Transcript Lectures: Even-aged Regeneration Methods

Common Even-Aged Systems
Clearcut
Seed Tree
Shelterwood
Clearcutting
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Clearcutting: A method of regenerating an even aged stand
in which a new age class develops in a fully exposed
microclimate after removal, in a single cutting, of all trees in
the previous stand.
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Regeneration is from natural seeding, direct seeding,
planted seedlings, and/or advance reproduction.
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Silvicultural clearcuts differ from ‘commercial clearcuts’
– The first removes all trees, the second only merchantable
trees
How Clearcutting Changes the Microenvironment
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Full sunlight conditions
Air and soil temperature near the surface increases
Humidity decreases and surface evaporation increases
Soil moisture increases because transpiration decreases
Precipitation interception decreases, more water reaches the surface
Water infiltration and percolation increases; subsurface flow increases
Decomposition increases (warmer and wetter), releasing more nutrients
Nutrients not taken up or bound to soil leach out of system
Clearcutting
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Edge effect
– Moisture increases on a gradient for 30-40 feet into a
clearing and then levels off
– Shade (in the northern hemisphere) is more pronounced
on the south edge of the clearing. East to west shade
depends on time of day.
Clearcutting
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Alternate clearcutting arrangements
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Strip clearcut
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Block clearcut
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Patch clearcut
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Use of alternative methods:
– Ensure good seed rain
– Manage shade patterns
– Protect against wind or ice/snow
– Improve aesthetics or meet policy-based constraints
Alternative Clearcutting Approaches
Block clearcutting
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All trees are removed in a single operation
Size limitations are based on policy and site conditions, not on
regeneration constraints
Alternative Clearcutting Approaches
Alternate strip clearcut
Progressive strip clearcut
Strip clearcuts, alternate or otherwise, are best oriented at right angles
to prevailing winds. The width of the strips will depend on seedfall
distances for the preferred species, wind hazard, and other factors
Alternative Clearcutting Approaches
Patch clearcutting
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Stand is regenerated in a series of clearcuttings made in patches
Patch size influences light availability within the patch and
should be chosen to match species silvics
Other considerations when using even-aged methods
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Stream Side Management Zones (SMZ’s)
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Typically leave an unharvested or partially harvest buffer
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Legacy trees
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Travel corridors for wildlife
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Management of viewscapes
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Alter shapes, adjacencies
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Orientation on landscape
Aesthetic buffers
Avoid straight edges and square corners
Limit harvest size
Clearcutting and Site Preparation
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Site preparation considerations for natural regeneration
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Some important questions
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Is it needed or would it be detrimental?
Do you need it for a desired species?
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Does species need a mineral seedbed
Site preparation and clearcutting: Considerations for natural
regeneration
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Scarification → mineral seedbed
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Control slash residues
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Partial shade or browse protection afforded by slash
Control slash cover to manage seed eating mammals
Mechanical or chemical vegetation control
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Competing vegetation that may inhibit or delay regeneration and
effect subsequent growth rates
Advantages Clearcutting with Natural Regeneration
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Good method for most shade intolerant species
Commercially attractive
Ease of administration and implementation of regulated forest
Clean site eases site preparation
Easy machine access eases harvesting
Total overstory removal reduces some pests (e.g. dwarf mistletoe)
Facilitates regeneration of species with serrotinous cones
Precludes blow down
Increases herbaceous cover (browse and cover for many wildlife species)
Shortcomings of Clearcutting with Natural Regeneration
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Problems with dependable seed sources and seedling establishment
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Issues associated with no high forest cover and high light environment
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Seed shortage limits regeneration to light seeded species
Poor seed years may lead to regeneration failure or irregular stocking
Overstory removal limits within stand seed production following harvest
Density and uniformity of a species is difficult to control
Lack of cover may adversely impact some tree species and may increase
competition by herbaceous and shrubs
Dense competition may require costly site preparation
Cold air drainage may damage reproduction
Dry sites may not have sufficient surface moisture to support germination
Reduced chance for genetic improvement
Shortcomings of Clearcutting with Natural Regeneration
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Impacts on soils and hydrology
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Wet sites may become wetter
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Wet soils may become unstable on steep slopes
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Mineral soil exposure may increase soil erosion
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Increased decomposition rates and potential nutrient leaching
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Decreased visual aesthetics
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Increased fuel loading and fire danger
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Decreased wildlife habitat for some species
Coppice Silviculture
Coppice
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The term "coppice" is commonly applied to any regeneration
arising from sprouts or suckers—typically hardwoods of
young to moderate age
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As a method, it is where regeneration is solely from sprouts
or root-suckers
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Associated with short rotation production of pulpwood or
fuelwood
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Historically associated with charcoal iron production
Coppice
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Some coppice principles:
– Low stumps produce better quality sprouts
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Best sprouts originate from the root collar
Sprouting vigor tends to decline with age and size of
stems
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smaller stems, better sprouting
Sprouting is most vigorous from dormant season cutting
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Least vigorous from late spring cutting
Coppice
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Coppice for energy, bioremediation, environmental cleanup
– Repeated crops without replanting
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Vegetative propagation maintains genetic integrity of
plantation
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Increased growth rates allow large volume production on
limited land base
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Short cycle provides quick return on investment
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Second and third rotation often produces greater biomass
in shorter time frame due to multiple stems from sprouts
Coppice
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The cutting cycle is set by when the MAI intersects PAI
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General shortcomings of coppice systems
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Financial success depends on access to markets for small diameter wood
Serve limited set of management goals
Frequent entry requires extra caution to minimize soil disturbance and may
increase loss of soil nutrients after repeated harvests
Coppices susceptible to freezing and browse
It takes time to convert from coppice to high forest methods
Coppice stands have limited non-market values
General shortcomings of short-rotation biomass plantations
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Require guaranteed markets
Require fertile soils with abundant moisture as well as fertilization to
maintain critical nutrients
May require protection from browse
Mechanized systems needed for efficient harvesting require fairly level sites
with uniform surfaces and highly trafficable soils.
Coppice with standards: scattered, individual stems allowed to
grow on through several coppice cycles
Seed Tree Method
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Definition: even-aged method retaining widely spaced,
uniformly distributed seed bearing trees
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Reproduction source: from seeds disseminated from trees left
after harvest
Seed Tree Method
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Remaining seed trees may be removed after suitable
regeneration is established, but this is not necessary to the
method's application
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Produces an even-aged stand
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Inherently works well for wind dispersed species, but not
hard seeded trees such as oaks and hickories
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The method removes size constraints on the regeneration area
(also shape and orientation issues)
Seed Tree Regeneration Method
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Regeneration must be established in a short period of years,
or else the site will be occupied by other plants
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Produces early successional conditions on the site (the same
as a clearcut):
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High light levels, high exposure to wind, and extremes in
temperature at ground level.
Retained trees do not provide enough canopy cover to alter the
stands microenvironment in comparison to open condition
– Density of retained trees that would alter microenvironment
is species-specific
Key considerations for the seed tree method
Number and spacing of seed trees depends on:
– Size and species of seed trees
– Amount of viable seed per tree
– Percent of seed trees that may survive
– Percent of seed that produces an established seedling
Considerations for number and spacing:
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Distance to which seed from desired species can be dispersed to fully stock
an area
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Nature of the seedbed
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Do not exceed maximum dispersal distances
If unfavorable (e.g., heavy duff or sandy topsoil), leave more seed trees (but,
better to prepare it by fire or disking)
Anticipated competition level
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Increase the number of seed trees if there will be a high competition level with
no or inadequate competition control
 Above all, know the silvics of species to be retained
Considerations for number and spacing:
• Light seeded trees can disseminate 2 to 5 times their height
• Amount of viable seed is usually limiting factor
• Influence of spacing on pollination alter total seed production
• Because of year-to-year variation in seed production, it is best
to ensure enough reserve trees to restock area in one moderate
seed year
• Usually, 4 to 20 trees per acre retained
8 seed trees per acre in a loblolly-shortleaf pine stand. Arkansas.
Recommended minimum number of seed trees for major
southern pines, by DBH class. Number per acre. (Average
distance between trees, in ft, shown in parentheses). Will provide
value for commercial removal
Species
DBH
(inches)
Loblolly
Shortleaf
Slash
Virginia
9
10
12
14
16+
NA
12 (60)
9 (69)
6 (85)
4 (104)
NA
20 (47)
14 (56)
12 (60)
12 (60)
NA
12 (60)
9 (69)
6 (85)
4 (104)
6 (85)
5 (93)
4 (104)
4 (104)
4 (104)
Characteristics of Quality Leave Trees
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Windfirmness
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Shallow rooted trees or species with weak wood are not desirable
Wide, deep crowns, with high live crown ratio
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Indicators of vigorously growing trees
Dominant or better codominant crown class
Seed production is linked to crown area
Height
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Height can influence distance of seed dispersion
Age
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Must be old enough to produce seed
Cutting Strategies – Seed Tree System
• Preparatory Cut: Optional initial treatment to increase tree vigor and seed
production
• Establishment Cut: Treatment to establish seedling reproduction within the
stand
• Removal Cut: Removal of final overwood to release established seedlings
– Multiple cuttings can be used and are the same as for a shelterwood except for
the density of the establishement cut (i.e. can have a preparatory cut and a
removal cut)
Cutting Strategies – Seed Tree System
• Additional Management Options:
– Reserve Cutting: Retain seed trees to help make an early thinning of
the next stand more economically feasible
– Not competing removal cut and retaining seed tree through next
rotation to meet multiple-use objectives
Seedtree Reserve Cutting Considerations
• Do economic gains out-weight positives to retention?
• Damage to established reproduction
• Is area fully stock with reproduction?
• Additional site preparation may be necessary if reproduction does not
develop
• If removal is chosen and growth of established reproduction is your primary
objective…
– Implement removal cut when site is fully stocked with seedlings of desired
height
– Level of stocking and seedling height required is species-specific
Site Preparation Considerations for Seed Tree Method
Site Conditions: An adequate seedbed and low level of
competition are required
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Some well-distributed exposed soil is desirable, since seeds are small
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Best case: thin, discontinuous litter, with some mineral soil exposed
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Dispersed skidding during logging may be sufficient, particularly if the
stand has been burned regularly
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Consider a prescribed burn (for pines) if a heavy litter layer exists
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Best if before harvest, and not between seedfall and a winter/early spring
harvest
Mechanical site preparation
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provides some density/distribution control
Site Preparation Considerations for Seed Tree Method
Reduce anticipated competition, if needed
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Logging operations can damage competition vegetation
present at time of harvest
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Pre- or post-harvest chemical control
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A burning regime prior to harvest
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Involves planning many years ahead
May be part of your silvicultural system for pines
May need to include one or more summer burns just before the
anticipated harvest
• Advantages to Seed Tree Method:
– Allows for the control of species and phenotypic characteristics of seed
source
– Seed source abundant and uniformly spaced
– Provides full sun growth conditions
• Disadvantages:
– Exposes seed source to increased risk of premature destruction.
– Does not provide protection to reproduction on harsh sites
• Application of Seed Tree Method:
– Southern Pines: slash, shortleaf, loblolly, sand
– Hardwoods: yellow-poplar, cottonwood, willow, ash
– Western Conifers
Shortleaf Pine
Slash Pine
Ponderosa Pine
Shelterwood System
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Definition: an even-aged silvicultural system where the
reproduction method removes mature community in two or
more successive cuttings, temporary leaving some old trees
to serve as seed source and to protect the regeneration.
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Characteristics:
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Relatively low density stand left of vigorous seed-bearing trees
Residual overstory provides sufficient canopy to mitigate
sensitive environmental conditions.
• Especially important on harsh or exposed sites.
Residual trees are removed once new reproduction reaches
adequate size (i.e. height) and density
Uniform Shelterwood Components
1. Preparatory Cut
An optional initial treatment to:
– Increase tree vigor and seed production in mature stand
– Remove undesirable seed sources
– Alter understory environment to promote development of
advance reproduction
Uniform Shelterwood Components (continued)
2. Establishment Cut
– Artificially moves stand into understory reinitiation phase
of stand development
– Promotes seed germination and establishment by creating
permanent openings in main canopy
– Opens the canopy for sufficient light availability to allow
regeneration
– Maintains some control (“shelter”) of understory
vegetation
– Generally, 25-60 ft2/ac residual basal area
• 30-40 ft2/ac southern pines, 50-60 ft2/ac for oak
– Should retain dominant, vigorous trees of favorable
phenotypes
Uniform Shelterwood Components (continued)
2. Establishment Cut
Considerations for success:
– Appreciably modify the understory environment
– Retain sufficient residual cover to create conditions that
favor target species and seed supply
– Understory environment must promote seedling
development of desired species
Tradeoffs Between Overstory Retention, Light
Environment, and Understory Competition
*Optimal level is dependant on species, site productivity, and
stand history
Uniform Shelterwood Components (continued)
3. Removal Cut
• A harvest to take away the overwood, so the reproduction can
develop uninhibited
– Conducted only after satisfactory establishment of reproduction based on
density, height, and distribution of seedlings
– Will impact (i.e. damage) established reproduction
– Remove the overwood before suppression of regeneration
Applicability of the Shelterwood Method
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Most flexible even-aged method
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A good method for heavy-seeded species
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A good method where the seed supply is irregular
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Obtain rapid increment of high quality wood
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Increase mast production
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Maintain aesthetics
Uniform Shelterwood Methods
• Three-cut Method: Preparatory, Establishment, and Removal
cut are used
• Two-cut Method: Establishment and Removal cut only
Other Types of Shelterwood Systems
Irregular or Reserve Shelterwood:
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Leaves residual overstory for an extended period of time into
new rotation – creates two-aged stand
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Has ecological/aesthetic vs. economic/operational tradeoffs
Other Types of Shelterwood Systems
Group Shelterwood:
• Takes advantage of existing patches of reproduction
• Removal cuttings done in patches containing reproduction
• Preparatory and establishment cuts done in areas lacking
favorable reproduction
Group Shelterwood
Other Types of Shelterwood Systems
Strip Shelterwood:
• Removal of mature age classes over a series of entries by
cutting narrow strips not exceeding the height of adjacent
standing trees
• Residual strips provide seed and partial shade to openings
• Strip Orientation:
– Long axis of strips at right angles to prevailing winds to
reduce blow-down
– Alignment in relation to sun’s path influences proportion of
direct and diffuse radiation
Strip Shelterwood
Application of the Shelterwood System
• Upland and bottomland oak forests
• Eastern pines: red pine, eastern white pine
• Southern pines: longleaf pine
• Rocky mountain conifers: western white pine, ponderosa pine, Douglasfir (Rocky Mountain variety), western larch (on harsh sites).
• Cascade and coastal range regions: western hemlock/Sitka spruce type
and Douglas-fir
Upland Oak Shelterwood
Shelterwood in longleaf pine
Brief Comparison: Clearcut, Seed Tree, and Shelterwood
Clearcut:
• Entire canopy removed in one operation
• Seedlings grow under open field conditions
Seed Tree:
• Canopy removed in 2 to 3 stand entries
• Residual trees retained to provide a seed source
• Seedlings grow under essentially open field conditions
Shelterwood:
• Residual trees retained to provide a seed source and modify understory
microenvironment
• Canopy removed in 2 to 3 stand entries