[Spacing Guidebook Table of Contents]

Spacing required by silviculture prescription

Maximum density

A maximum density number must be specified in each silviculture prescription for stands denuded:

Conifer stands with densities above the maximum density specified in the silviculture prescription, must be spaced. After spacing, the final density of healthy well-spaced coniferous trees must be within a maximum and minimum number of trees stated in the silviculture prescription. A stand is not free growing until its maximum density obligations are met.

Stand regeneration and spacing are the two management techniques that most significantly impact stand development. Regeneration and spacing manage stand density. Changes to stand density affects, among other things, tree diameter, stem volume, stand volume, and amount of shrubs and forbes. Density management is therefore the most significant means of shaping a desired future forest.

In timber supply analyses, managed stand yield curves are used to project stand densities and their resultant merchantable volume and piece sizes. If stand densities are not managed to these levels, forest level objectives may not be achieved within planned time periods.

The main objective of the maximum density requirement is therefore to ensure that stand densities are controlled, through early regeneration practices or spacing, so that minimum tree size, merchantable volume, and management objectives are achieved within an acceptable period of time.

The maximum density requirement allocates some of the costs and responsibilities for density management to licensees with silviculture prescriptions who are harvesting valuable stands today. In B.C., about 25 per cent of the regenerated areas come back to more than 5000 stems per hectare (sph), the usual number of countable stems to trigger maximum density spacing.

Stands that do not exceed prescribed maximum densities may be spaced, using other sources of funding, after the site has been declared free growing. Any spacing planned on free growing areas should be detailed in a stand management prescription.

Maximum density has nothing to do with repression densities or excessive densities that may reduce the potential site index or productivity of a stand. Repression is currently thought to occur when lodgepole pine stands densities exceed 10 000 sph.

Planning for maximum density

The Establishment to Free Growing Guidebook lists targets and minimum stocking standards based on ecosystems and species. These stocking standards are a starting point for density planning. The guidebook stocking standards provide numbers suitable for many stands. However, under optimal planning, these stocking standard numbers would be modified as necessary to meet specific stand objectives. A stand planned for multiple commercial thinning entries should have more stems left. A stand with forage creation as an objective may have significantly fewer stems. Forest health factors, recognized at the pre-harvest stage can also require modification of the guidebook stocking standards.

Maximum density spacing is required when a survey of countable stems (see section on countable stems) indicates greater than the maximum number of countable stems per hectare exist on an obligation area. The stand must then be spaced down to between a minimum and maximum post-spacing density of well-spaced trees. In most cases, the post-spacing densities should be identical to the establishment target and minimum stocking standards for a particular silviculture prescription. The common theme is that, by free growing, the stand should be within a pre-specified density range in order to meet required stand objectives. Whether this stand level objective is met by regeneration techniques or by spacing techniques should not greatly impact the density range required.

The following example taken from WinTIPSY (Table Interpolation Program for Stand Yields, Windows version 1.0) shows the minimal differences in two good site pine stands, one naturally regenerating to 5555 sph and spaced to 1200 sph and the other being planted to 1300 sph with no ingress.

The target stocking standard and the maximum post-spacing density can differ if there are clear reasons. These numbers still have to be written in the silviculture prescription. An example of this would be if porcupine feeding damage is a possibility in the area. A spacing operation may need to leave extra stems in the form of "sacrifice trees" to compensate for the feeding damage (see Pests of Young Stands Guidebook). The Operational Planning Regulation mandates the specification of these stocking and density standards within a silviculture prescription (see Figure 1). They include the following:

Each of these parameters is used to specify the free growing density and stocking necessary to produce or shape the desired future forest conditions. These parameters can be varied to produce different future forest conditions. For differing management regimes, it should be expected that the stocking standards and post-spacing densities will be modified.

Guideline 1

Setting density levels

The standards for maximum density for all conifers are generally uniform for the province. This is done to provide a consistent approach (equal treatment and obligations) to both industry and the Small Business Forest Enterprise Program, and across district and regional boundaries.

Different maximum density standards for different management units: Management unit-specific deviations for maximum density standards may be considered where there are specific forest habitat, forest health objectives, or target forest conditions approved in a TSA or TFL management plan. Different maximum density and minimum countable heights may be proposed on a management unit basis.

For example, a coastal TSA or TFL mangement plan has timber supply analysis units that assume that viable harvests occur when a minimum average stand diameter of 40 cm dbh and 400 m3/ha are achieved. The management plan yield curves assume that regenerating stands are grown at a density of no more than 1000 sph.

In the above example, density numbers may be proposed to require mandatory spacing when regenerating stand densities are too high to attain the desired stand conditions by the planned harvest date. Choose the maximum density number by analyzing the impact of increasing stand density on the time to achieve the desired stand conditions. The harvest age for a stand at maximum density should be about ten years more than a stand at target stocking.

Different maximum density may be proposed based on timber supply analysis units, management objective, species, site index, and management plan yield curve assumptions.

Proposals for change must be submitted to the chief forester for approval. No further approval is necessary for the specific changes to maximum density standards listed in this guidebook (see guidelines 4 and 9 for the CWH zone and guidelines 7 and 8 for wildlife habitat).

For areas that do not have approved management unit maximum density standards, the following standards apply:

Guideline 2

Trees must have a minimum height (see guideline 3) to be counted in a maximum density tally during a free growing survey.

Guideline 3

Guideline 4

Fulfilment of specific objectives may require stocking standards other than those listed in the Establishment to Free Growing Guidebook. Variations to the guidebook free growing stocking standards (target and minimum stocking standard) in a silviculture prescription will normally be reflected as changes to the maximum and minimum post-spacing densities.

The maximum post-spacing density may differ from the target stocking standard in a silviculture prescription if there are forest health, special wildlife habitat, integrated resource management, cattle grazing, or other objectives or concerns for an area. This is done at the silviculture prescription preparation stage. This variation is normally less than 600 well-spaced trees per hectare above the target stocking standard. The prescribed minimum number of well-spaced trees to be left after spacing should normally not be less than the minimum stocking standard.

Guideline 5

Guideline 6

For uneven-aged single tree selection systems that exceed the maximum density, spacing must be done on at least the layer 3 trees (between 1.3 m in height and 7.4 cm in diameter) to reduce the density to within the post-spacing density range.

For many of the larger mammals, there are approved guidelines for stand densities that should be established to maintain critical wildlife habitat. When field staff are setting up stocking standards for critical wildlife habitat, including maximum density and allowable post-spacing density ranges, the approved guides should be consulted to determine appropriate densities.

The following are guidelines for setting maximum density numbers for special wildlife habitat management objectives.

Guideline 7

Management for mule deer habitat in the Kamloops Forest Region is listed in both guidelines 7 and 8. For both the even- and uneven-aged management at least 40 per cent of the area should be spaced to a target equal to the minimum stocking standard listed in the Establishment to Free Growing Guidebook. This more open stand should allow development of the necessary crown morphology for snow interception. The remainder of the area, after accounting for non-spaced zones such as buffers and wildlife tree reserves, should have a target stocking standard as listed in the Establishment to Free Growing Guidebook. This variability in spacing densities can be managed either within each block or proportionately distributed among a number of small blocks. These blocks should be within the same vicinity, such as a drainage, or they may be a number of blocks managed within one spacing contract.

Guideline 8 shows stand densities for mule deer winter range in the Kamloops Forest Region for uneven-aged single tree selection systems. The maximum density is inversely related to the canopy closure of both the overstorey and pole layer. Stands with dense upper canopies will have a low maximum density number for layer 3. The goal is to keep overall canopy closure low enough to allow growth of forage species.

Salal-cedar hemlock sites

Maximum density may be modified for the salal phase of certain Coastal Western Hemlock (CWH) subzones, variants, and site series. Growth of the trees in these stands can be slowed excessively by salal. The current strategy for dealing with these sites is to ensure stands achieve crown closure as quickly as possible. This is accomplished by prescribing and leaving high stand densities. The salal dominance will be suppressed due to shading at crown closure.

Site preparation by broadcast burning, prompt planting, and fertilization at a very young age are techniques that may speed the attainment of crown closure. Suppression of the salal growth is not expected to occur until the stand has passed the optimum time for spacing.

The Forest Practices Code of British Columbia Act states that a maximum density number must be set. This requirement must be met while still allowing the necessary high stand density to suppress the salal on these sites. To accomplish this, maximum density can be set up to 10 000 countable trees per hectare for cedar/hemlock salal sites.

Guideline 9

Sites can be designated as having a salal dominated phase when:

To continue to achieve the objectives of early crown closure, the maximum and minimum post-spacing density should be set higher than normally prescribed. For these salal dominated ecosystems, a suggested maximum post-spacing density is 3000 sph. A suggested minimum post-spacing density is 2000 sph.

District and licensee staff should evaluate the current minimum and target stocking standards of 500 and 900 well-spaced trees/ha for these ecosystems. The evaluation is done to determine if the stocking standards will meet the objective of high establishment densities on these special ecosystems. Consider increasing the target stocking standards to 1500–2000 or more well-spaced trees per hectare, where high densities are desirable to achieve early crown closure. The minimum stocking standards should also be increased to 1000–1200 or more well-spaced trees per hectare.

These ecosystems often respond well to fertilization at time of planting or shortly after stand establishment. On appropriate salal dominated sites, fertilization may be included in a silviculture prescription and considered a basic silviculture obligation. See the Forest Fertilization Guidebook.

District and licensee staff should determine if modification of the earliest and latest free growing assessment dates is necessary for these ecosystems. When free growing surveys are carried out on these sites, the minimum countable height for a tree to be tallied should be 50 per cent of the median height of the preferred and acceptable well-spaced trees.

Earliest and latest free growing dates

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.

For some tree species and ecosystems, trees in densely regenerated stands may take longer to reach a preferred spacing height. Foresters may wish to specify earliest and latest free growing dates in a silviculture prescription that are different than those specified in the Establishment to Free Growing Guidebook. The earliest and latest free growing dates can be lengthened to cover a time period when coniferous trees would achieve a desirable spacing height and to allow for more suitable selection of crop trees during spacing.

After spacing has been completed, no delay is required before a free growing survey is done. The silviculture prescription obligations can be declared as being met once a free growing survey and report indicates that the prescribed post spacing densities and standards for the site have been achieved. The sampling can be a low intensity survey and/or a visual assessment. This can be accomplished with the post-spacing inspection as long as sufficient information is obtained to fulfil Major Licensee Silviculture Information System (MLSIS) reporting requirements for major licence obligations and Integrated Silviculture Information System (ISIS) reporting requirements for the Small Business Forest Enterprise Program. The sampling methodology for maximum density obligations is outlined in the Silviculture Surveys Guidebook.

Amendments for forest health concerns

Forest health concerns should normally be dealt with at the pre-harvest stage. However, during the life of the obligations a forest health concern may arise. Surveys may indicate incidence of root disease or other forest health agents on a site. Treatment modification, no treatment, or application of a forest health strategy, may be necessary if the incidence exceeds threshold levels (see the section "Forest health factors"). Where a survey indicates that spacing may increase the forest health agent incidence on an obligation area, the holder of a silviculture prescription may apply to the district manager for:

Where the holder of a silviculture prescription wishes to deviate from the approved prescription, they must prepare a silviculture prescription amendment and submit it for the district manager’s approval.

Implementing maximum density spacing

Spacing projects that are required to meet maximum density obligations do not significantly differ in implementation from an incremental silviculture spacing project (spacing project covered by a stand management prescription). Unspaced buffer strips or patches should be left, when necessary, to fulfil objectives such as hiding cover for wildlife (see the section "Maintaining stand level biodiversity"), or reduction of fire hazard (see the section "Fire protection"). The cumulative area of these buffers should normally not exceed 10 per cent of the standards unit area. The provision of these buffers is in keeping with the intent of the Forest Practices Code. Wildlife and biological diversity are listed as objectives in Part 2, Section 2(1)(a) of the Forest Practices Code of British Columbia Act.

The following sections of this guidebook are applicable to both maximum density and incremental silviculture spacing: "Fire protection," "Maintaining stand level biodiversity," "Considerations for post-treatment densities," "Methods of spacing," "Crop tree selection," and "Monitoring and reporting."

Return to top Back Forward