Common characteristics of evenaged stands

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Common characteristics of even-aged stands

• Diameters usually vary widely 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
• The crown canopy is generally limited to a single
  layer elevated above the ground

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Common characteristics of uneven-aged stands

• 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
• The crown canopy is generally comprised of
  multiple layers and commonly extends close to the
  ground

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Characteristics of the Selection Method

• This is not equivalent to "selective" cutting, as
  the term is commonly used!
• As commonly used, "selective" logging and
  "select-cut" merely mean that the harvest is not
  a clearcut
• So, these terms are imprecise -- they could be
  referring, for example, to a thinning, to a
  shelterwood establishment cutting, or to a
  high-grading cut

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Characteristics of the Selection Method

• 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
• Maintains a continuous high forest cover (though
  typically with a lot of irregularity).
• The entire stand remains under the influence of
  mature trees.
• Harvested opening widths are no more than 2 times
  the height of adjacent mature trees.
• Typically emphasizes the production of
  large-sized trees (sawtimber veneer) pulpwood
  production is relatively low

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Characteristics of the Selection Method

• 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
• If the stand is balanced, each harvest should
  remove an amount equivalent to the growth
  produced since the last harvest.
• One should seek a "reverse J-shaped" (negative
  exponential) distribution of stem density by
  diameter classes

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Characteristics of the Selection Method

• Rotation length is the average time period
  required to obtain crop trees of a specified
  target size
• Harvests occur regularly at short intervals
  (typically 3-10 years, but may vary) throughout
  the rotation
• The period between harvests (in years) is the
  length of the cutting cycle

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Characteristics of the Selection Method

• To avoid "high-grading", each cutting should
  include thinning and improvement cutting among
  trees other than those of the target size
• For a sustained yield, the method requires
  frequent and accurate inventory
• best, at the end of each cutting cycle
• an accurate stand and stock table is needed

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General procedure in the selection method

• 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
(i.e. maintaining a balanced stand)

• Area regulation
• Volume regulation
• Structural regulation

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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.
• Combined area of all trees removed in each
  cutting cycle
• (Cutting Cycle / Rotation Length)
• Example,
• Cutting Cycle 7
• Rotation Length 70
• (7/70) 0.10
• Interpretation 1/10 of the area will be cut
  every 7 years with a 70 year rotation age

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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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Volume Regulation An Example

• The Volume-Guiding Diameter-limit (VGDL)
  approach
• Determine a maximum stocking level for the stand
  to have just before each harvest.
• Estimate the annual stand growth rate of
  sawtimber-sized trees.
• Set the cutting cycle length
• The minimum feasible cutting cycle length is set
  by the minimum volume acceptable for an operable
  cut divided by the stand's annual growth rate

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• VGDL Approach (Continued)
• Annual growth multiplied by the cutting cycle
  length equals the allowable cut, if the stand is
  balanced.
• Using a stand and stock table at each cutting
  cycle harvest,
• Determine the maximum DBH that would be cut if
  the allowable cut is removed by starting with the
  largest diameter class and progressing from the
  largest downward.
• This DBH becomes the guiding diameter limit (GDL).

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• VGDL Approach (Continued)
• Harvest the allowable cut volume from trees of
  the GDL and larger.
• However, in practice, it is usually desirable to
  leave some fast-growing, high quality trees
  slightly above the GDL
• make up their volume by cutting from poor trees
  of smaller sizes.
• To avoid problems and imbalances, you must also
  apply thinning and improvement cutting to all
  size classes.
• In sizes below the GDL, always cut the worst,
  leave the best
• A few trees larger than the GDL are typically
  left, and some trees less than GDL are cut.
• If the stand is understocked, remove less than
  the allowable cut by reducing the harvest in
  under-represented sizes.

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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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Steps in Applying the BDq Method

• Determine the annual stand growth rate
• Basal area or board-foot volume
• Annual growth for loblolly/shortleaf pines will
  typically be 2.5-3.5 ft2 BA, 300-500 bdft (Doyle)
  per acre
• Decide on Cutting Cycle
• longer cuts more shorter cuts less
• Set maximum BA to be reached
• Example, 75 ft2 ac-1 for loblolly pine
• Set the residual basal area
• cut back to this level
• should be maximum BA minus BA growth during
  cutting cycle
• For loblolly/shortleaf pines use 45-65 ft2/acre

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Steps in Applying the BDq Method

• 5. Set the target minimum DBH (D) for harvested
  mature trees
• for timber production, this should reflect
  financial maturity size for the desired product
  (16-21)
• larger D means fewer total trees left, higher
  average DBH
• Select q define it for either 1 or 2-inch
  classes
• -For loblolly or shortleaf pine q is typically
  1.1 to 1.3, for 1-inch classes (1.2-1.7 for
  2-inch classes)

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• 7. Calculate the numbers of trees to be retained
  in largest diameter class
• Nmax
• Nmax the number of trees in the largest
  diameter size class
• BA target residual basal area
• di diameter class
• bai basal area of diameter class midpoint
• dmax largest diameter to be retained in the
  stand
• dmin smallest diameter class
• w width of diameter class (usually 1 or 2
  inches)

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• 8. Calculate the number of trees in each
  diameter class (the "target" distribution)
• calculate your target distribution by multiplying
  the number of trees by q to get the number of
  trees in each successively smaller size class
• Steps 7 and 8 can be simplified by using a
  spreadsheet that calculates target diameter
  distribution from residual basal area and q (link)

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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.
• Most applicable to very tolerant species
  (spruce-fir, beech-maple)
• Used loosely in
• loblolly-shortleaf pine, in the form developed at
  Crossett Experimental Forests (Arkansas)
• in xeric oak types as in the Missouri Ozarks
  (Pioneer Forest)
• Typically, not considered suitable for producing
  quality southern hardwoods

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Variations of the Selection Method

• Single Tree Selection in practice
• Logging is difficult and costly, and may result
  in a high degree of damage
• Aggressive control of competing tolerant species
  through herbicides or cutting is essential for
  success

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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 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
• Due to edge effects, including shading, the best
  success and growth of seedlings of intolerants
  may be restricted to 2/3 or less of the area in a
  small opening

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Application of group selection

• Shape the harvested openings to fit the stand
  conditions or to maximize objectives/constraints
  considerations
• rectangular openings will be more efficient for
  logging than circular or square ones-narrow
  rectangular openings provide more sun if oriented
  with their long axes east-west
• 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 (possibly pre- or post-harvest
  injection, basal bark herbicides, or cutting)
• Tend the remaining stand at each harvest, employ
  improvement, presalvage, salvage, and thinning of
  scattered trees in the uncut stand areas between
  group openings

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Issues associated with group selection

• A hybrid method using 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
• If groups are mapped, or located for harvest in
  the office rather than on the ground you are
  using even-aged silviculture at a small spatial
  scale.
• if selection is done without regard to the traits
  of the trees to be harvested, then you are not
  practicing selection

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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)
• It is not as vulnerable to complete stand
  destruction (fire, insects) as are even-aged
  methods
• 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
  (usually provides good wildlife habitat for many,
  but not all species)
• Maintains good site protection (although
  frequent logging may result in increased soil
  damage on sensitive sites)
• Maintains pleasing aesthetics without time gaps
• When balanced, produces more sawtimber sized
  trees than other methods

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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