After a stand has been designated for harvesting, the most important silvicultural decision is how the harvesting should be carried out.  Depending on the regeneration and growth requirements of the species for which the forest is principally being managed, stocking, and other environmental factors (e.g., wildlife or hydrological concerns), silviculturalists assign harvest treatments that fall on a continuum of intensities.  Where stocking, species composition, slopes, soils or other conditions are extremely variable, more than one type of harvesting might be prescribed for the same cutting unit.  Matching the harvesting regimes with silvicultural requirements is the paramount challenge for forest managers.  If harvesting is too intensive, future production is threatened and environmental damage is likely.  Light, selective harvesting, on the other hand, may not result in conditions appropriate for regeneration of the harvested species and necessitate expensive, post-harvest silvicultural treatments.

Single-tree selection

Harvesting isolated trees and thereby creating single-tree canopy gaps maintains pre-harvesting forest structure more than any of the more intensive harvesting regimes described below.  Single-tree selection is appropriate where there is substantial advanced regeneration of the harvested species that responds well to canopy opening and locally reduced root competition.  Minimizing damage to the residual forest during harvesting is critical for single-tree selection management.  Tree marking is also necessary to avoid reducing post-felling stand quality by harvesting; taking all the best trees and leaving just culls and otherwise non-merchantable trees, known as forest “creaming” or “high-grading” is a form of forest exploitation that should not be confused with forest management.  For just this reason, simply setting minimum diameter limits for felling is not sufficient to avoid forest degradation.

Where single tree selection is used, a stand is entered at intervals shorter than the time it takes for a seedlings to grow to be harvestable trees.  In other words, cutting cycles are shorter than full rotations.  An advantage of this sort of polycyclic, uneven-aged management is that stand disruptions at any point in time are minimal.  A disadvantage is that roads, bridges, and other components of the harvesting infrastructure need to be maintained permanently or frequently.  Such roads need to be built to a higher standard, which implies increased cost.  Permanent roads also provide ready access to the forest by wildlife poachers, timber thieves, and other undesirables.

Group selection

Where successful regeneration of the commercially valuable species requires openings larger than those created by harvesting single trees, clusters of trees can be harvested.  Regeneration in gaps created by felling groups of 3-6 trees can be from pre-existing seedlings or saplings, or from seeds dispersed in after gap formation.  For the same volume of timber harvested, fewer gaps are created by group selection than by single tree selection.  Where post-felling silvicultural treatments to promote regeneration and growth are applied in gaps, having fewer and larger treated areas is logistically advantageous.  But the likelihood of weed infestations needs to be considered because the large gaps favor both light-demanding commercial tree species and weeds.

Shelterwoods

Where regeneration of moderately light-demanding species is desired and a large proportion of the canopy trees are merchantable, the entire canopy can be removed in two stages with harvesting systems called shelterwoods.  During the first harvest in a typical shelterwood operation, about half of the canopy trees are removed.  This fairly intensive harvest is intended to promote seedling establishment in the understory with the residual canopy providing shelter and a seed source for regeneration.  When the regeneration is well established but before it gets large enough to be easily damaged during harvesting, the remainder of the canopy is removed.

Some researchers refer to strip clearcuts as strip shelterwoods.  Cutting narrow strips through the forest is analogous to the shelterwood system described above insofar as the adjacent standing forest provides shelter and a seed source for regeneration in the strip.  When the regeneration in the strip is well established, the adjacent strips can be harvested.

The prerequisite for profitable shelterwood management of having a high proportion of merchantable trees in the canopy is not satisfied in many tropical forests.  Often creative marketing is needed to make harvesting the entire canopy financially reasonable.  For example, the species that do not produce marketable timber can be used for charcoal or be impregnated with preservatives to make poles for construction or fence posts.

To an extent shelterwood management mimics the effects disturbances such as hurricanes or ground fires that destroy most but not all of the canopy trees.  Forests with extremely fire and wind resistant tree species, such as Swietenia macrophylla (mahogany), may regenerate in this way.

Shelterwood management is the first of the harvesting systems presented in this chapter that does not attempt to retain pre-intervention stand structure.  The radical changes in stand structure due to management with shelterwoods or clearcuts (see below) are likely to be associated with radical changes in species composition and to have major impacts on wildlife populations.  Care is needed to avoid infestation by vines and other light-demanding weeds, or stand domination by undesired stump sprouts.

Where harvested stands are small relative to the entire forest and are widely separated from one another in time and space, the forest-wide effects of shelterwoods are mitigated.  To reduce the environmental risks associated with severe stand treatments, attention has to be directed to these larger spatial scales.  Unfortunately, the environmentally preferable arrangement of stands is not always obvious.  For wildlife management, for example, for some species it is best to locate stands of different age close to one another whereas some species benefit from wide separation of young stands.

In defense of these more intensive approaches to management, recall that many forests now being harvested for timber actually regenerated after severe natural or anthropogenic disturbance.  Written evidence of these disturbances is often not available, but stand structure and composition can provide reliable cues.  For example, large-scale stand regenerating disturbances are likely to have occurred where the canopy is dominated by species that are not well represented in the understory but regenerate readily in abandoned agricultural clearings.

Clearcutting

Complete stand removal is often used to manage light-demanding species with well dispersed seeds or dense populations of small seedlings.  Clearcutting is also used in many coppice management systems.  The silvicultural and other environmental effects of clearcutting vary a great deal depending on a wide variety of factors including: stand size, shape and spatial arrangement; source of regeneration (e.g., wind dispersed seeds or advanced regeneration of seedlings); presence of productive seed trees; amount of mechanical soil disturbance associated with harvesting; and, post-harvesting site preparation treatments (e.g., roller-chopping or controlled burns).  If natural regeneration fails after clearcutting, planting of seeds or seedlings is a costly but potentially feasible option.

Clearcutting, like any other severe forest disruption, has many associated risks.  Regeneration can fail in portions of clearcut stands that are too far from seed sources.  In some cases seed trees die due to rapid and severe exposure to high light intensities, high bark temperatures, and high wind speeds.  Where soil conditions are inappropriate for the germination, establishment, and growth of the commercial species or seed predation and herbivore damage to seedlings are severe, stand management by clearcutting is unlikely to be sustainable.  Finally, often after clearcutting weeds grow rapidly and suppress crop tree seedlings.  Site preparation treatments, like roller chopping, herbicide application, or controlled burns are often required to reduce the vigor of resprouting herbs, grasses, shrubs, vines, and weed trees.