouraged to read Crop Tree Management in Eastern Hardwoods (Perky et al, 1993).
Identifying Goals and Objectives
The first and most important step in managing any resource is identifying ownership goals. In the case of woodlands, why do you own them? Do you like to hike and bird watch? Are you a hunter? Do you hope to produce income from timber now or in the future? Is it the fall foliage and spring wildflowers that grab your attention? Can you use your forest to improve the quality of water flowing through your property?
Clearly, identifying goals for your woodland is critical. Goals provide the basis for determining what should and should not be done in your woods to improve its ability to satisfy your needs and desires. Said another way, ownership goals define the target for management activities such as crop-tree management.
The next step is to translate the more general ownership goals into stand-specific objectives. A woodland stand can be defined as, “A group of trees set apart from others based on species composition, age, structure, quality, or geography” (Londo & Londo, 2022). For example, if enhancing the quality of wildlife habitat is an ownership goal, shortening the time of trees to mast (produce seeds or fruits used as food for wildlife) and increase the amount of mast they produce using crop-tree release might be an objective for a young oak-hickory stand. Or, if future income was a goal in a mixed species stand, increasing the growth rate of the economically more valuable trees through crop-tree release might be an objective.
Just as we can have more than one goal for our woodland, we most likely will have more than one objective for individual stands. We could, for example, have overall goals of improving the timber production and wildlife habitat of a 30-year-old mixed oak-hickory stand. In such a stand, crop-tree management could be used to selectively release trees that satisfy the timber production goal (high-value species, good quality, fast growing), some that satisfy the wildlife habitat goal (mast-producing species, variety of species, large crowns, etc.), and some that satisfy both goals.
Developing Crop Tree Selection Criteria
Once specific goals and objectives have been established, criteria can be developed for selecting crop trees. These criteria are characteristics looked for when selecting crop trees. Table 1 shows examples of selection criteria for landowners whose primary objective is wildlife, timber, or aesthetics. Remember, these are only examples. You can—and should—develop criteria to suit your specific goals. As noted earlier, several criteria that achieve more than one objective can be used in a single stand. For woodland owners interested in managing for wildlife, read Enhancing Food (Mast) Production for Woodland Wildlife in Ohio (ohioline.osu.edu/factsheet/f-60).
Table 1. Examples of Crop Tree Selection Criteria for Woodland Owners with Wildlife, Timber, and Aesthetics, Primary Objectives.
Wildlife
- Crown is large, healthy, and in or above the main canopy.
- Mast-producing species (prefer hard—acorns or other nuts—over soft—cherries, plums, berries etc.).
- Trees with dead branches and open cavities are desirable.
- Species variety is highly desirable.
- Expected longevity of 20+ years.
Timber
- Crown is large, healthy, and in or above the main canopy.
- High-value commercial species (black walnut, white oak, etc.).
- High-quality tree with:
- Butt log with high-grade potential.
- No sprouts on butt log.
- No lean, low forks, etc.
- Species well adapted to the site.
- Expected longevity of 20+ years.
Aesthetics
- Select trees and species that are unique in appearance or character, produce attractive flowers and colorful foliage, and have attractive or unique bark.
- Visible from travel lanes, vantage points, etc.
- Expected longevity of 20+ years.
Other Objectives
While wildlife, timber, and aesthetics may garner attention in the selection of woodland crop trees, woodland owners want to manage their woods for many additional reasons. Individuals wanting to produce maple syrup can select their best maple trees as crop trees and allow them to produce more sap each year with a higher sugar content. Those wanting to increase the biodiversity on their property can select a wide range of crop tree species to ensure those trees survive. Others may select trees that attract the most wildlife diversity. Additionally, unique trees (such as an American beech that has family member names carved into it) or trees rare to the area (such as the only cucumber magnolia tree on your property) can be selected and managed as crop trees.
Inventory—Identifying Crop Trees
Now it's time to inventory your property, utilizing your selection criteria, to see how many potential crop trees are present. How extensive and sophisticated this inventory needs to be depends on the size and character of the woodland and the planned intensity of crop-tree management. A woodland owner with 10 acres who intends to release three or four trees producing hard mast per acre to improve wildlife habitat may need to do little more than walk over the property and select the trees based on the selection criteria and their spacing throughout the woodland.
On the other hand, a woodland owner with 50 acres who intends on releasing 20–30 crop trees per acre to favor timber production and wildlife habitat may need a more detailed inventory to determine the number of suitable crop trees. This inventory identifies the trees’ location, evaluates their potential to respond to release, and indicates how many trees need to be removed to release the crop trees. Also, if the trees to be removed are merchantable and the removal is to be a commercial sale, some form of formal inventory is desirable for marketing purposes. Normally, crop tree release is considered a pre-commercial activity and is not meant to initially generate income. The methodology to perform a formal inventory is beyond the scope of this fact sheet. However, a complete or sample crop tree management inventory should include the species, diameter, height, and free-to-grow rating (described in the next paragraph) of the potential crop trees, along with a record of the trees competing with the crop trees.
The free-to-grow rating is an index of competition that rates the available growing space for the crop tree. This is accomplished by determining how many sides of the tree’s crown have room for outward growth. The rating is graphically depicted as a division of the tree’s crown into four sections and then determines how many of the sections have room to grow (Figure 2). Ratings range from 0 for trees with no room to grow to 4 for trees with room to grow in all four directions.
Application
The application of crop-tree release involves removing or deadening all trees that directly compete with the designated crop trees. Essentially, this involves removing or deadening all trees whose crowns touch the sides of, or overtop, the crowns of the crop trees. Trees whose crowns are below the crowns of the crop trees are usually not removed or deadened.
The number of crop trees released, and the number of competing trees is dependent on several factors including the number of potential crop trees occupying the site, the age and size of the forest stand, and the landowner’s preference. Ideally, a released crop tree should have a free-to-grow rating of 4, meaning it has been released from competing crowns on all four sides of its crown. Occasionally, two crop trees will be so close that both cannot be released on four sides. If the decision is made to keep both trees, each can be released on three sides. In general, younger stands require more crop trees to be released because all the crop trees will not survive until the stand matures. Additionally, older stands often contain fewer potential crop trees compared to younger stands.
The recommendation for crop tree release is when trees are pole-sized (5–11 inches in diameter measured at DBH-Diameter at Breast Height) (Perkey et al., 1993). In many Ohio woodlands, this equates to trees being 20–40 years old. Trees in this size range respond well to crop tree release. However, when woodlands reach this size and age, fewer potential crop trees are available, and opportunities are reduced to modify the species composition of the woodland moving forward. Releasing crop trees at a younger age provides more potential crop tree choices, resulting in an opportunity to manage trees that would have otherwise not existed. This early crop tree release method could prove particularly effective in maintaining oak and hickory in woodlands where they would be out-competed by red maple, yellow-poplar, or bigtooth aspen. Releasing younger crop trees, on average, requires controlling more trees to achieve a four-sided release as these trees are smaller and take up less growing space. Additional releases of crop trees may be needed to maintain the rapid growth rates achieved in these young stands.
Once the crop trees and trees to be removed have been identified, mark them with different colored flagging and evaluate the planned release. While a crop tree release of only a few trees per acre will have little effect on the woodland's appearance, a moderate to heavy crop-tree release may substantially alter its appearance. Evaluating the visual impact before cutting allows modifications if the anticipated results are not desirable.
If releasing all your crop trees will result in an unacceptable visual effect, release fewer crop trees. It is far more desirable to completely release a few crop trees than it is to partially release many. Partially released trees, with lower free-to-grow ratings, grow at a much slower rate than completely released trees. For example, using the data from Figure 3, a 10-inch-diameter tree with a free-to-grow rating of 4 would increase more than 14 inches in 10 years while the same-sized tree with a 0 free-to-grow rating increases less than 12 inches. Based on a conservative estimate, the tree with a free-to-grow rating of 4 would have approximately 80% more board foot volume after 10 years than the tree with a free-to-grow rating of 0. Larger trees not only produce more timber volume but also produce substantially more wildlife food and other benefits. Consequently, crop trees that are free-to-grow on four sides are much more likely to contribute toward reaching those woodland ownership goals.
Trees to be removed may be cut or deadened. In some instances, enough trees of suitable size may be cut to warrant a commercial timber sale. More commonly, the trees being removed are too few or too small to be marketable. In such instances, the trees can be cut for landowner use (firewood, fence posts, etc.) or deadened in place by girdling or other appropriate techniques. The most common technique for removing trees is through cutting or directional felling using a chainsaw. Directional felling is a critically important skill because it avoids damaging the crop trees. Remember, the trees being cut have a canopy touching the crop tree’s canopy. As a result, damaging crop trees is a real threat. Girdling can be accomplished using a chain saw to cut through the bark and about two inches into the wood of the tree around its entire circumference. Double girdling, with a second girdle about six inches above the first girdle, is even more effective. Properly girdled trees will die standing and will slowly decay, providing habitat for wildlife in the process. Depending upon the size and species of the girdled tree, it may remain standing for months to upwards of a decade.
Another common method for deadening trees is to apply herbicides using a stem injection technique. Stem injection applies a concentrated herbicide into strategically placed cuts into the tree. It is possible for the roots of crop trees to connect underground to the roots of the competing trees through root grafts. Where these root grafts occur, be aware that herbicide can enter the crop tree and cause significant undesirable effects. It is common for trees that are closely related to form root grafts with one another (Lev-Yadun & Sprugel, 2011). It may be acceptable to use herbicide on a red maple to benefit a nearby white oak crop tree. However, it would be unwise to use herbicide on a black oak to benefit a nearby red oak crop tree. For more information on girdling, stem injection, and other techniques to free your crop trees from competition, refer to Controlling Undesirable Trees, Shrubs, and Vines in Your Woodland (ohioline.osu.edu/factsheet/f-45).
Acknowledgments
The authors would like to thank former Ohio State University Extension Specialist, Randall B. Heiligmann, for his work on previous versions of this fact sheet.
References
Lev-Yadun, S. & Sprugel, D. (2011). Why should tree have natural root grafts? Tree Physiology, 31(6), 575–578.
doi.org/10.1093/treephys/tpr061
Londo, A. J. & Londo, H. A. Forestry terms for Ohio landowners [Fact sheet]. Ohioline, Ohio State University Extension.
ohioline.osu.edu/factsheet/f-0104
Perkey, A. W., Wilkins, B. L., & Smith, H. C. (1993). Crop Tree Management in Eastern Hardwoods. United States Department Of Agriculture, Forest Service.
cpb-us-w2.wpmucdn.com/u.osu.edu/dist/a/836/files/2016/02/CropTreeMgmt-1h35gph.pdf
Originally written in 2002 by David Apsley, former Ohio State University Extension Natural Resource specialist, and currently employed as Southern Ohio Woodland Owner Outreach, Ohio Department of Natural Resources, Division of Forestry; and Randall Heiligmann, Specialist, Forestry, Ohio State University Extension. Reviewed in 2024 by Kathy Smith, Program Director, Forestry; College of Food, Agricultural, and Environmental Sciences; Ohio State University Extension.