Temporary forest canopy openings are caused by various natural processes including wind events, ice storms, fire, and insect and disease outbreaks. Such disturbances are neither good nor bad, but merely inevitable. Silvicultural techniques seek to simulate these natural processes in order to release or stimulate the growth of the next cohort of trees to fill that canopy opening. By repeating this process over time in increments across a large forest, a mosaic of various age classes and ecological characteristics is developed, which should serve the habitat needs of the largest number of fauna and flora. Ecological integrity is enhanced by biodiversity.
The silvicultural techniques most closely associated with even-aged management in oak and white pine forests include shelterwood and clearcut systems. Clear cutting involves the cutting of all trees within the harvest area, perhaps leaving a certain number of standing trees for wildlife purposes. A clearcut exposes any seedlings or saplings present before the harvest and the litter layer to the maximum amount of available sunlight. Certain seeds present within the soil will germinate and produce new seedlings, and stumps of cut trees may resprout, sending forth new saplings. Under this system, rapidly growing shade-intolerant tree species are favored, such as oak, aspen, black birch, red maple, and tulip poplar.
The shelterwood system is slightly more complicated. The first harvest of a shelterwood system involves the partial removal of the trees within a site. The geometry of the trees to be harvested versus the trees to remain is carefully planned. The desired effect is to create large enough gaps in the forest canopy in order to promote existing seedlings and saplings, and to promote growth of new seedlings and saplings. However, enough canopy must be retained in order to partially shade the new growth at intervals during the day.
Once enough seedlings and saplings (also called regeneration) are present, the second harvest of a shelterwood system can be accomplished. Here, the remainder of the original forest canopy is removed, with certain trees left behind for wildlife purposes. The geometry of the original harvest is especially important, so that little or no regeneration is damaged during the second harvest. Shelterwood harvests favor mildly shade-intolerant species, but not strictly shade-intolerant species. Shelterwood harvests will regenerate oak, white pine, birch, and maple.
A third method, known as the seed-tree method, is located somewhere in the middle. In a seed-tree cut, trees are left to remain to provide seed to the regeneration area. However, the remaining trees are not adequate in number to significantly affect the microclimate for new seedlings.
Even-aged management has many positive attributes. First, certain species require sizeable areas of young forest for their habitat. Ruffed grouse, for example, needs at least five acres of young forest (aged 0 to 20 years), in order to reproduce. Many studies have also shown that early-successional forest is an important habitat need for some songbirds and woodpeckers. Secondly, larger regeneration harvest areas are more likely to succeed when deer browse is a problem. Deer consume a very large amount of leaves each year, particularly from seedlings of trees that foresters consider to be valuable or desirable. In areas of mild to moderate overpopulation of deer, small regeneration harvests may have the seedlings killed off by excessive deer browse, whereas the same number of deer may not be able to kill off all of the seedlings if a larger area is harvested.
For obvious reasons, these regeneration methods are reserved for larger forests and wildlife-savvy landowners.