Woodland Improvement and Crop Trees in Iowa

Woodland with trees in fall color
Healthy woodlands provide multiple benefits through good stewardship. Photo credit: Vladi Nillissen.

Vigorous and healthy woodlands in Iowa have the unique distinction of being able to provide a wealth of benefits for the landowner and residents of the state. Benefits from a healthy forest include timber and wood resources, watershed protection, fragile site protection, wildlife and bird habitat, aesthetics and beauty, and recreational opportunities. 

When good stewardship is practiced, landowners utilize their land in such a manner as not to diminish or destroy the potential of the land for future use. Woodland improvement operates under the umbrella of land stewardship and is a series of practices on the land with the intent of improving the amount and the quality of the benefits derived from forest land. One of the major tenets of good stewardship is protection; woodlands must be protected from uncontrolled livestock grazing and wildfire in Iowa. Grazing may reduce soil quality because livestock compact the forest floor and destroy desirable vegetation. Compaction causes the decline of woodland plants through the reduction of air and moisture retention in the soil. Grazing may also result in soil loss and erosion. Prescribed fire may be a potential woodland management tool; however, uncontrolled wildfire can destroy regeneration, damage woodland stems, and modify herbaceous and woody understory plant composition. Fire should only be utilized when recommended and planned by a professional forester to achieve a specific management objective in your woodland.

Illustration showing cross-section of tree trunk with tree rings that decrease in thickness towards the outside of the trunk o
Slowed growth rate (B) may indicate time for thinning or release. Illustration created by Andrea Da Silva Gonzalez.

Woodland Improvement Practices

Forest stand improvement practices are used in stands (a group of trees with similar characteristics such as species, age, size, health, and quality) or woodlands prior to their maturity or their final harvest to improve the quality of the trees for their intended uses. These practices may be designed to improve species composition, quality of the woodland, survival, growth rates, forest health, and wildlife habitat. Woodland improvement practices are based on the tenet that a given parcel of land has limited productive potential and with management this potential can be distributed to the desired stems. Practices used in forest stand improvement include thinning, crop tree release, pruning, control of invasive tree, shrub, and vine species, and protection from damaging agents. 

Forest thinning means cutting some of the trees in a stand or a forest. Noncommercial thinning refers to thinning in stands in which the thinned trees are too small to be sold for products. Commercial thinning is cutting of trees to improve the quality of the remaining trees in the stand. Commercial thinning results in cut trees, which are large enough to be marketed for sawtimber, firewood, or pulpwood.

Non-commercial thinning

Weeding is the removal of all trees and herbaceous vegetation that directly compete with seedlings of desired species. This is rarely done except in the case of very special products such as plantations, sugar from maple syrup, or black walnuts for nut production. Cleaning is the removal of undesirable trees which are directly competing with saplings of desirable tree species. The removal of trees that overtop seedlings or saplings is a liberation or release cutting. This may be required in a direct seeding to help the oaks become a component of the stand. Methods of tree removal include girdling, felling, and/or herbicide treatments.

Commercial thinning

Close up of tree trunk with two cuts that girdle the tree
Chainsaw girdling. Note cut depth is just deep enough to sever cambium and sapwood. Photo credit: Vladi Nillissen.

An improvement cutting is a thinning made in a stand to improve its composition by removing trees of less desirable species, form, and condition. Commercial thinnings are most often applied to even-aged stands (stands where all the trees are nearly the same age). Stands may contain more trees than the land is capable of supporting; tree tops in these stands often compete for sunlight and tree roots for water and nutrients. Trees in overcrowded stands often have slowed growth rates and smaller live crowns due to self-pruning and shading. Some crowding in young stands will help develop good form—straightness and self-pruning of side branches. Once the trees have developed the desired form, they should be thinned to stimulate diameter growth. Thinnings may be required periodically to keep the desired trees growing fast—often stands require thinning every 5-10 years, as the thinned stands grow
back to occupy the site. 

Trees, which are removed in the thinning process may be cut and marketed, cut and left in place, or girdled and left standing. Girdling entails creating a double chain saw cut around the tree, cutting through the cambium and interrupting sap flow. This may also be achieved by using a hatchet to remove a strip of bark 2–4 inches wide completely around the tree. Deciduous trees that are cut or girdled may resprout below the girdle or
from the stump. To prevent resprouting, herbicides may be applied to the fresh cuts or the herbicide may be applied through an injection site or frill (i.e., hack and squirt method). The use of herbicides will prevent resprouting, and should be used with care to minimize damage to desired, residual, trees. Risk exists that same-species trees may be connected by root grafts and herbicide may be translocated to a desirable tree. For more information on herbicides and their use see our Chemical Control of Unwanted Woody Vegetation article.

Close up of tree trunk with a hatchet cutting into the wood and a herbicide sprayer applying blue-colored herbicide into the cut from the hatchet
Hack and squirt with herbicide application. Note cutting deeper than cambium and sapwood is not needed. Photo credit: Vladi Nillissen.

Crop Tree Identification and Release

Working with young woodlands to identify and thin around crop trees has the greatest potential of any practice to improve the woodland to meet landowner objectives. The concept of crop trees and release is simple, but in practice requires some commitment from the landowner starting with the identification of landowner objectives and continuing to the removal of some trees to favor others. In addition, for maximum benefit, it should be started early in the life of a stand or forest and continue throughout the life of the stand. For a more in-depth discussion of crop tree management, see the article Technical Guide to Crop Tree Release in Hardwood Forests

Goals and objectives

Before going to the woods, a landowner should work through a planning process to identify goals for their property. Property goals may be fairly broad and for the whole woodland or property. Goals should answer the question: “What is the property used for? Do you like to hunt? Watch birds? Do you want income from your woodland? Are you enjoying wildflowers?” These broad goals become the basis for determining what management practices you will perform in your woodland. Goals do not need to be mutually exclusive. 

Close up of cut tree trunk with ring of blue herbicide around outer edge of stump
Application of herbicide to the outer sapwood will prevent resprouting of cut trees. Photo credit: Vladi Nillissen.

After you have developed your broad goals, the next step is to begin to think about more specific objectives to attain the broader goals. Objectives are more task-oriented and may relate to stands in the woodland. For example, if one of the goals was to “improve wildlife habitat,” a related objective may be to “increase mast production from the oaks in the pole-sized white oak stand.” Or, if “future income generation” was a goal, the objective might be “to increase growth rate of valuable trees through crop tree identification and release." 

Woodlands are wonderful resources, partly because they can provide multiple benefits to owners. You might have a 30 year-old oak-hickory stand with the overall goal of improving timber production and wildlife habitat. In this stand this may be achieved with crop tree management to selectively release some of the trees that satisfy timber production and release others that provide more and better wildlife habitat.

Crop trees

Graphic of tree trunks of increasing size indicating increasing free to grow rating on a scale of 0-4
Growth rate increase is dependent on how many sides of the crop tree are released. Illustration created by Andrea Da Silva Gonzalez.

Crop trees are trees in your woodland that you want to keep in the woodland because they will help attain your goals and objectives; they would normally be selected for their characteristics and because they will live at least 20+ years. Crop trees may have different selection criteria depending on your goals and objectives. 

Once the crop tree criteria have been developed and stands and areas to crop tree release have been identified, test your system on a stand or two and evaluate the results before cutting or removing any trees. As stands change, so might the objectives and the criteria for crop trees. Be careful not to select too many crop trees; never consider more than 50–100 crop trees per acre; younger stands tend to have more crop trees than older stands. Some stands may have as few as 5–10 crop trees, and parts of a stand may not have any desirable crop trees. Stands with a large number of crop trees will have more material removed than stands with fewer crop trees. It is very important to remember that if crop trees are to have the maximum response to release, they need to receive a complete crown touching release; release on all four sides of the selected crop tree. This concept is visualized by dividing the crown of the crop tree into four sections and then determining how many of the sections have room to grow (i.e., access to full sunlight). Trees with the most free-togrow quadrants will grow the fastest following release.

Crop tree selection and release should only impact the crop trees and trees which are competing with the crop trees. Trees that are not in direct competition with the crop trees are not removed from the stand. This includes trees not touching the crop trees and trees in the understory which are not competing with the crown of the crop trees. By default in the crop tree release, some other trees may receive benefit from the removal of competition around the crop trees.

Examples of crop tree selection criteria

Primary objective: timber crop tree

  • Dominant/co-dominant
    • large healthy crown
    • no dead upper crown branches
    • should be single stem or if multi-stem have a very wide U-shaped connection
  • Appropriate form
    • straight, good butt log, minimal forking (see our logging video to learn more)
  • No sprouts
  • Valuable species
  • Be well adapted to the site

Primary objective: wildlife crop tree

  • Dominant/co-dominant
    • large healthy crown
  • Mast (seed/fruit) producer
  • Cavities and dead branches are a plus

Primary objective: aesthetics and or diversity crop tree

  • Species with good fall color, flowers,  bark, or other visual characteristics
  • Well developed crowns
  • Allow some dead branches, stump sprouts, and consider mid and understory trees
  • Trees having low representation in stand/property

Primary objective: water quality crop tree

  • Timber, wildlife, and aesthetic crop trees may all act to enhance water quality
  • Also consider species and individuals (young trees) with fast growth rates for nutrient accumulation
  • Also consider species tolerant to flooding and shallow water tables, and trees along streambanks

Application of crop tree release

graphic showing crop tree in middle with a cross that splits the area around the tree into 4 quadrants
The space around a crop tree is divided into four sides or quadrants for thinning. Illustration created by Andrea Da Silva Gonzalez.

Before beginning to remove trees, use flagging to mark both the crop trees and trees to be removed. This will help visualize the affects on the stand when the trees are removed. With a large number of crop trees, the cutting will be heavy; with a smaller number of crop trees, fewer trees will be removed to obtain the desired release of the crop trees. If the cutting appears to be too heavy, reduce the number of crop trees; do not reduce the number of free-to-grow quadrants around the crop tree because of the resulting reduction in growth response. 

Spacing in woodlands is seldom ideal for crop trees. In some cases, two trees adjacent to each other are ideal crop trees. In this case, treat the pair of crop trees like they were one, and provide room-to-grow space around their common space. For each tree this means it is released on three sides, rather than on all four sides. 

Free-to-grow means more than a foot or two. When removing trees around a crop tree, if there is doubt about a potential competitor, remove it. Healthy tree crowns in Iowa expand laterally at least a foot per year. Therefore, 15 feet of spacing from a crop tree to a competitor will provide adequate crown release for seven to eight years. At that time, the process starts over again, with the identification and release of crop trees. It should be noted that providing too much growing space during thinning may result in epicormic branch formation on crop tree stems. Epicormic branches originate from dormant buds beneath the bark. Crop trees that are dominant/codominant with large healthy crowns are less likely to form epicormic branches. If the tree already has epicormic branching, thinning will only stimulate more branching.

graphic showing top down view of two crop trees growing close to each other and surrounded by other non-crop trees
Two crop trees growing close to each other are treated as a single tree when thinning around them. Illustration created by Andrea Da Silva Gonzalez.

Contact your professional forester for more information on crop tree release or to request an on-site consultation. Your professional forester will be able to guide you through the crop tree release planning process, and also work with you to select an appropriate forestry contractor to perform crop tree release activities. Go to our forestry contacts directory to find your local professional forester and a list of private forestry consultants and contractors in your area.

Additional Resources

This article is a web-based modification of the original publication "PM2002A Woodland Improvement and Crop Trees in Iowa" prepared by Paul H Wray (former ISU Extension Forester), Paul Tauke (former Iowa DNR Forestry Supervisor), and Jean McGuire (former ISU Extension Communication Specialist).