Competitive Exams: Forestry: Selection Cutting
Selection cutting is the silvicultural practice of harvesting a proportion of the trees in a stand. Selection cutting is the practice of removing mature timber or thinning to improve the timber stand. This system may be used to manage even or uneven-aged stands. Management objectives can include the protection of forest soils, maintenance or improvement of wildlife habitat, the increase of individual stem productivity, encouragement of regeneration/species diversity or the improvement of the visual amenity of plantations. Selection cutting may include opening up areas to allow tree species that require greater light intensity to grow but that are not large enough to meet the legal definition of a clearcut.
Selection cutting it is not recognized as a valid silvilcultural term since it does not define the objectives of cutting, such as, to improve the quality of trees in the residual stand (also known as an “intermediate treatment” ), or for the purposes of regenerating the stand (also known as a “regeneration treatment ‘). The term,” selection--or selective--cutting “means nothing more or less than the fact that someone has designated trees for harvest. Often the term is used to’ selL'the concept of harvesting to reluctant owners who usually then discover--after the fact--that only the best trees were ‘selected'leaving a largely defective stand in its wake. Used correctly, the term’ selection silviculture ‘implies the use of an uneven-aged method--either’ single tree or group selection'--to begin regenerating, or reproducing, a stand usually to more late-successional, or ‘climax’ species. Anyone who uses the term” selection cutting “or” selective harvesting" should be challenged to define his or her terms and, moreover, to identify the overall silvicultural purpose of harvesting trees.
The most common type of selection system is Single Tree Selection, in which scattered individual trees are marked and harvested. Typically, in north America, trees are selected for harvest using the Arbogast Method (after the method's creator). 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 tells the forester how many trees in each size-class should remain in the stand. An inventory of the stand is conducted, and excess trees are marked for harvest. The goal here is to regulate the diameter distribution into a form that is known to be sustainable. A distribution is sustainable if enough trees remain post-harvest that they can grow back all that was harvested before the next harvest. Sustainable distributions can provide a steady even-flow of timber over an infinitely long time-horizon.
Conducting a full stand inventory is rarely practical, and tracking the number of trees in small size-classes is tedious in the field. A typical single-tree selection harvest will involve an inventory from a number of sample plots, which is used to estimate which size-classes contain excess trees. Based on this estimate, and the smooth residual curve, a marking-guide is constructed based on larger, more tractable size-classes.
On the ground, the forester will use the techniques of Bitterlich Sampling to determine the basal area around a fixed point. If it is higher than the desired residual, trees are marked to bring it down. Whenever possible, they are marked from the surplus classes indicated on the marking guide. The result matches very closely the one given by a full inventory, but is much faster and more practical.
If production of high-quality sawlogs is a management goal, then crop tree management may be an appropriate technique. Under this method the highest grade trees are selected and then “released” by removing lower grade trees which would otherwise compete with the selected tree for sunlight and water. The selected tree is frequently pruned to grow logs with maximum value.
A similar approach, known as the ‘Frame Tree’ system, is employed in Western Europe. A number of high quality stems are identified at an early stage of the stand development and successive thinning interventions are aimed at releasing the growth potential of these trees. Commonly the final crop trees are harvested when they reach a specified size in order to maximize the financial return to the grower. Throughout the process natural regeneration is encouraged to infill the ground that has been opened up. This “continuous cover” approach can be seen as an alternative to clearfelling.
Another common but sometimes controversial method of selection is diameter limit, the removal of all trees above a certain diameter. Poorly planned diameter limit cutting is considered high-grading by some.
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. 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 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 midwest region of the US, 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 income 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 gets more profit 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.