Forestry

Forest harvesting refers to the methods used to cut and remove trees from the forest. Harvesting methods vary greatly in their intensity. Clear-cutting is the most intensive system, involving the harvest of all trees of economic value at the same time. The areas of clear-cuts can vary greatly, from patch-cuts smaller than a hectare in size, to enormous harvests thousands of hectares in area, sometimes undertaken to salvage timber from areas that have recently been affected by wildfire or an insect epidemic. Strip-cutting is a system involving a series of long and narrow clear-cuts, with alternating uncut strips of forest left between. A few years after the first strip-cuts were made, tree regeneration should be well established by seeding-in from the uncut strips, and the uncut strips would then be harvested. Shelter-wood cutting is a partial harvest of a stand, in which selected, large trees are left to favor particular species in the regeneration, and to stimulate growth of the uncut trees to produce high-quality sawlogs at the time of the next harvest, usually one or several decades later. In some respects, the shelterwood system can be viewed as a staged clear-cut, because all of the trees are harvested, but in several steps. The least intensive method of harvesting is the selection-tree system, in which some of the larger individual trees of desired species are harvested every ten or more years, always leaving the physical integrity of the forest essentially intact.

Usually when trees are harvested, they are de-limbed where they have fallen, the branches and foliage are left on the site, and the logs taken away for use. However, some harvest systems are more intensive in their removal of tree biomass from the site. A whole-tree harvest, usually used in conjunction with clear-cutting, involves removal of all of the above-ground biomass. A complete-tree harvest is rare, but would attempt the additional harvest of root biomass, as is possible on sites with peaty soils. These very intensive harvesting methods may be economically advantageous when trees are being harvested for the production of industrial energy, for which the quality of the biomass is not an important consideration. However, the whole-tree and complete-tree methods greatly increase the removal of nutrients from the site compared with stem-only harvests, and this can be a consideration in terms of impairment of fertility of the land.

Forest management refers to the activities associated with establishing new crops of trees on harvested sites, tending the stands as they develop, and protecting them from insects and diseases. As was the case with harvesting, the intensity of management activities can vary greatly. The least intensive management systems rely on natural regeneration of trees and natural stand development. Although relatively natural systems are softer in terms of their environmental impacts, the rate of forest productivity is often less than can be accomplished with more intensive management systems.

One natural system of regeneration utilizes the socalled advance regeneration, or the population of small individuals of tree species that occurs in many mature forests, and is available to contribute to development of the next stand after the overstory trees are harvested. Other systems of natural regeneration try to encourage the post-harvest establishment of seedlings of desired tree species after the site is harvested. For some species of trees, the site must be prepared to encourage seedling establishment. This may require burning of the slash and surface organic matter, or mechanical scarification using heavy machines. Depending on the particular nature of the forest and the tree species, either of the advance regeneration or seeding-in regeneration systems might be utilized along with selective harvesting, or with clear-cutting.

If the forester believes that natural regeneration will not be adequate in terms of density, or that it would involve the wrong species of trees, then a more intensive system might be used to establish the next stand of trees. Often, young seedlings of desired species of trees will be planted, to establish an even-aged, usually single-species plantation. The seedlings are previously grown under optimized conditions in a greenhouse, and they may represent a narrow genetic lineage selected for desirable traits, such as rapid productivity or good growth form.

Once an acceptable regeneration of trees is established on a harvested site, the stand may require tending. Often, non-desired plants are believed to excessively compete with the trees and thereby interfere with their growth. As such, these plants are considered to be silvi-cultural "weeds." This management problem may be dealt with by using a herbicide, or by mechanical weeding. Similarly, once the growing stand develops a closed canopy of foliage, the individual trees may start to excessively compete among themselves, reducing the overall growth rate. This problem may be dealt with by thinning the stand, an activity in which the least productive individuals or those with poorer growth form are selectively removed, to favor productivity of the residual trees.

In some cases, the regenerating stand may be threatened by a population outbreak of an insect capable of severely reducing productivity, or even killing trees. This pest-management problem may be managed by "protect ing" the stand using an insecticide. In North America, insecticides have most commonly been used to deal with severe defoliation caused by outbreaks of spruce bud-worm or gypsy moth, or with damage associated with bark beetles.