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Forests and Forestry Division
Woodland Enhancement For Wildlife
When you actively manage forest vegetation, you affect wildlife as well. Forest management is one of the most long-lasting ways to improve wildlife habitat because you can provide a good mixture of habitat types. Timber stand improvement (TSI) practices like thinning, optimizing the mixture of tree sizes, selecting the right kind of crop trees, crop tree release, understory treatments and retaining den trees are important wildlife management tools.
Because of its long life, high degree of effectiveness and low cost, TSI is a cost-effective way of improving wildlife habitat on your property. Factor in increased growth and future value of forest products and TSI becomes very beneficial for your woodlands.
TSI designed to increase the quality and growth rate of sawtimber and veneer trees is a long term investment that may not pay returns in the landowners life-time. However, TSI for wildlife habitat improvement often shows marked results within 1 to 2 years following treatment. Some of the benefits to expect:
Thinning Creates More Wildlife Food
Mast (acorns, walnuts, hickory and other nut-like tree seeds) is an important wildlife food source in oak-hickory woodlands. Mast production increases as a timber matures, then decreases as timber becomes over-mature. Consider, however, that properly stocked oak-hickory forests do not even begin to produce significant quantities of mast until they are 50 years of age or older. Consider also, that very few woodlands are properly stocked. Most pole-sized timbers, in which the trees are 4" to 10" in diameter, are overstocked and too crowded for optimum tree development or mast production.
Woodlands that attract and hold the greatest numbers of deer, turkeys and squirrels produce in excess of 100 pounds of acorns per acre per year. To do this, these areas must have a minimum of 22 to 25 dominant, good acorn producing oaks per acre that are at least 14" in diameter and have at least 1/3 of their total tree height in living crown.
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Few mixed oak-hickory, pole-sized woodlands have trees that produce measurable amounts of
mast. Typically, these stands are comprised of about 60 year old trees, 4" to 10" in diameter
with small crowns, that have stagnated because of overcrowding. Such stands are the least
desirable for attracting wildlife. However, excellent wildlife habitat can be created at very little
cost and effort simply by releasing crop trees. In fact, this system of thinning will increase mast
production, understory cover and browse for over 30 species of wildlife.
Crop Tree Release
Crop tree release is a method of thinning in which selected trees are released from the competition of other trees. It consists of selecting trees to remain in the stand, then giving them room to grow by killing adjacent trees. Their crowns can then expand to produce more wildlife mast at earlier ages.
Crop tree release works well in most Iowa woodlands where trees of different diameters, ages and species are present. Crop tree release can begin when trees are 3" to 4" in diameter, but can still be effective when trees average up to 12" in diameter.
Crop trees should be selected for their ability to produce mast and to eventually produce high quality forest products. Select species like oaks, walnut and hickory that occupy a dominant or co-dominant position in the canopy. When more than one type of oak or hickory is present, try to select some of each to promote diversity.
The best time to select crop trees is in the fall when you can see which oaks, walnuts and hickories are producing the heaviest mast crops. If none of the trees are producing mast, which can happen because crops are cyclical, look for trees with large, healthy crowns.
To counter the affects of over-crowding, the stand should be thinned to allow selected trees to thrive at the expense of their neighbors. Trees selected to be retained in the stand should:
Select as few as 50 or as many as 100 crop trees per acre. Spacing for 50 trees per acre is about 30 x 30 feet and for 100 trees per acre about 20 x 20 feet.
Mark crop trees with brightly colored flagging. Then step back to see how evenly distributed they are. If spots are lacking crop trees, make the necessary modifications.
Using a different color of flagging, identify trees next to crop trees that shade them and prevent crop tree crowns from expanding. Remove those trees that when they are cut will leave from 5 to 20 feet of open space on at least 2 sides of the crop tree crown. You may need to remove only 1 or 2 adjacent trees to properly release a crop tree.
Trees to be removed may be cut down or girdled and left standing.
In most cases, trees that do not interfere with the crown of a selected crop tree do not need to
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be removed. They are useful for promoting a multi-layered effect that is desirable for some birds. As crop trees are released, more light is able to reach into the understory. This promotes the rapid development of ground vegetation and adds more wildlife cover and browse to the woodland. Trees that are cut can be piled to create hiding places for rabbits and other wildlife.
Snags and Den Trees
Most woodlands contain snags and den trees. Snags are standing, broken trees, either dead or alive and den trees are those with nests and/or holes for cavity dwellers. Over 38 species of birds and 29 species of mammals use broken trees and tree cavities for nesting, food and shelter.
It is beneficial to leave den trees and snags scattered through a woodland, especially along the edges. The practice has little affect on woodland productivity and it greatly enhances the area for wildlife. Generally, 5 to 7 den trees per acre is adequate. Extending timber harvest ages to 100-150 years may increase numbers of snags and den trees.
Leave active den trees and snags in the woodland as long as possible and create den trees and snags by killing trees and leaving them stand.
Woodlands in which trees are small in size have few den trees. Habitat can be improved in such stands by using artificial nests and dens. Research shows that squirrel reproduction is 2.5 times more successful in dens than in nests. Plans for artificial nests and dens are available from your DNR District Forester or Wildlife Biologist for many wildlife species.
A recently introduced Stewardship Incentive Program (SIP) can assist with costs of managing a woodland for improved wildlife habitat. An eligible landowner can receive 75% cost-share. Contractors are available to do the work if the landowner finds that time is limited.
The program is administered by your DNR District Forester. Contact your forester for additional information about eligibility and practice specifications.