Intermediate Silvicultural Treatments
Summary of Intermediate Stand Treatments
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Thinning
Release
Improvement cutting (TSI)
Sanitation and Salvage
Fertilization
Pruning
Prescribed Fire
How intermediate treatments fit into a silvicultural system
Components of
even-aged
silvicultural systems
Stand Initiation
How intermediate treatments fit into a silvicultural system
Textbook Chapters by Topic
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Thinning: Ch 17, 18, 19
Release Treatments: Ch 16
Improvement, Sanitation and Salvage: Ch 22
Pruning: Ch 20
Thinning
What is Thinning?
• Defined by Nyland as:
“A treatment to increase the diameter increment of residual
trees, improve stand quality and health, and increase stand
level production by cutting excess and potential mortality
trees without permanently breaking the crown canopy”
When we thin a stand…
• We are reallocating growing space to the residual trees
• Which residual trees you choose depends on your objectives
Why do we thin?
• Residual trees increase DBH faster, maintain larger crowns and higher
vigor
• Reduces rotation length (age) required to produce trees of a given diameter
• Typically improves average quality of trees (more valuable)
• Provide intermediate income
• May avoid stagnation in densely crowded stands
• Reduce wood losses to mortality
• Reduce a stand's susceptibility to insect infestation
• Increase potential seed production
• Improve wildlife food production and availability
• Improve aesthetics, by creating a more open, park-like appearance
Effects of Thinning on Individual Growth
• Thinning increases diameter growth
– By allocating more growing space to residual trees, they are able to
capture more above and belowground resources
1959
1969
1979
Effects of Thinning on Individual Growth
• Species and canopy position influence magnitude of diameter growth
response
Effect of Thinning on Stand Growth
• Thinning recovers the yield in potential mortality trees and
excess growing stock
• Increases total realized sawtimber yield over an even-aged
rotation
– Net Volume Yield = Final Harvest Volume + Volume Removed in
Previous Thinnings
• Improved growth on residual trees compensates for the
increment lost from cut trees
– Ensuring full net production in stands with adequate residual stocking
Effect of Thinning on Stand Growth
Net Standing Volume
Production function for an even-aged stand thinned three times
Effect of Thinning on Stand Growth
Effect of Thinning on Stand Growth
Effect of Thinning on Stand Growth
Effect of Thinning on Stand Growth
Effects of Thinning
• Four major factors influencing long-term growth following
thinning:
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2.
3.
4.
Timing of each thinning
Crown condition and position of residual trees
Post-thinning spacing and its influence over crown vigor and
development
Interval between successive thinnings
• Timing of thinning effect diameter growth responses
Early thinning can yield increased diameter growth when
compared to later or no thinning
• Timing of thinning and site quality impact cumulative stand
yield
• Period of post-thinning growth increase vary due to spacing
and a tree’s competitive position.
Effects of Thinning
• Some potential negative impacts from thinning
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Tends to reduce total biomass production
Damage residual stems and site
May increase insect attack and disease losses
Temporarily increase wildfire hazard due to increased fuel load
Reduce tree quality due to increase limb retention and epicormic
branching
– Increased danger of wind and ice damage due to sudden exposure
What is epicormic
branching?
Methods of Thinning
• Primary thinning methods
1. Low thinning
2. Crown thinning
3. Selection thinning
4. Geometric thinning
5. Free thinning
Methods of Thinning: Low Thinning
• Low thinning
– Trees are removed from the lower crown classes
– Must be very heavy or done early and frequently to appreciably
increase growth of upper crown class trees
– Most applicable in stands which nearly all trees are merchantable
Methods of Thinning: Low Thinning
• Low thinning intensities
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Grade A (very light): remove overtopped
Grade B (light): remove overtopped and intermediate
Grade C (moderate): remove overtopped, intermediate, and some co-dominant
Grade D (Heavy): remove overtopped, intermediate, and most codominant
• Other names for low thinning:
• Thinning from below
• German
Methods of Thinning: Crown Thinning
• Crown thinning
– Removes trees from upper crown classes to favor the best trees in
those classes
• Removes mostly co-dominant trees, but intermediate or dominant trees
interfering with desired residuals also removed
– Unlike low thinning, no matter how lightly applied, principle cutting is
made in the upper crown classes
– Can provides greater income than low thinning
– Other names for crown thinning
• Thinning from above
• French
Methods of Thinning: Selection Thinning
• Selection thinning
– Removes dominant crown class trees in order to favor the
growth of trees in lower crown classes
• Trees favored in crown and low thinning are likely to be cut in selection
thinning
• If incorrectly applied, leads to high-grading and reductions in residual
stand growth and quality
– Most commonly used to remove poorly formed dominant
trees
– Applied in some two-layered stands to remove taller layer
– Repeated selection thinning is only appropriate for shade
tolerant and negatively geotropic conifers
Methods of Thinning: Geometric Thinning
• Geometric thinning
– Trees to be cut are chosen strictly on the basis of spacing or pattern
with no regard to crown class
– This results in removal from all crown classes and retention of trees in
all crown classes
– Most appropriate where there is little differentiation in crowns or in tree
quality
– Usually applied only as a first thinning in young stands
Methods of Thinning: Geometric Thinning
– Row thinning removes strictly by rows in a plantation
• Every fifth row is common practice
• Provides easy access for large machinery
• Provides no improvement in stand quality
A geometric thin performed by removing every 5th row in a 14-year-old slash pine plantation
in Georgia. Photo Credit: David Moorhead, UGA, Bugwood.org
After 5th row thinning with thin from below in leave rows in a 14-year old stand.
Initial Basal Area = 140 square feet per acre, Residual Basal Area = 70 square
feet per acre.
Methods of Thinning: Geometric Thinning
– Strip thinning (or corridor thinning) cuts trees in a strip of
set width
• Applied most commonly for precommercial thinning in young
overcrowded natural or direct seeded stands
• May use heavy mower or drum chopper in seedling stands
Aspen stand, 15 ft wide machine corridor, first summer post-harvest.
Methods of Thinning: Free Thinning
• Free thinning
– Objective is to select the best trees, regardless of canopy position, and
release them
– Differs from crown thinning in that spacing across the site need not be
uniform
– Typically, a set number of crop trees are released (e.g. 50 or 100)
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Number of crop trees should be sufficient to fully stock the stand at the end of the
rotation
Thinning Regimes
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Whether to thin?
When to thin?
How much to thin?
What thinning method do we use?
Often tough questions. Objectives, economics, growth rates, and
current stand conditions will drive the decision.
Timing of Thinning - When do we thin?
Merchantability
• To avoid capital investment, a commercial operation is
desirable rather than pre-commercial
– Southern pines:
• Commercial thinning tends to be possible at about age 12, depending on site quality
and initial density
– Upland hardwoods:
• Commercial thinning occurs later than for conifers due to slower diameter growth in
dense stands and higher minimum merchantability requirements
Timing of Thinning - When do we thin?
Objectives
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Pulpwood: thin late or preferably not at all
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Sawtimber: concentrate growth on best trees as soon as
possible
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Density must be sufficient to encourage self pruning to at least 17 feet
Wildlife: in general, thin early, frequently, and heavily from
below to increase light levels in the understory and increase
growth of shrubs, grasses, and forbs.
Timing of Thinning - When do we thin?
Site Quality
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Thinning can occur earlier on more productive sites
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May chose to thin only on sites of medium to high productivity
Timing of Thinning - When do we thin?
Live Crown Ratio
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Best simple indicator of tree vigor and ability to respond to thinning
What is Live Crown Ratio?
Timing of Thinning - When do we thin?
Live Crown Ratio
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Critical threshold for crown ratio of overstory trees may dictate when
thinning occurs
Example for southern pines
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First thinning, attempt to keep most potential crop trees at least above
35% LCR;
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40-45% is better for a high growth rate, but this slows self pruning
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For older stands, seek to keep LCR above 30% (higher for more
diameter growth)
How much do we thin?
Choosing a Residual Density
Objectives
• Pulpwood: maintain high residual density (light or no cut)
• Sawtimber: lower residual density to keep diameter increment
higher
– Density should be high enough to maintain good self pruning
• Wildlife or grazing: low density to encourage understory
vegetation
How much do we thin?
Choosing a Residual Density
Live Crown Ratio
• If crowns are small, thin lightly and frequent
Site Quality
• More flexibility in residual density is possible on good sites
– Productive sites can carry higher density without losing vigor, yet
openings close rapidly
Stand Density and Stocking
Quantitative tools for determining how much we should thin
• Stand density is an absolute measurement based on basal area or number of
trees per acre
– Stand density reflects the degree of crowding of stems within a stand
• Stocking is a relative term that relates a given stand density to a
management objective
– Because size-density relationships influence the amount of available growing
space in a stand
• Two stands with equal stand density may not have an equivalent stocking
– Example: 100, 4 inch trees do not occupy an equal amount of
growing space as 100, 18 inch trees
Simple Measures of Stand Density
• Trees per acre
– Simple
– Not helpful when describing uneven-aged stands but useful for
plantations
• Basal area per acre
– Basal area is a combined measure of both, number of trees and their
size
– Widely used because it is:
• Simple to measure via point sampling
• Highly correlated with volume and growth of even-aged forest stands
• Works well for single species plantations
Stocking
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Stocking Chart for Upland Hardwoods (Gingrich 1967)
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A graphical tool to allocate growing space on a relative basis for
upland hardwood stands in the Central Hardwood Forest Region
Properties of the Upland Hardwood Stocking Chart
Average Maximum Density
(A-Line) is set at 100%
Stocking
Shows the upper limit of a
fully stocked stand
Properties of the Upland Hardwood Stocking Chart
Area between the A-Line and BLine indicates that a stand is fully
stocked
A similar level of merchantable
volume produced within the
range of full stocking
Properties of the Upland Hardwood Stocking Chart
B-Line identifies the minimum density
of full site occupancy
Typically, residual stocking for
thinning in upland oak stands would be
at or just above B-Line
Below B-Line risks increasing
epicormic branching
Properties of the Upland Hardwood Stocking Chart
C-line is the lowest stocking that will
grow to the B-line within ten years
Thinning in the Central Hardwood Forest Region
• Silvicultural approaches:
1. Area-wide thinning to a target residual stocking
• Crown thinning preferred over low thinning
2. Crop tree release
• Free thinning that releases a predetermined number of crop trees
• Unlike area-wide thinning, a target residual stocking percentage is
not defined
– Crop tree release should not result in an understocked stand
• Preferred over area-wide thinning in cases where stand value is
concentrated on a small percentage of trees
Area-wide Thinning in the
Central Hardwood Forest Region
• When to thin
– Commercial thinning may begin as early as 30 years in
upland oak stands on site index 80 or better
• Site index 70 may have to delay the first cut until age 35
• Site index 60 we should be able to start by age 40
• Age of first commercial thinning could be between 40 to 50 if no
market for small diameter (< 12 in) wood exists
Area-wide Thinning in the
Central Hardwood Forest Region
• How much to thin
– Regardless of the site, the first thinning in young stands should be as
heavy as possible.
• Young stands respond most to thinning and the crowns close very quickly
– Typically, residual stocking for area-wide thinning in upland oak stands
would be at or just above B-Line
– The stand should approach average maximum density (A-line stocking)
before subsequent thinning
– Aim for a well stocked stand with at least 50 high quality trees per acre
for the final harvest cut
Area-wide Thinning in the Central Hardwood
Forest Region
• How Often to Thin
– Frequency of thinning depends on the intensity of the first
and subsequent cuts, site index, and rotation age
– Rotation age varies with site index if our goal is to produce
high quality sawlogs
• 80 year rotation for site index 80
• 100 year rotation for site index 70
• 120 year rotation for site 60
• Shorter rotations may not produce enough sawtimber size trees
Area-wide Thinning Mature Stands in the Central
Hardwood Forest Region
• Foresters generally consider upland oak stands mature when
they are 80 to 100 years old or have reached a specified
rotation age
• However, by the time stands are 50 to 60 years old and in the
large pole/small sawtimber size, they have generally slowed in
height growth, their annual basal area growth has leveled off,
and except for size, they have many of the characteristics of
older stands
Area-wide Thinning Mature Stands in the Central
Hardwood Forest Region
• In older stands with 80 percent or more stocking, a very light
thinning or improvement cut can be used to remove some cull
trees, undesired species, or some short-lived species or old
residuals from past logging
– Do not reduce stocking below 75 percent and do not make large holes
in the stand
– Trees 60 years old and older in dense stands will have relatively small
crowns and do not respond well to release
– Epicormic branching may become a problem when trees this age are
released too much
Area-wide Thinning Mature Stands in the
Central Hardwood Forest Region
• If current stocking is 60 to 80 percent, it is usually best to hold
off on thinning
– Any cutting in such stands will probably not benefit the entire stand
and may allow an unwanted understory to develop rapidly
– By simply waiting for about 10 years, stocking will increase and you
may be able to get an operable cut without depleting stocking
Crop Tree Release in the Central Hardwood
Forest Region
Miller, G.W., J.W. Stringer, and D.C. Mercker. 2007. Technical guide to crop tree release in hardwood forests. University
of Kentucky, Cooperative Extension Service. Southern Regional Extension Forestry Publication, Professional Hardwood
Notes, FOR-106. 23 p.
Crop Tree Release
• What is Crop Tree Release (CTR)?
– CTR is an intermediate silvicultural treatment and type of free thinning
– CTR is intended to provided increased growing space through the
removal of crown competition from adjacent trees
– CTR assures most site resources are focused on a small number of
selected trees rather than being widely distributed among all residual
trees
• ‘Low thinning’ and ‘crown thinning’ methods intend to reach area-wide residual
density targeting specific size or crown class trees
Crop Tree Release
Increasing growing space available to individual trees
Crop Tree Characteristics by Management Objective
How Crop Trees Respond to Release
• Increased growing space provides more light, water, and nutrients
• Released crop trees respond with faster root and crown growth, then faster
dbh and volume growth
• Although CTR can produce a significant response in the first growing
season, maximum growth usually occurs 2 to 3 years after release
Approach to Releasing Crop Trees
• “Crown-touching” release applied to deaden or fell adjacent
competing trees whose crowns touch that of the crop tree
Approach to Releasing Crop Trees
• Lower crown class trees
need not be removed
– Minimal influence on crop tree
growth
– Help discourage epicormic
branching
Intensity of Release and Crop Tree Growth
Crop Tree Release
• How many crop trees to manage?
– Nearly all economic value in hardwood stands is found in a relatively
small number of trees
– Maximum of 60 to 70 crop trees per acre
• Release strategies:
– Full crown-touching release for rapidly growing species and young
trees in the sapling/pole stage
– Less than a full crown-touching release (at least three sides released)
for small sawtimber to limit the risk of epicormic branching
– Sub-canopy trees retained to protect crop trees and add other benefits to
the stand, unless they conflict with management objectives
Economic value in hardwood stands found in a relatively small
number of trees
Declining number of crop trees
with increasing stand age
Timing of Crop Tree Release Treatments
• Best time for application is when canopy begins to close or
within 10 to 15 years following canopy closure
– High quality sites, canopy closure 8 to10 years
– Poorer sites, canopy closure 13 to 15
• Can be successful used in large poletimber and sawtimber
stands to promote vigor in selected crop trees
– Number of crop trees greatly diminished in older stands
Application Techniques for Crop Tree Release
• Mechanical fell or fell and herbicide stump treatment
– Difficult to fell trees in young stands
• Chainsaw girdle: double girdle 1” deep, 6 in apart
– Diffuse porous trees are poor candidates for girdling without herbicide
due to there resilience to girdling
• Cut surface herbicide application
• Basal bark spray (trees < 6 in)
• Cost: $40 to 60 per acre ($100 to $150 per acre contract cost)
• Focus on best available crop trees regardless of spacing
– Arbitrary spacing rules may lead to inefficient investments in
substandard trees
Benefits of Crop Tree Release
• Increase diameter growth and volume of crop trees
• Increases stand value by increasing the proportion of highvalue crop trees
• Greatest economic gains are when large differences exist
between values of crop trees and their competitors due to
species or quality
Risks Associated with Crop Tree Release
• Epicormic branching
– May reduce log grade and tree value
• Windthrow or damage from ice or snow