Transcript Multi-Aged and Irregular Silviculture Methods

Multiaged Systems
Uneven-aged and Irregular Silviculture
Uneven-aged Regeneration Systems
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Uneven-aged regeneration systems often referred to as
selection systems
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Not equivalent to "selective" cutting
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“Selective" logging and "select-cut" merely means harvest is not a
clearcut
• Terms are imprecise as they could refer to systematic silvicultural
methods or exploitive high-grading
Characteristics of Uneven-aged Systems
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Selection methods produce an uneven-aged stand (with at
least 3 age classes or distinct cohorts)
Characteristics of Uneven-aged Systems
• Maintains continuous high forest cover
• Typically emphasizes sawtimber production
• Regulation methods allow sustained yield at recurring intervals
– If balanced, each harvest removes amount equivalent to
growth produced since the last harvest
• Useful for putting an irregular stand under productive
management without losing existing stocking
Characteristics of Uneven-aged Systems
• Rotation length is the average time period required to
obtain trees of 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
• Intermediate treatments should be completed to ensure
continued recruitment and favorable growth among residual
trees
• For sustained yield, systems requires frequent and accurate
inventory
Potential Objectives/Benefits in Using a Selection System
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Maintains structural diversity within stand
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Maintains good site protection
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although frequent logging may result in increased soil damage on
sensitive sites
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Pleasing aesthetics without time gaps
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Frequent periodic income
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Good flexibility
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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 Drawbacks/Disadvantages of Selection Systems
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Involves a high level of complexity, requires higher
management costs than other methods
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Produces less pulpwood than other methods
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Harvesting more difficult and costly per unit area than with
even-aged methods
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Results in more logging damage to residual trees than with
even-aged methods
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Due to frequent entry of harvesting equipment
Leads to long-term depletion of growing stock if not applied
carefully
Characteristics of Uneven-aged Systems
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Uneven-aged regeneration methods provide openings for
establishment and recruitment of new age classes (cohorts)
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Selection methods are traditionally classified as either
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Single-tree selection
Group selection
In practice, selection methods can create a continuum from
very small to large regeneration openings
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
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More commonly applied in:
– Shade tolerant species
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Norway spruce, beech, silver fir (central Europe)
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Sugar maple, American beech, birch (Northern hardwoods)
– Restrictive sites where pronounced seasonal water limitations favor
natural monocultures
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Ponderosa pine
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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
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Central and southern upland and bottomland hardwoods
– Generally, without intensive competitor control, single tree selection
has resulted in a transition to shade tolerant species
Variations of the Selection Method
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Group Selection: removes clusters of adjacent trees from a
predetermined proportion of the stand area
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Group selection was developed to create larger openings needed to
regenerate shade-intolerant and intermediate species
Application of group selection
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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
– In the Central Hardwood Forest Region, opening sizes are typically
between 1 and 2 times the height of surrounding trees
• Locate harvest groups among the oldest or largest trees in the
stand
• Appropriate tool for other objectives—wildlife openings,
aesthetics, salvage/sanitation
Application of group selection
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Opening shape to fit the stand conditions or to maximize
objectives/constraints
– Rectangular openings will be more efficient for logging than circular
– Rectangular openings provide more sun if oriented with their long
axes east-west
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Group selection is easier to plan and keep the stand balanced
than with single-tree
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Logging is more efficient and less damaging to residual trees
than with single-tree
Application of group selection
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Complete felling of all trees in the openings is crucial to
allow for good regeneration
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Control of undesirable species should be considered
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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
• 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
• Openings in group selection should not exceed 2 times the height of
adjacent mature trees
• Difficult (or impossible) to locate groups within a stand
following second or third entry
Regulating Selection Systems
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.
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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
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4
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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)
Silvicultural Approaches to Creating Irregular Stand
Structures
Creating Irregular Stand Structures
• Irregular silviculture seeks to create stands with:
– Continuous canopy cover
– Variation in age structure and spatial arrangement
– Multilayered canopies with tree crowns at various height levels
(stratum)
– Practices increase or create heterogeneity within stands and allow
complexity to develop through time
• While uneven-aged selection systems create irregular stands,
all irregular silvicultural practices do not necessarily seek to:
– Balance the age classes
– Maintain stable diameter distributions
– Realize consistent yield at regular intervals (cutting cycles) through time
Creating Irregular Stand Structures
• Irregular silviculture differs from selective cutting (an
exploitive strategy) by:
– Tending and regenerating economically and ecologically important
species and sustaining values and interests through time
– Practices do not compromise future production or ecological function
• While irregular silviculture does not have to closely regulate
spatial or temporal arrangements of age classes, it must:
– Plan for deliberate regeneration of replacement trees to maintain the
irregular age class and structural attributes
– Invest in tending to nurture the recruitment and growth of target trees at
different stages of development
– Make treatments financially feasible and provide a revenue stream to pay
ownership and management costs
Creating Irregular Stand Structures
• First step in developing irregular stand structures is to create a
two-aged stand
• Irregular silviculture practices are referred to as:
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Continuous cover forestry
Femelschlag
Dauerwald
Close to nature forestry (Pro Silva Europe)
Ecological forestry
Variable retention harvesting
– Methods utilized in irregular silviculture include: selection systems
(group and single-tree), irregular and group shelterwoods, variable
density-thinning, “free” selection
Creating Irregular
Stand Structures
Creating Irregular
Stand Structures
Creating Irregular
Stand Structures
Two-Aged Silviculture
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
Benefits of a Two-Aged System
• Increased structural diversity and retention of habitat
components compared to even-aged methods
• Production of a wide range of forest products from pulp to
large-diameter sawtimber in the same stand at the same time
• Ability to regenerate shade-intolerant and intermediate shadetolerant species
• Improved aesthetics compared to clearcutting
Benefits of a Two-Aged System
• Increased initial revenue compared to other types of nonclearcut regeneration techniques
• Development of old-growth structural characteristics
• Maintenance of seed production in reserve trees throughout the
entire rotation
• 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
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
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
Two-aged Regeneration Methods
• Two-aged stands can be created in a single treatment or
through multiple entries
– Single entry: deferment or leave-tree methods
– Multiple entry: Reserve shelterwood
• Basal area of reserve trees (i.e., the oldest age class) does not
typically exceed 25 ft2 ac-1
Deferment or Leave-Tree Approach
Uncut Stand
Reserve trees
(10-15 ft2 ac-1 BA)
Deferment or Leave-Tree Approach
Reserve Shelterwood
Uncut Stand
Establishment Cut*
(45-60 ft2 ac-1 BA)
Reserve trees
(10-15 ft2 ac-1 BA)
*Similar to the uniform shelterwood (even-aged), an optional preparatory cut
may proceed the establishment cut
Variable Retention Harvesting
Variable Retention Harvesting
• Variable retention harvesting aims to retain legacies from the
past and enrich the new community with structural features
reminiscent of older forests
• With regeneration units of older stands,
– Retain large and decedent trees, maintain course woody debris, and
reduce disturbances to forest floor
– Retained attributes as dispersed elements or aggregated in small patches
of residual forest
– Results in an irregular forest of at least two ages
Variable Retention Harvesting
• In intermediate aged stands
– Intermix thinned patches with variable levels of residual stocking
• Stimulates residual tree development, creates horizontal
heterogeneity in stand conditions and structure, and establishes gaps
of various sizes to establish and release regeneration
• Attempts to mimic disturbances considered natural for the region
Variable Retention Harvesting
Dispersed Retention
Variable Retention Harvesting
Aggregated Retention
Irregular Shelterwood Systems
• Three general classifications:
– Expanding-gap irregular shelterwood
– Continuous cover irregular shelterwood
– Extended irregular shelterwood
Irregular Shelterwood Systems
Expanding-gap irregular shelterwood “Aims to regenerate new cohorts in groups that are gradually
enlarged until the stand is totally removed”
Expanding-gap irregular shelterwood
Expanding-gap irregular shelterwood
Initial Gaps
Expanding-gap irregular shelterwood
Preparatory/establishment cutting
around gaps
Expanding-gap irregular shelterwood
Preparatory/establishment cutting following gap expansions
Expanding-gap irregular shelterwood
Final removal of areas receiving preparatory/establishment cutting
Expanding-gap Irregular Shelterwood
Irregular Shelterwood Systems
Continuous cover irregular shelterwood –
“Sequence of cuttings is applied more freely in space and
time, which permits maintenance of a multicohort structure
and a continuous forest cover ”
Continuous cover irregular shelterwood
Irregular Shelterwood Systems
Extended Irregular Shelterwood –
“Aims to regenerate the whole stand while … two cohorts
are maintained for at least 20% of the rotation length”
Extended irregular shelterwood
Other Partial Cutting
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 (selective) 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
Rehabilitating Cutover Stands
• Stand assessment initial step in rehabilitating cutover stands
• Assessment determines:
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Abundance and distribution of suitable residual trees and their potential
Status of advance reproduction
Patterns of interfering understory vegetation
Characteristics of suitable residuals
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At least lower codominant
> 20 to 25% live crown
No (to few) epicormic branches on lower bole
No apparent decay or damage of the main stem
< 25% of major branches dead or dying
< 10% lean
Rehabilitating Cutover Stands
• High degree of stocking variability and patchiness makes stand-wide
treatments impractical in many cases and prescription may use combination
of methods within a stand
Silvicultural Methods for Rehabilitation
• Complete tree removal as regeneration method for places
lacking acceptable growing stock
• Liberation cutting to release a younger age class already in
place
• Leaving widely space trees as shelterwood establishment cut
or initiate a two-aged arrangement
Silvicultural Methods for Rehabilitation (cont.)
• Thinning even-aged patches with acceptable growing stock
• Releasing widely spaced crop trees within main canopy
without additional tending between (i.e., crop-tree release)
• Low-density selection systems for cutover uneven-aged areas