Uneven-aged Regeneration Systems
Uneven-aged Regeneration Systems
•
Uneven-aged regeneration systems often referred to as
selection systems also called
–
This is not equivalent to "selective" cutting, as the term is commonly
used
–
“Selective" logging and "select-cut" merely mean that the harvest is
not a clearcut
• These terms are imprecise
• They could be referring to a thinning, to a shelterwood
establishment cutting, or to a high-grading cut
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
Characteristics of Uneven-aged Systems
•
Maintains a continuous high forest cover
–
–
•
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 sawtimber sized trees
–
Pulpwood production is relatively low
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:
– If the stand is balanced, each harvest should remove an amount
equivalent to the growth produced since the last harvest
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
– Harvests occur regularly at short intervals throughout the rotation
– Cutting cycle is normally between 5 to 20 years
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
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"
Approaches to regulation in the selection method and maintaining
a balanced stand with sustainable yield
1. Area regulation
2. Volume regulation
3. Structural regulation
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:
Stand Area
Cutting Cycle
Rotation Length
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
• Structural regulation: use a reverse J-shaped curve of
residual diameter distribution as a guide.
Balance vs. Irregular (unbalanced) uneven-aged stands
Structural regulation and Guiding 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 shape (steepness or flatness) of
the structural regulation guiding curve and is called the q
factor (or quotient)
q=
Ni
N i 1
where,
Ni = number of trees in the ith diameter class
Ni+1 = number of trees in next largest diameter class
Influence of q on Target Diameter Distribution
140
q = 1.3
q = 1.5
q = 1.7
Trees per Acre
120
100
80
60
40
20
0
2
4
6
8
10
12
14
16
18
20
Diameter Class (in)
• 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)
Uneven-aged Regeneration Methods
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
Variations of the Selection Method
Single Tree Selection
•
More commonly applied in:
– Shade tolerant species
•
Norway spruce, beech, silver fir (central Europe)
•
Sugar maple, American beech, birch (Northern hardwoods)
– Restrictive sites where pronounced seasonal water limitations favor
natural monocultures
•
Ponderosa pine
•
Has been used for other forest types
– Upland oak forests of the Missouri Ozarks (Pioneer Forest, MO)
– Loblolly-shortleaf pine (Crossett Experimental Forest, AR)
– Longleaf pine, southern Coastal Plain region
Variations of the Selection Method
Single Tree Selection
•
Central and southern upland and bottomland hardwoods
– Generally, without intensive competitor control, single tree selection
has resulted in a transition to shade tolerant species
Depiction of Uneven-age Stand Managed Using Individual Tree
Selection
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
Group Selection Method
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 shading effects of edge, best success and growth of intolerant
seedlings may be restricted to 2/3 or less of the area in a small opening
• Group selection in the Central Hardwood Region generally
uses open sizes between 1 and 2 times the height of
surrounding trees
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
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 uncut stand areas between group
openings
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 objectives—wildlife openings,
aesthetics, salvage/sanitation
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
Potential Objectives/Benefits in Using a Selection System
•
Can provide frequent periodic income from the stand 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
Potential Objectives/Benefits in Using a Selection System
•
Maintains high diversity within the stand
–
•
Maintains good site protection
–
•
usually provides good wildlife habitat for many, but not all species
although frequent logging may result in increased soil damage on
sensitive sites
Maintains pleasing aesthetics without time gaps
Potential Drawbacks/Disadvantages of Selection Systems
•
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
Two-Aged (Hybrid) Silvicultural Systems
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
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 shadetolerant species
• Improved aesthetics compared to clearcutting
• Increased structural diversity and retention of habitat
components compared to clearcutting
Benefits of a Two-Age System
• Increased initial revenue compared to other types of nonclearcut 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
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
– Reserve trees must be carefully selected
• Lack of appropriate long-lived species to maintain the system
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
Irregular or Reserve Shelterwood:
•
Leaves residual overstory for an extended period of time
into new rotation – creates two-aged stand
–
In central hardwood region, reserve tree density is approximately 10
to 15 ft2 ac-1 of basal area
•
Has ecological/aesthetic vs. economic/operational tradeoffs
•
Characteristics of reserve trees are important
Irregular/Reserve Shelterwood
Uncut Stand
Irregular/Reserve Shelterwood
Uncut Stand
Establishment Cut
(45-60 ft2 ac-1 BA)
Irregular/Reserve Shelterwood
Uncut Stand
Establishment Cut
(45-60 ft2 ac-1 BA)
Reserve trees
(10-15 ft2 ac-1 BA)
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
Reserve Tree Criteria
• Long-lived commercial species
• Appropriate crown characteristics including live crown ratios
(typically > 40 for hardwoods), well-balanced crown
proportions and overall crown vigor
• Stem form and maintenance of potential veneer or high-quality
sawtimber
• Ability to withstand harvest
• Located to avoid wind-throw and other post-harvest
perturbations
Other Partial Cuttings
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
Other Partial Cuttings
(non-silvicultural treatments)
• Non-silviculture, exploitative partial cutting treatments
– Commercial clearcutting: removal of only commercially salable trees
– High-grading: removal of choice species or trees larger than a specified
diameter limit if they fit common utilization standards
– Diameter-limit cuttings: removal of all trees larger than a specified
diameter
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
– In the Central Hardwood Region, repeated exploitative cutting yield a degraded
stand composed primary of low-value, shade-tolerant species
• Ignores silvical requirements of desired species with respect to
regeneration and long-term growth
• Removes acceptable growing stock and leaves defective and
unhealthy trees