Even-aged vs. Uneven-aged Systems

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Even-aged vs. Uneven-aged Systems
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Common characteristics of even-aged stands

• Crown canopy is generally limited to a single
  layer elevated above the ground
• Diameters vary widely only if shade-tolerant
  species are present
• Only old stands have sawtimber sized trees
• Small trees have short live crown length when
  compared to total height
• Largest trees often have 25-40 percent live
  crown, depending on stand density

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Common characteristics of uneven-aged stands

• Crown canopy is generally comprised of multiple
  layers and commonly extends close to the ground
• Diameters range from seedling-sapling to
  sawtimber sizes, regardless of species present
• Trees of all diameters have a large live-crown
  ratio, often as high as 40 to 60 percent in
  managed stands
• Tree heights vary with tree diameter, with short
  ones having small diameters and tall trees having
  larger diameters

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Even-aged vs. Uneven-aged Diameter Distributions
Bell-shaped (normal distribution)
Reverse J-shaped
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Reverse J-shaped does not always indicate a true
uneven-aged stand (3 age classes)
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• Timeline of practices in an even-aged
  silvicultural system
• During the rotation age r, treatments are
  applied across the entire stand to meet
  silvicultural objectives that are related to tree
  age

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• Concurrent application of individual practices
  of an uneven-aged silvicultural system during a
  cutting cycle harvest in a balanced uneven-aged
  stand
• Treatments are applied to subunits of the stand
  depending on conditions within each subunit
• Each cutting cycle harvest will support similar
  treatments

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Uneven-aged Regeneration Systems

• Uneven-aged regeneration systems often referred
  to as selection systems

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Characteristics of Uneven-aged Systems

• Selection methods produce an uneven-aged stand
  (with at least 3 age classes or distinct cohorts)
• For regeneration, trees are harvested as
  individuals or in small groups
• Single-tree selection method removing individual
  mature trees more or less uniformly across a
  stand
• Group selection method removing mature trees in
  small groups or clusters

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Characteristics of Uneven-aged Systems

• Maintains a continuous high forest cover
• Typically emphasizes the production of sawtimber
  sized trees

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Characteristics of Uneven-aged Systems

• Selection is particularly useful for putting an
  irregular stand under productive management
  without losing existing stocking
• A selection system can be designed to obtain a
  sustained yield at recurring short intervals
• For sustained yield in a selection system

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Characteristics of Uneven-aged Systems

• Rotation length is the average time period
  required to obtain crop trees of a specified
  target size
• The period between harvests (in years) is the
  length of the cutting cycle

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Characteristics of Uneven-aged Systems

• To avoid "high-grading", each cutting should
  include intermediate treatments among trees other
  than those of the target size
• For a sustained yield, the method requires
  frequent and accurate inventory

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General Procedure in Uneven-aged Systems

• Harvest mature trees, either single trees or in
  small groups
• This provides openings for regeneration of a new
  age class (cohort)
• "Tend" the remaining cohorts to maintain
  approximately equal total area in each -- among
  these remaining sizes, "cut the worst, leave the
  best"

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Approaches to regulation in the selection method
and maintaining a balanced stand with sustainable
yield

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• Area regulation this is the simplest, and is
  fairly easy with a group selection system, but it
  is difficult with the single-tree approach.

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• Volume regulation harvest the allowable cut each
  cutting cycle -- if a stand is balanced, this is
  equal to the growth during the cutting cycle
  period

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• Structural regulation use a reverse J-shaped
  curve of residual diameter distribution as a
  guide.

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Balance vs. Irregular (unbalanced) uneven-aged
stands
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Structural regulation and reverse J-shaped curve

• In balanced uneven-aged stands with an reverse-J
  shape distribution, a constant ratio exists
  between the number of trees in successive
  diameter classes.
• This relationship defines the curves shape
  (steepness or flatness) and is called q (or
  quotient)
• q
• where,
• Ni number of trees in the ith diameter class
• Ni1 number of trees in next largest diameter
  class

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Influence of q on Target Diameter Distribution

• A smaller q value more large trees and fewer
  smaller trees
• A larger q leaves fewer large trees, more
  smaller tree (i.e. less sawtimber)

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Structural regulation BDq Method

• The BDq Method of Regulation
• B is the target residual basal area (after
  harvest)
• D is the maximum retained (after harvest)
  diameter class
• Maximum diameter or largest diameter tree)
• q is the ratio of numbers of stems (target-after
  harvest) of each DBH class to the next higher DBH
  class

BDq Method is being researched at the Crossett
Experimental Forest (Arkansas) for loblolly and
shortleaf pines. Information and recommendations
from their research is used as examples for the
following discussion.
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Uneven-aged Regeneration Methods
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Variations of the Selection Method

• Single Tree Selection removes individual trees
  of all size classes more or less uniformly
  throughout the stand to maintain an uneven-aged
  stand and achieve other stand structural
  objectives

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Variations of the Selection Method

• Single Tree Selection
• More commonly applied in
• Shade tolerant species
• Restrictive sites where pronounced seasonal
  water limitations favor natural monocultures
• Has been used for other forest types

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Variations of the Selection Method

• Single Tree Selection
• Central and southern upland and bottomland
  hardwoods

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Variations of the Selection Method

• Group Selection removes clusters of adjacent
  mature trees from a predetermined proportion of
  the stand area
• Group selection was developed to regenerate
  shade-intolerant and intermediate species
• Group selection is easier to plan and keep the
  stand balanced than with single-tree (if area
  regulation is used)
• Logging is more efficient and less damaging to
  residual trees than with single-tree

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Group Selection Method
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Application of group selection

• Locate groups to be harvested among the oldest or
  largest trees in the stand
• Uses area regulation to maintain balanced stand
• Openings must be wide enough to allow good
  regeneration establishment
• Group selection in the Central Hardwood Region
  generally uses open sizes between 1 and 2 times
  the height of surrounding trees

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Application of group selection

• Shape the harvested openings to fit the stand
  conditions or to maximize objectives/constraints
  considerations
• Complete felling of all trees in the openings is
  crucial to allow for good regeneration

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Application of group selection

• Control of undesirable species should be
  considered
• Tend the remaining uncut stand areas between
  group openings

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Issues associated with group selection

• Uses area regulation for structural control
• Difficult (or impossible) to locate groups within
  a stand following second or third entry
• Appropriate tool for other objectiveswildlife
  openings, aesthetics, salvage/sanitation

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Issues associated with group selection

• Group selection is often confused with patch
  clearcutting
• If groups are managed as an individual stand
  and tracked through time as such, you are using
  even-aged silviculture at a small spatial scale
• In group selection, harvested opening widths are
  no more than 2 times the height of adjacent
  mature trees

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Potential Objectives/Benefits in Using a
Selection System

• Can provide frequent periodic income from the
  stand (3 - 10 years), with no long time gaps
• Has good flexibility maintains a reserve of
  large trees on the stump (thus one can take
  advantage of market fluctuations)
• Requires only a low investment in regeneration

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Potential Objectives/Benefits in Using a
Selection System

• Maintains high diversity within the stand
• Maintains good site protection
• Maintains pleasing aesthetics without time gaps

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Potential Drawbacks/Disadvantages in using a
Selection System

• Involves a high level of complexity, requires
  higher management costs than other methods
• Produces less pulpwood than other methods
• Harvesting is usually more difficult and costly
  per unit area or product than with even-aged
  methods
• Typically, selection results in more logging
  damage to potential crop trees than with
  even-aged methods, due to more frequent entry of
  equipment into the stand
• Can lead to high grading if not applied carefully

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Two-Aged (Hybrid) Silvicultural Systems
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Two-Aged Silviculture

• Two-aged management is a hybrid between even-aged
  management and uneven-aged management
• Regeneration is accomplished (in general) two
  times over a standard rotation.
• Two age classes
• Referred to as irregular shelterwoods, reserve
  shelterwoods, leave tree systems, and deferment
  methods

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Benefits of a Two-Age System

• Development of large-diameter sawtimber or veneer
  trees
• Production of a wide range of forest products
  from pulp to veneer in the same stand at the same
  time
• Ability to regenerate shade-intolerant and
  intermediate shade-tolerant species
• Improved aesthetics compared to clearcutting
• Increased structural diversity and retention of
  habitat components compared to clearcutting

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Benefits of a Two-Age System

• Increased initial revenue compared to other types
  of non-clearcut regeneration techniques
• Development of old-growth structural
  characteristics
• Maintenance of sexual reproduction in reserve
  trees throughout the entire rotation and the
  ability to life boat species that would
  otherwise be eliminated if the area was clearcut

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Constraints/Undesirable Features of Two-Aged
System

• Reducing older age classes to low densities and
  wide spacing increase the danger of blowdown
• Residual trees may be prone to epicormic
  branching
• Lack of appropriate long-lived species to
  maintain the system

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Constraints/Undesirable Features of Two-Aged
System

• Forest fragmentation and habitat effects similar
  to clearcutting
• Reduction in initial revenues compared to
  clearcutting
• Limited development of shade-tolerant species
• Damage to new age-class trees if a portion of
  reserve trees are removed prior to the end of the
  second rotation length

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• Irregular or Reserve Shelterwood
• Leaves residual overstory for an extended period
  of time into new rotation creates two-aged
  stand
• Has ecological/aesthetic vs. economic/operational
  tradeoffs
• Characteristics of reserve trees are important

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Irregular/Reserve Shelterwood
Uncut Stand
Establishment Cut (45-60 ft2 ac-1 BA)
Reserve trees (10-15 ft2 ac-1 BA)
Two-age stand development
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Application of Irregular or Reserve Shelterwood

• Two-aged system typically initiated using a
  shelterwood cut

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Reserve Tree Criteria
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Other Partial Cuttings
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Other Partial Cuttings

• Timber harvesting vs. Silviculture
• Timber harvesting extracts a product
• Silviculture involves a determined effort to
  regenerate mature trees or tend immature ones and
  to provide by the future by using harvesting to
  recover products that become a byproduct of
  systematic management

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Other Partial Cuttings (non-silvicultural
treatments)

• Non-silviculture, exploitative partial cutting
  treatments
• Commercial clearcutting
• High-grading
• Diameter-limit cuttings

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Disadvantages of Non-Silviculture Partial
Cuttings

• Does not move forests toward a controlled age or
  size class distribution that ensures long-term
  sustained yields at predicable levels or
  intervals
• Does not ensure adequate regeneration in terms of
  number, species, or distribution
• Ignores silvical requirements of desired species
  with respect to regeneration and long-term growth
• Removes acceptable growing stock and leaves
  defective and unhealthy trees