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Forest Service logo Pre-commercial Thinning Operational Guidelines
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This paper provides operational guidelines for implementing pre-commercial thinning activities that have been designed after careful consideration of the biological, economic and forest-level benefits and need for spacing. It has been been derived largely from the Spacing Guidebook (1995), which has been replaced by the Guidelines for Developing Stand Density Management Regimes.

Density management required in silviculture prescriptions

Specifying maximum density levels

The Establishment to Free Growing Guidebook identifies reference stocking standards based on biogeoclimatic subzone (according to the Biogeoclimatic Ecosystem Classification [BEC] system), ecosystem and tree species. These stocking standards are suitable for achieving timber production objectives on many sites. Modification of the reference values may be required in the silviculture prescription (SP), however, depending on specific site conditions and forest management objectives and silviculture strategies.

Similarly, a maximum density value must be specified in the SP based on landscape and management unit objectives and silviculture strategies (see "A structured decision process" in the Guidelines for Developing Stand Density Management Regimes). Factors such as the desired type of forest habitat, preferred levels of natural biodiversity, timber production objectives and local forest health conditions must be considered. A maximum stand density level (expressed in number of conifer trees/ha) must be specified in each SP for all stands denuded:

  • after October 1, 1987 for lodgepole pine and dry belt Douglas-fir
  • after April 1, 1994 for all coniferous stands.

Stands exceeding the specified maximum density at the latest free growing survey date must be pre-commerciallyprecommercially thinned. The tactical density option selected for the stand must be specified in the SP, and becomes the post-treatment target should pre-commercial thinning precommercial thinning be required.

Evaluating stand density

Survey determination of stand density prior to free growing status must be based on a scheme that considers total tree number and competitive status. The earliest and latest dates for the free growing survey in relation to the maximum density requirement will normally be the same as identified in the Establishment to Free Growing Guidebook. Stands of some species on certain ecosystems may exhibit slow juvenile growth rates, requiring an extended free growing period before they can be realistically juvenile spaced. Under these circumstances, the earliest and latest free growing dates specified in the SP should be adjusted accordingly.

Maximum density juvenile spacing

Once the density of a stand is determined in a free growing survey, only those portions (or survey strata) of a stand not conforming to the maximum density specified in the SP must be pre-commercially. The final density or range of density of healthy, well-spaced trees following juvenile spacing must comply with that specified in the SP. Strata that fall below the maximum density are not required to be spaced.

Once pre-commercial thinning has been completed, a free growing survey may be undertaken immediately. The sampling methodology for maximum density obligations is outlined in the Silviculture Surveys Guidebook. A stand (or survey stratum) is not considered free growing until it meets the tactical density specifications of the SP, and the reporting requirements of the Ministry of Forests.

Operational considerations of maximum density spacing are similar to density management treatments specified in Stand Management Prescriptions (SMPs), as described later in this document.

Operational considerations

Density management costs

A variety of operational factors affects the costs of juvenile spacing projects including:

  • Project/block size - the proportion of fixed costs (e.g., administration, supervision, safety) to variable costs (e.g., labour, equipment costs) usually decreases relative to project size.
  • Access to and within the block - the cost of improving access should be weighed against the benefit of reducing the travel time to the work site.
  • Slope and terrain characteristics - stands occupying steep, rugged terrain are more costly to treat than those on flat, gentle terrain.
  • Post-juvenile spacing slash height - if a maximum slash height is set for fire hazard reasons, additional cutting may be necessary to reduce slash height.
  • Stand density and height prior to spacing - the number of stems to be cut and the ease or difficulty of cutting them and placing them on the ground will impact costs.
  • Worker movement - pre-treatment debris accumulation, slope, heavy, low branches and brush can affect the ease of work and travel within the block.
  • Worker availability - prices can be affected by the supply of contract workers. For example, prices may be higher in communities with a small work force, or for projects scheduled for late in the field season. In interior stands, fall viewing of the next year's projects generally leads to lower bids.
  • Fire season - fire prevention regulations must be strictly applied in the fire season. This will normally entail a fire watch and extra equipment on site.
  • Special contract requirements - for example, buffer zones along travel rights-of-way and wildlife migration corridors.

Fire protection

Concentrations of slash following pre-commercial thinning juvenile spacing operations can present a fire hazard within the treated stands as well as to surrounding areas. Natural processes of compaction and breakdown will diminish this hazard over time. In the meantime, it is prudent to predict the potential hazard and attempt to minimize the risk of accidental fire. Silviculture or stand management prescriptions should prescribe a suitable slash management procedure that addresses all potential fire hazards for the stand, including considerations of fuel accumulation and distribution, as well as adjacent adjacent stand values and hazards.

Estimating fire hazard

The FS 748 form Pre-Stand Tending Survey, and the FS 770 Pre-Stand Tending Site Description summary, are useful planning templates for determining and documenting potential fire hazards and risks for a stand prior to pre-commercial thinning juvenile spacing. They use the following criteria to assign an overall fire hazard category:

low hazard - Less than 10% per cent of the original number of trees/ha will be felled.

moderate hazard - 10 to 30% per cent of the original number of trees/ha will be felled.

high hazard - Greater than 30% per cent of the original number of trees/ha will be felled.

The Forest Fire Prevention and Suppression Regulation requires preparation of fuel management strategies where the predicted fire hazard is moderate or severe following juvenile spacing. Most, if not all, juvenile spacing projects will exceed the low hazard category.

The following site-specific factors should also be considered when determining the level of fire hazard associated with a juvenile spacing project:

  • Location - the local climate (seasonal moisture, insolation, lightening storm frequency), proximity to settlements, industries and other forestry operations, and the current and future ease of access and frequency of travel near the stand determine the natural and human associated fire hazards to the project stand
  • Slope - the rate fire of spread of a fire increases with the degree of slope
  • Aspect - determines the amount of sun exposure (insolation); greater hazard is associated on south- and west-facing slopes
  • Block size - determines the extent of contiguous accumulations of juvenile spacing slash; large project stands present a greater hazard for fire initiation and spread than do small stands
  • Stand structure - the volume and depth of slash, and therefore the fire hazard, increases with the stem size of the thinnings and proportion of the stand felled
  • Tree species - slash combustibility and resulting fire intensity vary by tree species in the following declining order of magnitude: western redcedar, pine species, Douglas-fir, spruce species, true fir species, hemlock species, deciduous species.

Fire prevention/control strategies

When evaluating fire prevention and control options, the benefits of hazard reduction must be weighed against the costs. Some methods are easily incorporated into the prescription at minimal cost, while others involve substantial additional effort and costs. Wherever possible, the following strategies should be incorporated into the density management prescription to reduce the fire to an acceptable level:

  1. Access restriction - deactivate or temporarily close roads adjacent to the stand during the peak period (five to 10 years) of fire hazard in the stand.
  2. Seasonal prevention - initiate forest closures, campfire bans or increased fire patrols during the peak seasons of fire hazard.
  3. Project size - restrict the size of proposed projects, depending on a consideration of fixed project costs vs degree of hazard.
  4. Fuel breaks - leave unthinned strips (buffers) along roads. The width of fuel breaks should vary from five to 20 m (measured from the edge of the road running surface), depending on the severity of the hazard and the frequency of road use. Roadside features such as cutbanks can increase the fire prevention characteristics of the fuel break.
  5. Roadside fuel reduction - where roadside buffer strips are not employed as fuel breaks, pull thinning slash to the road side where it can be piled and burned, or chipped. Alternatively, pull debris into the stand and disperse it. The width and measurement of the resulting fuel-free strip should be determined in the same manner as for fuel breaks (see point 4 above) varying from 0 to 20 m measured from the edge of the road running surface.
  6. Fuel modification - thinning slash may be lopped and scattered to reduce the depth of fuel. In certain limited cases, stands of fire-resistant species may be underburned during periods of low fire hazard.
  7. Thinning utilization - use of roadside thinnings for minor products such as firewood, posts and rails will reduce the fuel accumulations and fire hazard.

Refer to the Fire Management Guidebook for information regarding these and other strategies and tactics for fire prevention and control.

The total size of contiguous, thinned stands should not exceed 500 hectares until the area becomes fire resistant. Stands proposed for juvenile spacing should be separated by fire resistant stands or areas that exceed 400 m in width. The Fire Management Guidebook describes the characteristics of fire resistance areas.

If circumstances require, larger areas of contiguous thinning should receive exceptional fuel management, fire prevention and control strategies. These should be developed in consultation with Ministry of Forests protection staff.

Maintaining stand-level biodiversity

Recommendations for maintaining forest-living species diversity are discussed under "Other resource values" in the "Biological concepts of timber production" section of the Guidelines for Developing Stand Density Management Regimes. Strategies and specific tactics should be clearly identified in silviculture and stand management prescriptions, and specified in juvenile spacing and thinning contracts. Consider incorporating the following approaches in density management projects:

  • Non-competitors - the Silviculture Practices Regulation requires that a tree which is not a significant crop competitor should not be felled during juvenile spacing. Non-competing trees may be specified in terms of their proximity or size relative to crop trees.
  • Designated species - valuable or protected tree species may be designated as non-competing and retained in the stand regardless of its proximity or size. Such a tree is called a "ghost tree," and is specified in the prescription and juvenile spacing contract as reserved from cutting (e.g., western white pine or western yew).
  • Wildlife trees - a wildlife tree is a live or dead standing tree with special characteristics that provide valuable habitat for conservation or enhancement of wildlife. Silviculture and stand management prescriptions should incorporate wildlife tree management considerations, including the retention of suitable wildlife trees at the time of harvest and during silviculture activities, and the future recruitment of wildlife trees over the rotation period.
  • Variable stand density levels - creating a variety of stand density levels provides structural variability in the stand. Unthinned compartments and a range of high and low density compartments should be considered as a means of creating variability. Unthinned areas may be necessary for security cover and thermal cover for wildlife.
  • Visual screens - should be considered along main and secondary roads in stands located near settlements, or where game hunting pressure on local wildlife populations is high. A visual screen may be provided by a combination of topography and vegetation in a manner that blocks the direct view of the interior of the stand.
  • Native tree species conservation - where possible maintain the original range of native conifer and hardwood tree species in the stand.
  • Riparian areas - these habitat features require special management. Refer to the Riparian Management Area Guidebook for information.
  • Debris accumulations - large wildlife experience difficulty in moving through stands with heavy accumulations of thinning slash. Gaps through the debris may facilitate wildlife movement. Stands identified as winter range may require additional work to reduce the slash accumulation to an acceptable level. Slash should be removed from water bodies (streams, rivers and lakes) and wildlife trails.

Refer to the Biodiversity Guidebook and Guidelines for Maintaining Biodiversity During Juvenile Spacing for additional information.

Range management

Juvenile spacing in stands used for cattle grazing require special measures to protect the range habitat. Existing cattle trails should be kept clear of debris. Fence lines should be cleared of slash accumulations for 2 m on each side of the fence. The height of thinned tree stumps should be kept cut as low as practical (e.g., less than 20 cm high) and with as low a cut angle as possible (e.g., not exceeding 1:2 rise/run) to reduce the risk of injury to cattle and ranchers. Accumulations of slash should be avoided to prevent the entrapment of cattle and calves.

Pre-commercial thinning methods

Three methods of pre-commercial thinning juvenile spacing are recognized:

  1. Manual techniques employing a wide range of cutting tools including long-handle shears, chainsaws and brush saws
  2. Chemical techniques employing a variety of herbicide formulations applied to thinned trees by injections, cuts or sprays
  3. Mechanized techniques employing a variety of heavy machinery for severing and/or mulching thinned trees.

The equipment selected is based on the structural characteristics of the stand and site such as the density and size of trees, ruggedness of terrain and the amount of debris in the stand.

Mechanized equipment has been developed to improve the economics of spacing in dense immature stands. The majority of equipment utilizes a non-selective, strip thinning technique where standing trees are severed and mulched in alternating cut and leave strips. The leave strips are usually thinned using manual techniques one to three years later.

Several machines have also been developed to do selective spacing where the operator determines the suitable crop trees based on spacing distance and tree characteristics. These machines are usually more cost effective than manual techniques in stands with a density over 25 000 sph.

So-called "pop-up" spacing is a technique designed for use in stands infected with root disease. A variety of heavy machinery tears the thinned trees out of the ground, thereby removing fungal inoculum and decreasing root contact between adjacent trees.

Mechanized equipment may be restricted by site conditions. Efficient operation requires large treatment areas with gentle terrain, preferably minimal site debris and with soil conditions that are resilient to heavy equipment. When mechanized density management treatments are proposed, the SMP must describe the treatment season, method of treatment, and estimate the extent of detrimental soil disturbance. Refer to the Soil Conservation Guidebook for information pertaining to soil disturbance regulations.

Crop tree selection

Crop tree selection is a critical factor in pre-commercial thinningjuvenile spacing. Incorrect selection may degrade stand vigour and timber production potential. Recommendations for appropriate selection are discussed in the "Biological concepts of timber production" section of Guidelines for Developing Stand Density Management Regimes. The silviculture or stand management prescription must specify the species, density, and characteristics of trees to be removed and retained during the juvenile spacing project. Residual trees should exhibit the following characteristics:

  • be of a preferred species
  • expressing a dominant or co-dominant position in the original stand
  • free of injury and disease
  • exhibiting a full crown and a straight stem
  • free from forks or multiple top
  • exhibiting small diameter, fine branching
  • producing terminal shoot growth which reflects the site potential
  • exhibiting good foliage colour
  • growing on a productive microsite.

In addition, the following general guidelines regarding tree removal or retention should be adhered to:

  • Single trees within openings should be retained, although they might not otherwise qualify as crop trees.
  • The inter-tree spacing at the periphery of openings should be reduced to compensate for trees missing from the opening; do not, however, retain an overly dense stand in one spot to compensate for a shortage of trees in another.
  • Preferably, forked or double-topped trees should be cut. However, if selected to leave, they should be left wholly uncut.
  • Selection of superior crop trees should supersede strict adherence to the specified spacing interval.

Monitoring and reporting

A spaced area may be monitored for compliance with standards set out in the SMP or SP. The Ministry of Forests may perform monitoring of spacing projects using the methodology described in the Ministry of Forests Silviculture Contract for Stand Tending - Juvenile Spacing, Schedule A (FS 751A). The minimum acceptable performance quality is 85%. Monitoring will be conducted on Crown land, and private land within tree farm licences and woodlots, using standard ministry procedures.

The aim of project monitoring is to ensure that the spacing reduces the number of stems per hectare to achieve the tactical objectives defined in the SP or SMP. The following are considered pre-commercial thinning juvenile spacing project errors:

  • cut trees leaning against crop trees
  • live limbs on stumps
  • high stumps
  • excess trees
  • incompletely cut ("hinged") trees
  • designated trees left uncut (e.g., tree with stem infection)
  • stump cut angle greater than 30o (thirty degrees) from the horizontal
  • crop tree damage (e.g., power saw nicks, broken tops)
  • felled trees left in streams, lakes, roads or areas designated for slash removal.

Pre-commercial thinning completed to fulfil the requirements of a SP for major licence holders must be reported quarterly in Form B to the district manager. Form D reports describing silviculture expenditures for the preceding year are due May 15. Juvenile spacing completed to fulfil SP obligations for a woodlot licence holder is annually reported to the district manager on or before April 30.

Pre-commercial thinning completed to fulfil a SP prepared by the Crown must be reported annually to the regional manager on or before April 30 (beginning in 1996). Information for this report will originate from the corporate database of the Ministry of Forests. The quarterly submission dates are April 15, July 31, October 31 and January 15.

Major licence holders are required to report SMP accomplishments on free growing stands. The Silviculture Practices Regulation requires that a treatment report be submitted quarterly to the district manager. This report must:

  • include a summary of the silviculture treatments, including juvenile spacing, accomplished in specified stands. Pre-commercial thinning information should include stand structural characteristics (average dbh, height, total stems per hectare), and a location map
  • cover the period of two weeks before the last report date and ending two weeks before the current report's submission date
  • be signed and sealed by a professional forester.

Pre-commercial thinning of free growing stands conducted on behalf of the Small Business Forest Enterprise Program should be reported following ministry procedures.

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© 1998 B.C. Ministry of Forests,
Forest Practices Branch

Comments to: Tim Ebata <>

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