[Spacing Guidebook Table of Contents]
Considerations for post-treatment densities
The following are several factors that should be considered when prescribing post-spacing densities in a stand management prescription.
Forest health factors
Where levels of pest attack are low and guidelines recommend spacing (see "Forest health factors" in the section on "Spacing for stand management prescriptions"), extra stems should not normally be left to counteract the impact of a low incidence of a damaging agent. There are several specific exceptions to this general recommendation:
For all other forest health factors, the loss in merchantable yield and value due to higher stand densities is likely to outweigh the attack damage.
- comandra blister, stalactiform blister, and western gall rusts (see "Forest health factors" in the section on "Spacing for stand management prescriptions" and the Pine Stem Rust Guidebook)
- lodgepole terminal, spruce, and Warren’s collar weevils (see the Terminal Weevils Guidebook).
The effects of site quality is sometimes a consideration in prescribing post-spacing densities. At a given stand density, both height and diameter growth are generally lower on a poor versus a better site. Where wood flow problems are a consideration, a density lower than what would otherwise be prescribed on a better site, will lead to a shorter technical rotation. However, the lower residual densities on poorer sites generally have the following trade-offs:
- lower merchantable volumes at the technical rotation
- larger average diameters
- more taper
- larger branches/knots, more juvenile wood content.
Target stand conditions
Post-treatment density levels must be determined based upon the target stand conditions desired for the stand. The objectives are normally stated in the silviculture prescription or the stand management prescription. An objective for a stand may be attainment of a target diameter sawlog and minimum volume within a rotation of a specified number of years. This objective is set considering the site quality of the stand. It will take longer to attain a target average stand diameter from a low site quality stand than a high site quality stand at the same stand density. The post-treatment density must be set so that the objective can be met. Refer to managed stand yield tables or growth models to determine appropriate leave tree densities to achieve desired stand structure objectives.
Post-treatment densities will impact the type of wildlife habitat the area will provide. If the area is designated as significant for wildlife habitat refer to the sections "Setting density levels" and "Species management."
If cattle management is identified in a range resource plan as an important resource on the site, densities may be kept low for growth of forage. Spacing of areas where cattle graze will require measures to protect the range resource. Existing cattle trails should be kept clear of slash accumulations to ensure continued cattle access. Fences should be cleared of slash accumulations for two metres on each side to provide access. Stumps throughout the area should be cut as low as practical (i.e., 20 cm high) and as flat as practical (i.e., stump angle not exceeding 1:2 rise/run) to reduce the risk of injury to cattle and range riders. Accumulations of heavy slash should be avoided so that cattle, particularly calves, can use the area without getting trapped by slash accumulations. The impact of heavy slash can be reduced by directional falling, limbing, bucking, and cutting access trails.
If commercial thinning is planned, more stems may need to be left on site. If, for example, 500 Douglas-fir stems/ha would be left with no commercial thinning planned, then 800 to 1100 stems/ha after spacing may be appropriate where a subsequent commercial thinning is planned.
There should be a commitment to perform the planned commercial thinning. Stand objectives cannot be met without the thinning entry once additional stems have been left. Confirm if a planned commercial thinning is necessary to meet higher-level objectives before prescribing higher stand densities. See the Commercial Thinning Guidebook.
Pruning and fertilization
Stands identified for pruning or fertilization should be at, or spaced to, proper stocking. The residual stand densities for fertilization must be low enough to allow room for crown expansion after treatment. Stand densities for pruning must be low enough to allow maximum growth of the pruned trees in order to obtain the optimum financial benefit. If required, spacing should occur before or in conjunction with fertilization or pruning to optimize treatment impacts and minimize treatment costs.
If pruning is planned, lower densities are more appropriate. For example, the best economic returns for coastal Douglas-fir (good and medium sites) occur when 400 to 500 sph are pruned. See the Pruning Guidebook and the Forest Fertilization Guidebook.
Methods of spacing
Methods used for spacing may be:
The equipment selected is based on the pre-spacing density, size of trees, terrain, and the amount of slash and brush on the site.
- manual, using a wide range of tools including long-handle shears, chain saws, and brush saws
- chemical, (rarely used in B.C.) applications to individual trees through hack-and-squirt or lance injectors
- mechanized chopping, mulching, brushing, or slashing.
Manual treatments utilize a wide assortment of tools ranging from inexpensive shears to costly brush cutters. Although some tools only require minimum worker skills, experienced well-trained workers will ensure efficiency and worker safety during spacing operations.
Mechanized equipment has been developed to improve the economics of spacing in dense immature stands. The majority of equipment uses non-selective, strip thinning where standing material is destroyed in alternating cut and leave strips. The leave strips must then be spaced manually. The manual treatment may be delayed 1–3 years after strip thinning. This delay will allow damage caused by the machines or windthrow to become obvious. The cost of this manual spacing must be counted in the total cost when assessing this treatment. Wait until the live crown has lifted at least one whorl above the ground before doing any strip thinning. Strip thinning of stands with full live crowns will likely leave live branches after the treatment.
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. A B.C. trial using the Silvana Selective showed that, for the machine to be a cost-effective alternative to chainsaws, the stand should have more than 25,000 sph and relatively flat terrain.
Pop-up spacing is a technique designed for use on sites with root disease. This technique breaks up root contact and inhibits the ability of the fungus to spread.
Mechanized equipment may be restricted by site conditions. Efficient operation requires large treatment areas with gentle terrain, preferably minimal site debris and soil conditions suitable for use of heavy equipment. The amount of detrimental soil disturbance should be close to zero. Cumulative detrimental soil disturbance over a complete rotation must not exceed that listed in the Soil Conservation Guidebook.
Mechanized operations are often well suited for remote areas and areas where there is a shortage of field workers.
Crop tree selection
Crop tree selection is critical to maintaining stand health and vigor by the selection of the healthiest, best formed, and fastest growing trees in the stand. A stand management prescription will specify the species, densities, and conditions of trees to be left after spacing. Trees left after spacing should combine as many of the following characteristics as possible:
- a. preferred species
- b. dominant or co-dominant tree
- c. free of injury and disease
- d. full crown
- e. straight stem
- f. free from forks or multiple top
- g. small branches
- h. good terminal growth
- i. good color
- j. growing on a good microsite.
In addition, the following general guidelines regarding tree removal or retention should normally be adhered to:
- Single trees within openings should be left, although they might not otherwise qualify as crop trees.
- The spacing on the edge of openings should be reduced to one half the specified distance to compensate for trees missing from the opening. The number of additional trees left should be no more than the number of trees that the opening could accommodate at the target inter-tree distance.
- Forked or double-topped trees should preferably be cut. However, if selected to leave, they should be left wholly uncut.
- All trees not selected as leave trees should be removed if they are taller than a minimum height (e.g., 0.5 m, 1.0 m).
- Selection of superior crop trees should usually supersede strict adherence to the specified spacing interval.
- In multi-storied, mixed conifer stands, size will normally take priority over species preference in crop tree selection, particularly where the size difference is substantial (e.g., 3 m).
- Do not retain an overly dense stand in one spot to compensate for a shortage of trees in another.
Monitoring and reporting
A spaced area may be monitored for compliance with standards set out in the stand management prescription or silviculture prescription. The B.C. 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 per cent. The main considerations are those listed in guideline 26.
Ensure that the spacing reduces the number of stems per hectare to achieve the specified densities and minimum inter-tree distances. Make sure spacing projects are well managed so that spacers choose the best crop trees and leave the residual stand in good condition.
Spacing done to fulfil requirements of a silviculture prescription for major licence holders must be reported quarterly in Form B to the district manager. Form D reports, describing silviculture expenditure for the preceding year, are due May 15. Spacing done to fulfil silviculture prescription obligations for a woodlot licence holder is annually reported on or before April 30 to the district manager. Spacing done to fulfil a silviculture prescription prepared by the Crown must be reported annually on or before April 30 (beginning in 1996) to the regional manager. Information for this report will be from the Ministry of Forests corporate data base.
The quarterly submission dates are April 15, July 31, October 31, and January 15.
Reporting of stand management prescription accomplishments on free growing stands is required for major licence holders. The Silviculture Practices Regulation requires that a treatment report is submitted quarterly to the district manager. This report must:
- include a summary of the silviculture treatments, including spacing, accomplished on specified areas. Spacing information should include 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.
Spacing on free growing stands done by the Small Business Forest Enterprise Program should be reported following ministry procedures.