Intensive Silvicultural Practices

After a new crop of trees has been established and reaches a free growing condition, the future timber and other resource values of the forest stand can be further enhanced by intensive silvicultural practices.

Pre-commercial Thinning (PCT)

Any given stand has a limited capability to produce an annual volume of timber, when the site is fully occupied by trees; pre-commercial thinning accentuates the volume production on fewer trees and trees of desired species. It is usually done manually using brush and chain saws or mechanically with tractor-mounted flails or saws. PCT homogenizes the stand, increases mean tree size, and lowers the age at which the stand can be harvested. The practice is difficult to mechanize and is very expensive; more than one person-day per hectare is commonly required. In spite of its high cost, this practice is common. Accelerated harvest of younger stands often allows for accelerated harvest of older overmature stands in a forest management unit, thus yielding more than enough revenue to pay for a PCT program. PCT also reduces logging costs and increases product values. Wildlife habitat and landscape values are often improved. Without PCT treatment, stands on lower-fertility sites may never be operable. Stands may be left at final crop density or may be designed to allow for one or more commercial thinnings.

The practice is called pre-commercial since it is done shortly after crown closure, the time at which the available crown space has become fully occupied. The trees are relatively small and there is no market for the cut trees.

Commercial Thinning (CT)

Up to 30% of the total volume production in a stand can be lost due to competition-induced mortality (smaller trees in dense stands die because there is not enough growing space for all of them). Commercial thinning attempts to recover this mortality loss and to provide early income from a stand by harvesting trees big enough to have product value. In practice, it is almost impossible to recover all potential loss through mortality because repeated light thinnings are necessary if residual stand volumes sufficient to maintain stand growth rates are left after each thinning.

Because of present Canadian market conditions and the high logging cost of thinnings, most of the repeated light thinnings desired are unmarketable and too expensive; even a single thinning may not be economical. Commercial thinnings are more likely to be economical if the stand has had density control at establishment or by subsequent precommercial thinnings. Currently, very little commercial thinning is done in Canada, primarily due to unsuitable stand densities and species, high logging costs, and low stumpage values.

Pruning and Shearing

After stands have been reduced in density to the point where valuable final crop trees can be identified at an early age, pruning is possible. Pruning increases the value of individual trees by prematurely removing the lower branches so that clear wood, free of knots, is laid down around an unpruned knotty core. Several valuable tree species have branches that do not rot off readily and remain persistently on the tree even after death due to shading.

Dead and live branches are cut off flush with the trunk using manual handsaws; mechanization is very difficult. Repeated prunings are needed to maintain a cylindrical knotty core and yet not remove too much live crown.

Pruning is very expensive; for it to be economical the increase in stumpage value of clear timber over knotty timber must be great and the stand must have been thinned. Since these conditions are rare in Canada, pruning is little practiced. As more and more PCT-treated stands accumulate and as supplies of clear old-growth timber diminish, pruning will become more economically attractive. The practice requires a 20- or 30-year lead time before clear wood in sufficient quantities is produced by a tree. The most common species pruned are white pine and Douglas fir.

Shearing is the practice used in Christmas tree culture: a shaping of the form of the tree to make it more salable. It is usually done with sharp knife blades. Pine trees are treated while in candle, that is, while the leaders are growing. Other conifers are treated after bud formation.

Timber Stand Improvement (TSI)

Timber stand improvementÛcutting down or poisoning all deformed and unwanted trees within older standsÛis usually done in previously untended hardwood stands containing valuable trees mixed with less valuable ones. It is usually a noncommercial practice, in that no revenue is generated, although some fuelwood may be produced. The objective is to concentrate the growing capacity of the site on the most valuable trees, without excessive growing-stock reduction and loss of growth. Eastern Canada has very extensive areas of tolerant hardwood forests, often previously subjected to cuttings that removed the most valuable trees and left the least valuable. Such stands are of low quality and should be treated by TSI practices. Currently, TSI is little practiced because of the high cost and lack of immediate return.

Fertilizing and Drainage

The growth rates of individual trees and stands can be increased by providing additional supplies of limiting nutrient elements. In contrast to agricultural crops, forest ecosystems recycle nutrients; however, many forest stands are mainly limited by their nutrient supply, rather than by their climate, temperature, or moisture regime.

Granular fertilizers are spread by helicopter on designated forest stands, an expensive practice. Because of the high cost of the extra fertilizer-grown wood, the fertilized trees must be final crop (the stand thinned to final crop density) and the stumpage values high. In addition, there may need to be a shortage of stand volumes in certain forest age classes in a management unit. It is essential that the forest stand be known to be responsive to such treatment.

Most stands are fertilized after crown closure, usually at final crop density. Some fertilizing at the time of planting or immediately after planting is also done. To be successful, vegetation management is required, together with accurate diagnostic tests of regeneration nutrient status.

Drainage of forest sites, especially organic soils, has a great potential to improve stand growth. Although it is widely practiced in the boreal forests of Finland, where timber and growing sites are in short supply, it is used only experimentally in Canada, usually in black spruce muskeg conditions. The practice requires large forest ploughs or specially designed backhoes. The distance and depth of drainage ditches must be carefully matched to the organic matter classification and hydrological characteristics .

Special Silvicultural Practices

Most of the previous intensive silvicultural practices are used to enhance timber production. In addition, there are cultural practices designed to attain other objectives.

Seed Orchard Management

Designed to maximize cone production, the practices involved in seed orchard management are drainage to control soil moisture; standpipe mist irrigation to delay bud formation until local pollen sources have gone; repeated nitrogen fertilizing and basal scarring to stimulate female flower bud formation; radical top shearing for convenience of cone picking; and cross-pollination breeding and progeny testing followed by roguing.

Nursery Management

These techniques are not considered silvicultural practices. They are numerous and complex and many are considered to be more horticulture than silviculture.

Recreation and Landscape Silviculture

TSI, PCT, and CT and vegetation management practices are applied to meet the needs of tourist campers, hikers, and motorists in parks, recreation areas, and commercial timber forests. Landscape design criteria often require a major modification of the cut-block layout and influence the choice of silvicultural system and even of species for planting. Pruning of trees along roadsides in forests provides views into stands. Forests stands are deliberately manipulated by silvicultural practices to improve wildlife habitat. Stand-level practices include PCT, CT, and fertilizing to encourage understory forage production; use of selection silvicultural systems along streams and rivers to protect riparian habitat and keep water temperatures cool; and the retention of snags, dead trees, and wolf trees for nesting. Cutovers, forest meadows, and stands are set on fire to create forage and set back succession. Grass seeding is a common practice on cutovers on open-range situations in western Canada where cattle graze.

Silvicultural Surveys and Stand-History Record Keeping

One of the essential features of forestry practice is the planning of silvicultural actions in a way that meets the owners' stand management objectives. Planning requires excellent record keeping and monitoring of stand performance by surveys. Silvicultural survey practices include those of regeneration and backlog assessment surveys, plantation surveys, and free-to-grow surveys. All data are entered into stand-history record keeping systems, which are in turn components of forest inventory and planning systems.

Silvicultural surveys of every cutover area are required during a 10 to 40-year period until the new stand has reached crown closure. This means an annual workload of 1-3 million hectares of silvicultural surveys in Canada. These surveys involve fieldwork or the use of remote-sensing techniques. Fieldwork varies from field walk-throughs to the sampling of numerous small, temporary and permanent plots established in grid patterns on cutover areas. The plots are measured for species frequency; tree density and height; occurrence of insect, disease, and other damage; and competing brush invasion.

Since most commercial forest land in Canada is under lease for timber harvesting with a legal obligation for successful regeneration or free-to-grow status, the silvicultural surveys represent inspections of performance. Such inspections are done by provincially licensed or approved silvicultural surveyors in some provinces.

Stand-history record-keeping practices are usually computer-based storage and retrieval systems on a stand-by-stand basis. These systems are often linked to a geographic information system (GIS) and a broader total forest inventory and planning system. Such systems are maintained and developed by both corporate licensees and provincial governments. Canada has no national system of stand-history record keeping.

Preparation of Silvicultural Prescriptions

The final major field of silvicultural practices, apart from the business and legal components of silvicultural contracting is that of preparing silvicultural prescriptions.

This silvicultural practice involves assessing the stand for growing stock, stand structure, species composition, fertility and moisture regime, amount of regeneration, seedbed and seed supply situation, brush invasion, and landscape, recreation, and wildlife habitat values; planning stand establishment and crop tending; writing a detailed prescription; implementing and monitoring the planned actions at the stand level; and determining the impact on forest-level objectives. Such formal analyses and prescriptions are mandatory in some provinces for every cutover area and must be signed by a professional forester (thousands are required every year). They have become necessary because of controversies over the use of forests for multiple purposes, such as timber, recreation, wildlife, landscape scenery, and nature conservation, and because the biological variation and complexity of forests do not allow for standardized successful treatments.

The stand inventory is related to various feasible stand management alternatives that meet landowners' objectives and are economical. The best prescription is chosen and a detailed year-by-year plan of implementation is designed with recommendations on costs and details of procedure. The whole procedure is similar to the work done by engineers and architects in design work. The preparation and implementation of a successful silvicultural prescription is the ultimate test of the professionalism and ability of a Canadian silviculturist.

Adapted from publication prepared by Dr. G. F. Weetman, Professor of Silviculture, University of British Columbia.