Selection cutting is the silvicultural practice of harvesting trees in a way that moves a forest stand towards an uneven-aged or all-aged condition, or 'structure'. Using stocking models derived from the study of old growth forests, '"Selection cutting"', also known as 'selection system', or 'selection silviculture', manages the establishment, continued growth and final harvest of multiple age classes (usually three) of trees within a stand. This type of silviculture is generally considered to be more difficult to implement and maintain than even-aged silviculture, due to the difficulty of managing multiple age classes in a shared space, but there are significant ecological benefits associated with it. Uneven-aged forests generally exhibit higher levels of vertical structure (key for many species of birds and mammals), have higher levels of carbon sequestration, and produce a more constant flow or market and non-market forest resources than even-aged forests. This method of silviculture also protects forest soils from the adverse effects of many types of even-aged silviculture, including nutrient loss, erosion and soil compaction and the rapid loss of organic material from a forested system. Selection silviculture is especially adept at regenerating tolerant species of trees (those able to function under conditions of low solar energy), but can also be modified to suit the regeneration and growth of intolerant and mid-tolerant species.
Selection cutting is often confused with "selective" cutting, a term synonymous with the practice of highgrading (the removal of the best trees in a forest, often with a disregard for the future of the residual stand). Often the latter term is used by foresters or loggers to imply the former (which has a generally positive connotation in forestry circles) and mislead landowners into stripping their woodlot of its most valuable timber. Used correctly, the term 'selection cutting', 'selection system', or 'selection silviculture' implies the implementation of specific silvicultural techniques—usually either 'single tree selection', 'group selection' or a combination of the two—to create an uneven-aged or all-aged condition in a forest stand, one more akin to a late successional or 'climax' condition.
The most common type of selection system is single-tree selection, in which scattered individual trees of multiple age classes, whose canopies are not touching, are harvested. This type of selection system generally produces small canopy openings especially conducive to the establishment and growth of tolerant tree species.
Another variation of selection silviculture is group selection. Under this system, a number of 'groups', or small openings created by the removal of several adjacent trees, are created in complement to the harvest of scattered individual trees. If the groups created are large enough, and if seed-bed conditions are favorable, this can allow species which are intolerant of shade to regenerate. Group selection is designed to mimic larger, multi-tree mortality events, which in some environments may represent natural disturbance regimes.
The maximum size of a group (before it becomes a patch, or clearcut) is debatable. Some say it may be up to 2 acres (0.8 hectares) in size, whereas others limit it to a maximum of 0.5 acres (0.1 hectares).
Implementing A Selection System
In North America, trees are selected for harvest in an selection system with reference to the Arbogast Method (named after the method's creator). This is also known as the BDq method. Under this method, a harvest is specified by defining a residual basal area (B), a maximum diameter (D), and a q-ratio (q). The q-ratio is the ratio of the number of trees in a diameter class to the number of trees in the next larger class. Typically diameter classes are either 4 centimeters or 2 inches.
Given the BDq, a curve representing the state of the residual stand is computed. This curve is compared to the inventory data from a stand, specifically the curve of the diameter classes of the trees in the stand (diameter used as a surrogate for age) against the number of trees in each diameter (age) class . The comparison of these two curves tells the forester how many trees of each age-class should remain in the stand. The excess trees are marked for harvest. The goal of the use of a DBq curve is to ensure the continued development of trees in each age class, and the continued availability of mature timber to harvest on a relatively short cutting cycle.
- McEvoy, T.J. 2004. Positive Impact Forestry - A Sustainable Approach to Managing Woodlands. Island Press, New York, DC. 268p.
- R.D. Nyland (1998). "Selection System in Northern Hardwoods". Journal of Forestry 96 (7): 18–21.
- C.R. Webster and C.G. Lorimer (2002). "Single-tree versus group selection in hemlock-hardwood forests: are smaller openings less productive?". Canadian Journal of Forest Research 32 (4): 591–604. doi:10.1139/x02-003.
- Arbogast, C (1957). Marking Guides for Northern Hardwoods Under the Selection System. USDA Forest Service.