Iowa's 2.1 million acres of forests are among the most productive and valuable hardwoods in the United State. Iowa soils, especially those in Northeast Iowa are capable of producing some of the best oak, walnut and ash in the world. Much of Iowa's forest is not managed for optimum production of fine hardwoods.
Eliminate Grazing by Domestic Livestock
The first step in good management and stewardship of the forest is to eliminate grazing by domestic livestock. Grazing has undesirable effects on the forest soil, the forest vegetation and many times even the livestock causing the damage to the woodland. Forest soils compared to prairie soils are usually shallower more subject to erosion, and less stable.
Soil compaction caused by grazing results in less water and air infiltration into the soil and more water runoff and erosion; on the average, soil loss rate on grazed woodlands is ten times greater than from ungrazed woodlands. Grazed woodlands, usually, have little desirable regeneration because of continued browsing. Livestock can be harmed by grazing woodlands either by consuming poisonous plants or because of low forage production in the woodland.
Identify "Crop Trees"
The second opportunity to improve the woodland occurs when the trees are young. With periodic thinning, the growth rate of desirable trees can be increased; these faster growing trees can be marketed at a much younger age. In addition, through crop tree identification and thinning, better quality and more valuable species of trees can be produced.
First, identify the "crop trees". Start at any location in the forest stand and on the average of every 20 feet, identify and mark with a non damaging marker such as plastic or cloth flagging the best trees. The best trees are generally of high value species and trees with the best form and potential to develop into high quality sawlogs when they are mature. A species priority list from high to low might include black walnut, red oak, white oak, black oak, bur oak, ash, maple, basswood and hickory. Crop tree spacing will vary from less than 10 feet to more than 30 feet, but always try to select the best 100 trees per acre.
Goal of Periodic Thinning
After the crop trees have been identified, mark the trees which should be removed to give the crop tree more room to grow. Work with each individual crop tree and looking at its crown or foliage with respect to its competitors. The goal of thinning is removal of competing trees on 3 to 4 sides of the crop tree if they are crowding or overtopping the crop tree. Do not overthin; overthinning may result in reduced quality of trees by stimulating epicormic branching or reduced natural pruning.
Ensure Desirable Reproduction for the Future
The third opportunity for improving your woodland occurs when the trees are ready to harvest. The main goal in the harvest should be practices which will ensure desirable reproduction for the future. In many cases, simply removing the harvest trees will result in inadequate regeneration of oak, walnut and ash. Some pre-harvest cultural activities may be necessary to regenerate Iowa's woodlands.
For all forestry practices, on site assistance is available from your Iowa Department of Natural Resources district forester.
For more information check out the Iowa State University Extension Publication Woodland Improvement and Crop Trees in Iowa or visit the Native Iowa Woodland Understory Restoration website.