Silvicultural Systems Appendix 2

Advantages and Disadvantages of Various Silvicultural Systems

Advantages

  • Allow for establishment of a more uniform crop (includes the benefits of uniformity and even-aged management).
  • Allow for easier and efficient operations, because it is the simplest method to use.
  • May have lower costs for forestry activities including: planning, layout, supervision, harvesting, site preparation, and intermediate treatments. Harvesting may be less expensive due to the higher volume/hectare removal.
  • May more easily accommodate highly specialized equipment designed for harvesting and site preparation.
  • Avoid damage to regeneration since felling and extraction are done before establishment.
  • May provide a means to most rapidly achieve a free growing plantation when combined with plantation forestry techniques and fast-growing shade-intolerant species. Note: Often shade-intolerant species are more desirable due to their growth and yield and wood quality considerations.
  • May allow for easier control of insect and disease problems:
  • Root rots - clearcut and remove stumps
  • Dwarf mistletoes - remove overhead infection sources
  • Mountain pine beetle - remove susceptible stands.
  • May more easily allow for amelioration of site/soil through site preparation (although it may be argued that amelioration may not be necessary if another silvicultural system is used).
  • Enhance worker safety because most or all trees are removed.

Disadvantages

  • Sometimes negatively perceived as being systems that fight against nature, regardless of the ecological conditions, by encouraging uniformity, especially when agricultural techniques, such as site preparation and planting, are used.
  • May not be suited to wildlife species where overhead cover or more structurally diverse habitats are required at a stand level.
  • May expose the site to erosion, particularly if soils are compacted and moisture inputs are high on steep slopes with significant amounts of exposed fine-textured soils.
  • May increase mass wasting hazard on steep slopes with fine soil and high moisture inputs or with smooth geologic bedding planes that are parallel to the ground surface.
  • May exacerbate adverse environmental conditions for regeneration such as microclimate (frost, drying winds, extreme temperatures), soil moisture and perhaps nutrients, competing vegetation, predators (insects/animals). This adverse situation is only created on extreme sites where trees are very difficult to re-establish.
  • May prevent full growth and yield potential of individual trees (as in single tree selection management). During a significant portion of the rotation the growing space is not fully occupied by crop trees.
  • May not be considered visually pleasing.
  • Not well suited to shade-tolerant species that grow slowly in the juvenile stages, even if they are planted.
  • Pioneer vegetation may have a great advantage and overtop these trees.

Advantages of strip clearcut system over block clearcut system:

  • Relies mostly on natural regeneration thereby possibly reducing regeneration costs.
  • May have less impact on visual and other resource values (temporary benefit) because strips are smaller in scale than other clearcut systems.

Advantages of block clearcut system over strip clearcut system

  • Allows more flexibility to meet site-specific circumstances because some planting is often used and therefore boundaries can be determined using considerations other than seed dispersal.
  • Larger units may make administration, planning, layout, and execution of activities less costly.
  • Greater flexibility to deal with large-scale catastrophic events (e.g., fire, insects, and disease).

Advantages

  • Follows nature's model by retaining part of the forest after harvesting.
  • Retains structural features -- snags, large woody debris, live trees of varying sizes, and canopy layers -- as habitat for a wide variety of organisms.
  • Retention structures can be dispersed throughout a cutblock (individual trees or small groups) or aggregated (clumps or patches) depending on the objectives.
  • Can mitigate factors such as visual viewscapes and wildlife habitat that might constrain amounts of timber available.
  • May be advantageous for wildlife that require large living and dead trees for habitat.
  • May be well suited to shade-tolerant species or species requiring a shaded environment for establishment.
  • Lower regeneration costs if natural regeneration is easy to achieve.
  • May allow for establishment of species that are hard to regenerate artificially if retention of some advanced regeneration is possible.

Disadvantages

  • Pioneer vegetation may have a great advantage and may inhibit and overtop regeneration.
  • Site preparation and some vegetation management activities may be restricted or more difficult.
  • Work is less concentrated, so harvesting and/or associated planning will be more costly.
  • Requires more worker skill to achieve goals and involves more training at extra cost.
  • Increases risk of exposed residual trees to windthrow.
  • Some diseases (e.g., dwarf mistletoe) or insects (e.g., spruce budworm) can easily spread from the retained overstorey to regeneration.
  • May require more roads unless layout is carefully done.

Advantages

Similar to clearcutting except:

  • Lower regeneration costs if natural regeneration is easy to secure.
  • Better manipulation of species and genetics than with other partial cutting systems when natural regeneration is relied on (more choice for leave-trees).
  • Resolves issue of regeneration for species that are difficult to regenerate artificially (e.g., larch in the Kootenays).
  • Aesthetically better than clearcuts where the number and arrangement of leave-trees is visually pleasing.
  • May be advantageous for wildlife, especially species that use large living or dead trees for habitat depends on the size, species, and vigour of the leave-trees and their duration on the site.
  • May have some growth and yield advantages since seed trees will grow while regeneration is being established.

Disadvantages

  • Of all silvicultural systems involving partial cutting, exposes leave-trees to the most wind. Should not be used with species, sites, or stand types with a high wind hazard. Losses should be expected on high hazard sites (will depend on individual tree characteristics).
  • Higher harvesting costs, compared with clearcut systems, if seed trees are removed (two-stage harvest). Also, silviculture costs may be higher if regeneration damage is excessive during seed tree removal.
  • May not have an advantage over clearcutting in situations where maintaining amoderate crown cover is desirable (for aesthetics, recreation, water, wildlife, soil, or microsite objectives).
  • May generate lower harvest volume than with clearcut systems if seed trees are notremoved.

Advantages of grouped seed tree system over uniform seed tree system

  • Easier to protect from harvest damage.
  • Easier to harvest in final cut.
  • May be more windfirm if initial stand was irregular (clumpy) and an entire clump is left or, if clump is left in a protected area.
  • May make it easier to achieve other non-timber objectives (e.g., maintaining wildlife trees).
  • Advantages of uniform seed tree system over grouped seed tree system
  • Easier to choose the species and individual trees with the best characteristics for seed production, windfirmness, genetics, or any other criteria related to timber production or other resource objectives.

Advantages

  • Protect new regeneration that is sensitive to frost, drought, and cold winds. Such protection is not found in clearcuts except in small cuts.
  • May more efficiently use the productive growing space since the sheltering overstorey will add growth as regeneration establishes. Generally trees not capable of further increases in volume and value are cut first to make room for regeneration.
  • May provide some protection of soil from erosion and mass wasting since precipitation inputs to soil may be reduced via interception and evapotranspiration. This effect will depend on many factors including the amount of overhead cover, skid trail density and location, and the amount of site disturbance.
  • Usually preferred aesthetically to clearcuts and seed tree systems through the regeneration phase.
  • May be more beneficial for wildlife, recreation, or water objectives where significant overhead cover is desired. However, this will depend on leave-tree characteristics and their duration on-site.

Disadvantages

  • Require more skill and time to secure regeneration than with clearcutting or seed tree systems.
  • Work is less concentrated, so harvesting and associated planning will be more costly.
  • Potential to damage young trees through the removal cut, although this risk can be reduced by careful planning and system design.
  • Cutting rates and regeneration establishment and growth may be more difficult to regulate and control than with clearcutting and seed tree systems. This could complicate sustained yield goals.
  • Major problems can develop with some diseases (e.g., dwarf mistletoe) or insects (e.g., spruce budworm), which easily spread from the overstorey to the regeneration.
  • In the BC interior, some forest managers prescribe cutting mature trees that overtop regeneration to reduce budworm problems.
  • Trees to be harvested in known Armillaria infection centres have been "pushover logged" or roots have been extracted afterwards in some parts of the interior.
  • May be more difficult to conduct silviculture treatments like site preparation and vegetation control.

Advantages

  • Felling is simpler than in most shelterwood systems.
  • Produces regular, even-aged stands with uniform, straight stems.

Disadvantages

  • May damage regeneration during harvesting of overstorey during removalcuttings.
  • Vulnerable to windthrow where hazard is high and overstorey is susceptible. Future crops will also be susceptible due to regularity and uniformity.

Advantages

  • May adjust conditions of shelter and seedling microsite within systematically arranged transition zones along the leading edge of strips.
  • May provide side protection in a specific direction against wind and sun.
  • Can provide side protection from sun, which is generally more effective than overhead cover on sites subject to drought and radiation frost.
  • Allows for extract of harvested timber through the mature forest and avoid residual damage in areas of tall regeneration.
  • May easily plan and execute harvesting, regeneration, and tending operations in a systematic, logical fashion.
  • May easily control the progress of regeneration and growth and determine the impact on yield.
  • May have advantages for biodiversity and aesthetics through variation in sizes of trees and diversity of small habitats near one another.

Disadvantages

  • Requires a specialized and rigid layout.
  • Requires careful design to gain aesthetic benefits since straight lines may not be considered natural.

Advantages

  • May provide suitable light conditions for all tree species in a particular biogeoclimatic subzone.
  • May have advantages for biodiversity and aesthetics through variation in sizes of trees and diversity of small habitats near one another.
  • Young stands may develop more naturally than in uniform systems (in forest types where small-scale "gap disturbance regimes" are common).
  • Can be used as group openings to concentrate and plan the pattern of small openings, thereby meeting more closely the management objectives (e.g.,release and use advance natural regeneration, maximize habitat, water, or aesthetic benefits).
  • May protect against snow breakage and sliding on steeper slopes due to the irregular nature of the regeneration.
  • Felling is directed away from gaps with regeneration, thereby avoiding damage to regeneration in early stages.

Disadvantages

  • Regulation and control of cutting and regeneration, is more time consuming due to small scattered centres of regeneration.
  • Regeneration must be able to tolerate exposure to open conditions on north edges as gaps enlarge.
  • Planting may be necessary in the simple versions.
  • Requires dense extraction network to offset scattering of activities. However,excessive roading may be mitigated or even eliminated through careful planning.

 

 

Shelterwood Systems Defined by Timing of Overstorey Harvesting

Advantages

  • Very flexible system.
  • Best possible use of each small site.
  • Highly diverse forest structure with potential advantages for wildlife, biodiversity, recreation, and aesthetics objectives.

Disadvantages

  • Planning and execution made difficult with scattered activities.
  • Requires dense extraction network to offset scattering of activities. However, excessive roading may be mitigated or even eliminated through careful planning.
  • Requires a high level of skill in planning and execution.
  • Favours shade-tolerant species unless opening size is large enough to promote shade-intolerant species.

Advantages

  • Saves planting costs and time. A 10- to 15-year-old stand may already be established after harvest, significantly shortening the regeneration delay period.

Disadvantages

  • Requires a well-stocked, acceptable understorey.
  • Requires a vigorous, acceptable regeneration in the understorey that will be able to release and grow to a merchantable size. It is generally considered necessary to have 2 times the target stocking to allow for harvesting damage.
  • Must be economically possible to harvest the overstorey and save the regeneration.
  • May require some quality slashing or spacing after harvesting and some fill planting.

 

 

Shelterwood Systems Defined by the Management of Different Species in Different Canopy Layers

Advantages

  • May logistically fit with natural succession and therefore works with, rather than against, succession.
  • Overstorey provides protective cover for establishment of species.
  • May allow for "heavy" thinning of overstorey, without excessive reduction to growing space occupancy, to get early returns at a mid-point in the rotation.

Disadvantages

  • Can considerably damage understorey when overstorey is thinned or harvested. The understorey can be protected by harvesting on the snowpack and carefully planning the road network. Also, the understorey can be thinned to allow future overstorey fellings to fall into gaps.
  • Must establish a stratified stand with different species in each layer. This may require planting for one or both layers. Combinations of other systems may preclude this requirement and still allow for a stratified species mixture.

Advantages

  • Well suited to uneven-aged stands that cannot easily be converted to even-aged without wasting considerable growing stock.
  • Fulfills management objectives that require maintenance of some large trees on-site for aesthetic and/or wildlife habitat reasons.
  • Desirable on sites where climatic conditions are seldom conducive to regeneration and growth is too slow to justify cost of planting. Because trees on-site produce seed whenever circumstances allow, this system is favored in dry ponderosa pine or interior Douglas-fir stands.
  • Can make the best possible use of the site since the system is both flexible and intensive.
  • May have advantages for small landowners who wish to harvest smaller yields over more frequent intervals while minimizing initial development/regeneration costs.
  • May be more flexible in reacting to changes in markets that influence management objectives for tree size and quality.
  • May provide some soil protection through the presence of a continual, variable canopy. Soil damage, however, can still be influenced by the nature and extent of roading, and the maintenance of roads. Note: With ground-based harvesting, this system may require extensive roading.
  • May provide some protection from fire once the stand structure is established due to the broken irregular structure that may disrupt rapid spread through the upper canopy. However, fire hazard may increase in fairly dense stands during initial entries due to the slash and multi-age classes acting as ladder fuels.
  • Tend to minimize snow and wind damage. This is not necessarily the case if attempts are made to convert even-aged stands to uneven-aged stands using group selection or other methods.
  • May allow for merchantable volume gains, by capturing mortality from frequent harvesting entries and from continuous stocking.

Disadvantages

  • Considerable silvicultural skill is required in planning and executing of selection systems.
  • Felling and extraction must be done with extreme skill and care. A range of equipment has been used successfully in British Columbia, including:
    • a variety of small ground skidding machines
    • feller bunchers in the Okanagan
    • light cable systems in the southern interior
    • helicopters on the Haida Gwaii.
  • Costs are often increased in planning and execution.
  • Tend to favor the most shade-tolerant species for the site when single tree selection systems are used. (May not be advantageous if shade-intolerant species are preferred.) However, shade tolerance is relative and depends on the species and subzones. Single tree selection has been used to regenerate ponderosa pine (in the ponderosa pine zones in the United States).
  • Highgrading is a potential problem.
  • Stands often regarded as uneven-aged and more or less balanced when they are not. This assumption is made because a wide range of diameter classes or mixed species may be present.
  • May incorrectly assume that the allowable harvesting rate is equal to the periodic annual increment (multiplied by a factor for the cutting cycle).
  • Stands resulting from highgrading will have a predominance of middle-aged trees, even though an inverse "J" diameter distribution is achieved. If these stands are cut at a rate determined by periodic annual increment, the average diameter will decline for every cutting cycle and the trees available will decrease.
  • Stands often undercut. Quantitative guides are needed to prevent stands from being overcrowded with middle-aged trees.
  • The "balanced uneven-aged structure" may become the major goal instead of a means of attaining objectives. This may lead to reckless cutting of age classes with desirable, but surplus, trees and careful retention of poor trees in sparse diameter classes. This blind adherence to diameter distribution may interfere with resource objectives.
  • May be difficult to handle insect and disease problems. The risks from rootrots, dwarf mistletoe, spruce budworm, bark beetles, etc., must be considered.
  • While harvest volumes per unit area are low, harvesting is spread over more of the landbase at a given time, with more frequent entries on the same unit of land. This continuous harvesting may not be perceived favorably by the public.
  • Increased site degradation is possible when using certain harvesting methods on specific sites.
  • The risk of damage to the remaining trees is present during each harvest entry.

Advantages

  • Generally better than group selection if discernable openings are to be minimized.
  • May be better than group selection at reducing wind, snow and, in some cases, fire damage.

Disadvantages

  • Crown closure of adjacent trees may occur before the regeneration in the small openings can occupy a place in the canopy. This risk makes frequent assessment and light cuttings necessary. Also, the growth and branching characteristics of the overstorey trees should be considered before developing a prescription.
  • More difficult to protect regeneration and immature age classes than with group selection as operations are scattered and mixed into a mosaic of treatment units.
  • Logging may be more difficult and costly than with group selection. Very large trees or difficult terrain further increase difficulty.
  • best for light equipment and suitable terrain where permanent skid trails or skidding corridors can be used on a continuous basis
  • suited to either ground-based or light cable systems.
  • More complicated to manage regulation, planning, and layout for single tree selection than group selection.
  • May be more difficult to meet Workers' Compensation Board (WCB) regulations when managing for wildlife trees because frequent harvesting entries are made throughout most of the stand.

Advantages of group selection system over single tree selection system

  • Many forests in BC with natural uneven-aged structures have developed with clumped distributions, making them more conducive to group rather than single tree selection.
  • Easier to plan, control, and therefore reduce costs of harvesting, site preparation, planting, and intermediate treatments.
  • Trees develop in clearly defined even-aged aggregations, with associated advantages for form.
  • May benefit some wildlife and increase diversity of stand species due to increased heterogeneity, with increased edge effect and maintenance of good cover in the same stand.
  • May satisfy regeneration requirements for a greater range of species (e.g., the classic Kootenay mix: Fdi/Lw/Pw/Cw/Hw).
  • May be better able to meet WCB requirements when managing for wildlife trees.

Disadvantages of group selection system over single tree selection system

  • Increased edge between young and mature trees may create problems such as:
    • large edge trees compete for light (and moisture on dry sites) and may shed snow on to the regeneration encouraging damage
    • more windthrow may be encouraged, depending on size of openings and wind hazard and risks
    • insects and diseases could be spread to the regeneration
    • more browsing damage to regeneration may be encouraged.

Advantages

  • Timber is scattered throughout the stand and often must be harvested across areas of regeneration in both single tree and group selection. This problem may be avoided by concentrating each age class in a long narrow strip.
  • May progressively develop a complete set of age classes across the landscape.
  • Similar to strip shelterwood.
  • Enables transport of logs through next strip to be harvested.
  • Road location and consideration of harvesting patterns ahead of time is very important.
  • Will want to progress into the most dangerous winds, or towards the south if harvesting allows.
  • If wind protection wanted, resulting stand has an "aerodynamic effect."
  • May get an increased water yield from snow-melt in both strip and groupsystems.

Disadvantages

  • More time consuming to implement.
  • May be less aesthetically pleasing than group selection depending on scale, viewpoint, and viewing distances.