[Biodiversity Guidebook Table of Contents]
Stand management to maintain biodiversity
This chapter describes how specific objectives for maintaining stand structure, tree and vegetation species composition, and coarse woody debris can be determined.
A fundamental premise for maintaining biological diversity is to implement strategies at both the landscape and stand scales. There is a linkage between how much retention of stand structure is required at the stand scale and how much should be retained at the landscape scale. When landscape units have been designated and landscape level biodiversity objectives have been established then the requirement for maintaining biodiversity in individual stands can be reduced and the appropriate retention levels determined from Table 20(a). As a result, the development of landscape unit objectives will allow for greater flexibility at the stand level. When no landscape unit biodiversity objectives have been established, then appropriate retention levels should be determined from Table 20(b).
The recommendations in this chapter should be applied in the preparation of:
- forest development plans
- silviculture prescriptions and stand management prescriptions
- logging plans, fire management plans, and range use plans.
Objectives for wildlife trees and coarse woody debris must be included in the contents of a Forest Development Plan (OPR 15 (7) (b)). A Forest Development Plan (OPR 15 (2)) and a silviculture prescription (OPR 39 (2) (w)) must contain a reasonable assessment of non-timber resource values known to be on or adjacent to the plan area and must describe the actions to be taken to accommodate those values.
Stand level practices in this chapter are the recommended minimum requirements needed to meet the structural characteristics of natural openings (OPR 21(3)(b)).
When wildlife tree patches (group reserves) are larger than 2 ha (a patch that is isolated within the cutblock boundary and not included within the net area to be restocked) and also meet the age and structural requirements of old seral forest (see “Establishing landscape unit biodiversity objectives”) then these larger, within-block patches can contribute to old-seral stage forest requirements within the landscape unit and be used in landscape level retention calculations (OPR 39 (3)(d)). A wildlife tree patch is synonymous with a group reserve in silvicultural terminology.
Given the high degree of ecological variability in our forests, managers need to consider biological diversity on a site-specific basis in order to most effectively apply the recommendations presented in this chapter. This variation may require exceeding the minimum retention requirements recommended in this guidebook.
The recommendations for maintaining biodiversity are an important component of ecosystem health and will have to be integrated with other objectives for forest health.
Maintaining stand structure
Stand level recommendations are designed to maintain or restore, in managed stands, important structural attributes such as wildlife trees (including standing dead and dying trees), coarse woody debris, tree species diversity, and understorey vegetation diversity. Appendix 5 discusses in more detail the importance of these structural attributes to the maintenance of biodiversity. The Stand Level Biodiversity Course for Forest Workers and Stand Level Biodiversity Course for Forest Managers offer a more in-depth approach to stand level biodiversity management. The Wildlife/Danger Tree Assessor’s Course Workbook contains single stem safety assessment procedures.
Safe work practices
Safe work practices, as established in conjunction with Workers’ Compensation Board, must be followed at all times when implementing the recommendations in this guidebook. Forest workers should have freedom to remove obstacles whenever necessary to maintain a safe working environment. Trees that are marked to leave or are outside the cutblock boundaries can be felled for safety reasons, but should be left on the ground as future coarse woody debris. This will retain the benefit these trees can have on site and remove any incentives for felling and harvesting.
A wildlife tree is any standing live or dead tree with special characteristics that provide valuable habitat for conservation or enhancement of wildlife. These trees have characteristics such as large size (diameter and height) for site, condition, age, and decay stage; evidence of use; valuable species types; and relative scarcity. They serve as critical habitat (for denning, shelter, roosting, and foraging) for a wide variety of organisms such as vertebrates, insects, mosses, and lichens.
Maintaining wildlife trees within harvest and silviculture units can be ecologically beneficial in a number of ways. While standing, they provide habitat for many species (birds, bats, and other small mammals) that perform roles in maintaining ecosystem functions. Standing green trees can provide for future wildlife tree recruits. Wildlife trees will, over time, become sources of coarse woody debris and finally, through decay and nutrient cycling, become incorporated into second-growth forests.
Wildlife tree management strategies can range from the retention of existing wildlife trees, as scattered individuals or in patches, to the creation of new wildlife trees. Many approaches can be applied within a single cutblock, though retention of patches is recommended as the priority approach in most cases. Wildlife tree requirements apply to the use of all silvicultural systems.
Wildlife trees patches (group reserves) and individual live tree retention
Wildlife tree patches (WTP) provide several advantages over other retention strategies. Snags or other potentially dangerous trees are more easily retained in patches than as individual trees and operational inconvenience is minimized. There is also evidence that clumps of trees provide better habitat for birds than do scattered individual trees, as well as an area of relatively undisturbed forest floor within cutblocks. However where scattered individual wildlife trees already exist they should be retained.
Wildlife tree patches should be well distributed across the landscape. The maximum distance between WTPs (500 m) has been based on territory size and dispersal requirements of wildlife.
Area and distribution of patches or individual trees:
- The retention of wildlife trees should be based on the pre-activity assessment of the wildlife tree values and requirements on or adjacent to the proposed cutblock; and on the described actions that are required to accommodate these values.
- The amount of retention to be applied should be determined from Table 20(a)
. It is based on the proportion of each biogeoclimatic subzone in the landscape unit that is operable, and the degree of development that has occurred before the recommendations presented here are applied. The higher the proportion of operable area in a landscape or the more that previous development has reduced wildlife tree abundance, the greater the amount of retention required.
- Where landscape unit objectives have not been established, the application of
Table 20(b)should be based on an interim landscape unit or a portion of a forest development plan that forms a contiguous geographic unit. In these cases, given the absence of landscape objectives for biodiversity, the percentages for wildlife tree patch retention in Table 20(b) are 3% higher than in Table 20(a).
- Suitable areas outside the cutblock, such as riparian management areas, can contribute to the required retention provided the inter-patch spacing requirements are met, and they are mapped and designated as wildlife tree patches in the forest development plan. It is assumed that up to 75% of wildlife tree patch area requirements on the coast, and up to 50% in the interior, will be met in riparian management areas and other constrained areas.
- Good candidates for retention: individual live wildlife trees that provide special wildlife habitat value (such as nest trees); veterans; and other large trees. Patches of deciduous or unmerchantable trees provide opportunities to accomplish this objective. Defective trees of the largest size are often most valuable. These large, live, unhealthy trees (Class 2 trees that are well branched) are important as future habitat since they provide recruitment wildlife trees over time (see Appendix 6 “Classes of wildlife trees”).
- Wildlife tree patches can range in size from individual trees to patches of several hectares. Individual trees, outside patches, can contribute to the required retention area on a basal area equivalency basis. However, simple stem basal area does not equate to patch area. For example, a 1 ha patch containing trees, shrubs, rocky or wet sites, and other ground level diversity is not ecologically equivalent to “x” number of single green stems (such as seed trees) distributed around a harvest block. Therefore, the use of individual trees to contribute to required retention area targets must be planned and selected carefully on a site-specific basis.
- Single green trees chosen for retention should exhibit some characteristics of a valuable recruitment wildlife tree (such as being large and well branched). In order to optimize their habitat value within the harvest block, these trees should be located, wherever possible, near areas that already contain some structural elements of stand level diversity (for example, near a riparian area, rocky outcrop, gully, hardwood patch, or meadow opening). Single live trees left near a shallow gully running across a cutblock are more valuable ecologically than those same green trees distributed randomly throughout the block.
- Wildlife tree patches should be mapped and recorded as part of the documentation of the silvicultural or stand management prescription for the cutblock and be removed from the net area to be restocked.
- No timber harvesting is permitted in designated wildlife tree patches.
- The importance of wildlife tree patches within cutblocks increases with the size of the cutblock. Patches should generally be centered around the most suitable trees and distributed throughout the cutblock, with distances between patches (or other suitable leave areas outside the block) not to exceed 500 m.
- Patches should be located in a way that minimizes windthrow risk. Advantage can also be made of locating patches along small streams, wet areas, gullies or other locations that are likely to pose harvesting, yarding or regeneration problems (Figure 10).
- When partial cutting silvicultural systems are used, sufficient leave trees should be retained throughout the rotation, to meet retention objectives.
- Wildlife trees must be retained at least until other suitable trees can offer equivalent replacement habitat and structural diversity. In most cases this will take at least one rotation.
Patch and live tree retention characteristics:
- A range of tree diameters should be included within patches, favouring larger stems when possible. Care should be taken to include the upper 10% of the diameter distribution of the stand (taken from cruise data), because these are the most valuable wildlife trees.
- Both live and dead trees (subject to safety requirements) should be included in patches representing a range of wildlife tree classes (Appendix 6).
- A variety of tree species, including deciduous, should be represented.
- When possible, trees showing wildlife use or presence of heart rot, and those with a large size and well-branched structure should be retained.
Management principles for wildlife trees
- Wildlife tree management includes both the retention of suitable wildlife trees at the time of harvest and during silvicultural activities, and the provision for recruitment of suitable replacement wildlife trees over the rotation period.
- Generally, the most operationally feasible and biologically advantageous method for retaining wildlife trees is to leave patches of live and dead trees as no work zones and to exclude these from the net area to be reforested.
- The amount of wildlife tree area required for a specific cutblock depends on the level and distribution of existing and planned harvesting in the surrounding landscape (see Table 20(a) and (b)).
Table 20(a). Percentage of a cutblock area required as wildlife tree patches when landscape units have been designated and landscape level biodiversity objectives have been established
Table 20(b). Percentage of a cutblock area required as wildlife tree patches when landscape units have not been designated
Application of table 20(a) and (b)
This is a one-time calculation for each biogeoclimatic subzone within the landscape unit (or interim landscape unit or portion of a forest development plan forming a contiguous geographic unit) unless the landscape unit objectives change, a new landscape unit is designated, or operability limits change (changing the area available for harvest). A separate prescription is made for each subzone within the landscape.
X-axis numbers (columns) are the proportion of the subzone within the landscape unit (or interim landscape unit or portion of a forest development plan forming a contiguous geographic unit) that is identified as available for harvest (that is, not inoperable or in some sort of reserve status, such as riparian or protected area). Y-axis numbers (rows) are the proportion of the available landscape (above) that has already been harvested without application of this guidebook’s recommendations or similar prescriptions.
For each biogeoclimatic subzone in the landscape unit (Table 20(a)), calculate the area available for harvest (the X-axis). For example, if 30% of the SBSmc area is available for harvest, then, using the 30% column, the recommended minimum proportion of each cutblock to be managed for wildlife trees is between 1 and 9%. If 50% of the available area has already been harvested without application of these or similar guidelines (Y-axis), then 5% of each new cutblock would need to be left for wildlife tree patches. This can be distributed adjacent to cutblocks in riparian or other long-term leave areas when feasible. Where landscape units have not been designated, the same calculation can be done using Table 20(b).
Examples of how to apply this process to a proposed cutblock are shown in Figures 8 to 10.
Creating wildlife trees
One method of creating wildlife trees is to high-cut stumps during the use of feller bunchers. This creates standing dead trees called “stubs” (see Figure 11). These provide structure within second-growth forests and create future coarse woody debris. They cannot, however, replace all attributes associated with full height wildlife trees and thus can not be used as a complete substitute.
- If mechanical harvesters are being used, snags and cull trees that must be felled should be left as high as can be reached safely. For dead trees, the maximum allowable height must be according to Workers’ Compensation Board regulations.
- Stems suitable for stub creation should have some visible defect (such as canker, scar or conk) in the lower bole and little or no lean.
- Creation of wildlife trees by topping, blasting, and other methods should also be considered.
Figure 8. Applying wildlife tree management recommendations to a proposed cutblock.
Figure 9. Reserves for wildlife trees incorporated into four silvicultural systems.
Figure 10. Harvesting areas with wildlife trees maintained in a riparian management area and a gully complex.
Figure 11. Artificially created wildlife trees (stubs).
Other stand structure recommendations
Any action that encourages structural diversity within managed stands can have value to biodiversity.
- Some vertical and structural variability should be maintained throughout stands or rangelands. Within larger stands, this can be achieved by the use of variable stocking densities at planting, in combination with spacing or thinning activities. In rangelands, components of shrub communities should be retained.
- In vegetation management or thinning treatments, patches or strips can be left unthinned on some sites so that dense patches develop.
- Wider spacing of patches can be used to maintain a partially open canopy that will promote understorey vegetation.
- Non-merchantable defect trees should be left as recruitment snags rather than removed.
- Patches of advance regeneration can provide structural diversity.
- Range management should be designed to maintain a component of shrubs within rangelands, based on the shrub component found in undisturbed rangelands.
- Forest encroachment on rangelands can be reduced through management practices such as prescribed fires.
- During tree pest and disease treatments, some areas should be left untreated.
- Openings should be irregularly shaped to most closely reflect natural disturbance patterns.
Maintaining tree and vegetation species composition
The maintenance of the diversity of naturally occurring plant species is key to the maintenance of biological diversity within landscape units. Within cutblocks, several actions can help maintain that diversity.
- The variety of native understorey plants and plant communities should be maintained across the landscape units.
- Vegetation management treatments can be designed to create variability among or within treatment areas. Effects on non-target plants should be minimized.
- A component of the deciduous species, both immature and mature, should remain after harvesting, spacing, vegetation management, and site preparation activities.
- Extensive conversion from climax to young seral species (such as Douglas-fir to lodgepole pine) or from young seral to climax should be avoided.
- Where suited to the site, stands should be regenerated with a mixture of tree species (natural and planted) rather than with a single species.
- Where mature hardwoods form a minor component of the stand (<20%), greater emphasis should be placed on maintaining these either singly or in clumps.
- Minor tree species such as yew, birch, alder, aspen and cottonwood should be maintained.
Maintaining coarse woody debris
Maintaining coarse woody debris after harvesting is a critical element of managing for biodiversity. However, it is recognized that this requirement conflicts with existing utilization standards. Work is under way to resolve this policy conflict. Until this issue is resolved, utilization standards will take precedence over requirements for coarse woody debris.
Despite this policy conflict, some existing practices can be modified to help address the requirements for coarse woody debris. For example, preliminary indications are that post-logging residue and waste can meet the volume requirements for coarse woody debris if it is well distributed across the entire stand. This will not be the case in situations of whole-tree harvesting, clean site preparation practices, or excessive salvage of material not considered merchantable under current utilization standards.
- Modify whole-tree harvesting by limbing and topping on site.
- Maintain residue and waste well-distributed across the stand (avoid practices such as piling and burning).
- Leave non-merchantable material on site.
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