As the glaciers receded from our region over 12,000 years ago, Native Americans moved into the landscape and began shaping it with fire. They burned the woodlands, savannas, wetlands, and prairies regularly to promote the growth of food plants and to attract game species. These fires ensured that fire-tolerant oaks and hickories dominated the landscape. The open canopy of these habitats allowed sufficient sunlight to reach the ground so that a diversity of fire-tolerant wildflowers, grasses and shrubs could thrive. These plants in turn supported a diverse array of wildlife, including beneficial insects, songbirds, reptiles, amphibians, and mammals.
Changed woodlands. Fire suppression during the past 170 years, deer overbrowse and human-created disturbances have caused many changes in these woodlands. Without fire, the woodlands have been invaded by aggressive, shade-tolerant understory trees such as maple, ash, cherry, elm, basswood, and box elder. These trees closed the canopy, creating a darker, more forest-like environment. Little light remained for oak seedlings and diverse ground layer plants.
In addition, a growing deer population browsed the oak seedlings and native plants. These unbalanced ecosystems were ripe for invasion by non-native species such as buckthorn, honeysuckle and garlic mustard, which are not controlled by our native pests.
Breaking point. Our oak-hickory woodlands are now nearing a breaking point. Deer browse and dense shade combine to combine to create a situation in which oak seedlings have a difficult time sprouting and surviving; once the old trees die, the ok woodland will expire. In order to save our woodlands we must take an active role, including the removal of invasive species, thinning out aggressive non-native tree species, initiating a regular fire regime and reestablishing and promoting native woodland plants.
Thinning operations should be included in any woodland management plan and overseen by an arborist. The timing of thinning will depend on the species and rate of growth. Canopy coverage over an area should range from 30% to 80%, depending on the soil type, slope and aspect (compass direction) of the woodland being restored.
Visual assessment. Each property requires individual assessment. A visual check will show where tree crowns are overlapping and thinning is needed. The aim should be to create a “ring of sky” around each tree that is retained, and into which it will spread. The woodland edge should normally be left unthinned to create dense, branching growth that shelters the woodland from the wind. Trees Can be selected to give a variety of form and structure and to break up planting lines.
Selective thinning. Selective thinning involves individually selecting trees for thinning, normally removing those that are weak, diseased, forked, or dead and retaining the strongest, straightest and healthiest trees. Where growth is good throughout a property, the removal of viable trees may be necessary. Selection of trees should be made in winter when the crown and upper stem can easily be seen.
Village program. Riverwoods has a cost-share program for residents wishing to carry out a canopy or subcanopy tree thinning. To obtain a permit for the project an ecological consultation with the Village Ecologist is required. If approved by the Ecologist, a permit will be given by the Village and, once the Village is notified that the work is completed and the Ecologist verifies that the work has been properly completed, the resident will be reimbursed according to the provisions of the program.