Selection cutting is the silvicultural practice of harvesting trees in a way that implements an uneven-aged or all-aged condition in a forest stand. 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 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 many of the adverse affects 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. This type of silviculture is especially adept at regenerating tolerant species of trees, but can also be modified to suit the regeneration and growth of intolerant and mid-tolerant species.
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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 harvesting their woodlot. 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 stand, one more akin to a late successional or 'climax' condition. [1]
The most common type of selection system is single-tree selection[2], in which scattered individual trees of multiple age classes, whose canopies are not touching, are harvested. Typically, in North America, most trees are selected for harvest using the Arbogast Method (after the method's creator[3]). This is also known as the DBq method. Under this method, a harvest is specified by defining a maximum diameter (D), a residual basal area (B), 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 required DBq, a residual curve is computed. This curve is compared against the curve of the trees currently in a stand, which is computed by graphing the diameter of trees in a stand (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 in 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.
A popular variation of single-tree selection is group selection. Under this system, a number of 'group openings' are created in addition 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.[4] Under typical single-tree selection, it is rare for shade-intolerant species to do well.
Sometimes these group openings can be quite large, prompting critics to say that group selection is tantamount to clearcut.
Overstory removal or shelterwood cutting is a variation of selection cutting. In this method all the large trees are taken and the understory of saplings and smaller trees are left for regeneration. Overstory removal requires care be used to avoid residual stand damage.
At least in the Midwestern United States, it is typical for shelterwood cuts to be a two-step process. The first step is a thinning down to ~75% crown cover. This provides growing space for new and vigorous regeneration, without removing the benefits of a forest-floor micro-climate and abundant seed source. After the new regeneration layer is well established (25-30 years), the residual overstory from the first step is removed. Once this new tree layer reaches maturity, the process is repeated.
The retained trees can be damaged by and otherwise impede the logging operation, necessitating a higher density of logging roads and skid trails. Depending on the regional topography, it is sometimes economical to use cable logging or helicopter logging as an alternative to skidding logs.
The practice of high grading, which targets only the highest quality trees, is commonly confused with selection system silviculture. This gives higher profit at first but lowers the quality of the remaining forest by leaving undesirable tree species standing, as well as poorly formed trees from desirable species. This can create conditions where no substantial quality timber will naturally grow for many decades.
High grading most often occurs when foresters employed by the logging company mark the stand or in cases where no forester is employed. The logging company profits more from a higher-volume harvest, so it is in their economic best interest to take as much as they can. A small minority of logging companies operate this way; it is by no means the norm. However, the effects of this mistreatment are long-lasting, so only a few 'rogue' companies can do quite a bit of damage to the landscape.
One way to avoid high grading is to contract with an independent forester to mark the stand. Typically marking contracts pay the forester a flat fee for any marking job. This removes the economic incentive to over-harvest a stand. More than that, there's an economic incentive to practice responsible management so that the forester might receive another contract the next time a particular stand is harvested.