Transcript Lectures: Ecological Basis for Oak Silviculture

Ecological Basis for Oak Silviculture
• Quercus is a dominant genus throughout North America and
has been for the past 10,000 years or more
• Oak distribution has shifted in response to changes in climate,
disturbance regime, and human population and culture
• A recent successional trend is the replacement of oakdominated ecosystems throughout oak’s range
Ecological Basis for Oak Silviculture
• Periodic disturbances maintained oak dominated forests of the
eastern United States prior to European settlement.
• In the 19th and early 20th century, large-scale post-settlement
disturbances such as land clearing, logging, intense wildfire,
and chestnut blight likely increased the dominance of oak on
the landscape.
• Fire suppression efforts in the early to mid 20th century
essentially removed fire as a disturbance mechanism in oak
dominated forests.
Ecological Basis for Oak Silviculture
• Due to shifts in disturbance patterns, dense midstory canopies
of shade tolerant species such as sugar maple, red maple, and
American beech have invaded many eastern hardwood forests.
• This alteration of stand structure and the resulting understory
environmental conditions has led to a decline in oak seedling
development
Ecological Basis for Oak Silviculture
• The large decrease in oak recruitment over the last 50 to 100
years could have a substantial impact on the perpetuation of
oak dominated systems.
– Presence of advance reproduction and stems capable of stump
sprouting is generally required to successfully regenerate oak forests.
• If current stand development trends remain, shade tolerant,
mesophytic species would replace oak as the dominant canopy
species in many stands.
Ecological Basis for Oak Silviculture
• The inability of oak reproduction to compete with large shade
tolerant advance reproduction or aggressive pioneer species is
the fundamental cause of problems in oak regeneration and
sustainability.
• Oak regeneration problems and reductions in oak stocking are
most likely on higher-quality mesic sites (site index > 60 feet).
• Oaks appear to be successionally most stable on xeric sites.
– Increased competition from shade-tolerant trees and shrubs may
influence oak regeneration potential even on xeric sites.
The Oak Regeneration Window
Sources of Oak Regeneration
• Reproduction by Seed
– Most oak seedling establishment occurs in years of good acorn
production
– Seed production is highly variable among oak species, between
individual trees, over the years, and from one location to the next.
– In general, oaks have large seed crops at 2-to 10-year intervals.
Sources of Oak Regeneration
• Reproduction by Seed
– Most oak seedling establishment occurs in years of good acorn
production
– Seed production is highly variable among oak species, between
individual trees, over the years, and from one location to the next.
– In general, oaks have large seed crops at 2-to 10-year intervals.
• Reproduction by Seedling or Seedling Sprout (Advance
Reproduction)
– Seedling sprouts arise from vegetative propagation of seedlings that
experience shoot dieback.
• Reproduction by Stump Sprouts
Role of Light Relationships in Oak Silviculture
• Acorn germination and initial seedling development are not
limited by light levels, because the seed is relatively large and
supplies the bulk of the carbohydrates for growth until seed
reserves are exhausted.
• Inadequate light often limits oak regeneration and recruitment
into the overstory
Role of Light Relationships in Oak Silviculture
• Development of midstory canopies of shade tolerant species
has lead to low understory light availability (< 10% full
sunlight) in many stands of the Central Hardwood Region.
• Lack of large advanced oak reproduction has been attributed to
the low light environment created by midstory canopies.
• Problematic oak regeneration has coincided with the
development of midstory canopies as presence of advance
reproduction and stems capable of stump sprouting is
generally required to successfully regenerate oak forests.
Role of Light Relationships in Oak Silviculture
• The minimum light level required by oak seedlings to produce
enough carbohydrate to meet their respiration needs is low,
about 2% to 5% of full sunlight.
• Height and diameter growth of oak seedlings is near
maximum at light intensities approaching 50% to 70% of full
sunlight
• In contrast,
– Shade-tolerant species (e.g., red maple and beech) show maximum net
photosynthesis at light intensities as low as 5% to 10% of full sunlight
– Shade-intolerant species require near full sunlight for light saturation
of net photosynthesis, which promotes maximum growth rates that
exceed those in oak
Question: If stand structure and associated light
environment are limiting development of oak
reproduction, what can silviculturists do to enhance
oak reproduction?
Question: If stand structure and associated light
environment are limiting development of oak
reproduction, what can silviculturists do to enhance
oak reproduction?
Use tools in our silvicultural toolbox to manipulate
stand structure and light availability.
Silvicultural Tools That Affect Understory Light
Availability
• Regeneration Methods
– Clearcut
• Completed at stand level
• Patch clearcutting
– Seed-tree
– Group selection
– Shelterwood Approaches
• Uniform shelterwood
• Midstory removal
• Irregular or Reserve shelterwood (i.e. two-aged management)
– Single-tree selection
Oak Regeneration Patterns in Mature, Undisturbed
Stands
Regenerating Oak in the Central Hardwood Region
• Due to absence of periodic disturbances within the Central
Hardwood Region, dense midstory canopies of shade tolerant
species have developed
• Light levels found below these canopies are typically less than
3 to 5% full
– Below the light compensation point for many native trees and
herbaceous plants
Typical mature, undisturbed upland oak stand in the Central Hardwood Foret
Region, overstory dominated by oak and dense midstory canopy of shade
tolerant species
An another example…
So how does oak regeneration develop in these stands?
So how does oak regeneration develop in these stands?
Summary of Oak Regeneration Cycle on Intermediate to
High Quality Sites with Undisturbed Canopy
•
•
•
•
Bumper acorn crop
Seedling establishment
Slow loss of seedling cohort
Minimal height growth
– Gradual loss of seedling vigor
• Ultimate loss of seedling cohort
The Bottom Line
In the absence of period disturbance on intermediate
and high quality sites, large advance oak
reproduction does not develop
Pillars of Oak Silviculture
• Regeneration
– Establishing oak through regeneration practices to meet desired levels
of stocking to meet management goals
• Recruitment
– Ascension of oak reproduction into the overstory (i.e., they successful
reach the dominant and codominant crown classes)
– Maintenance of oak in the overstory crown classes during the stem
exclusion stage
Laws of the Oak Regeneration Pillar
1) The presence of competitive sources of oak regeneration
– Successful oak regeneration after harvest will come from advance
reproduction that exists in the current stand and stump sprouts from
trees that are harvested from the current stand
2) Timely, sufficient release of these oak regeneration sources
– Concerns the timing and pattern of tree removal from the existing
stand to ensure the regeneration sources develop, ultimately, into
overstory trees
Laws of the Oak Regeneration Pillar
• Competitive oak regeneration sources can result from:
– Natural stand processes including natural disturbances
– Silvicultural treatments to develop competitive regeneration sources
• Silvicultural treatments should:
1. Make oaks more competitive
2. Reduce competition from other species
Timing of Regeneration Treatments in Oak
Dominated Stands
• "If you go into a 10-year-old hardwood stand and wonder what
you might do to change the species composition, you may be
at least 20 years too late."
– Dr. Don Beck, US Forest Service (received from Dr. David Loftis, US
Forest Service)
– Interpretation: Planning for an oak regeneration treatment should begin
at least 10 years prior to final overstory removal in the stand.
Silvicultural Systems for Oak Dominated Forests
At least 10 years before
harvest:
1) Evaluate reproduction
2) Plan regeneration
3) If necessary, implement
treatments to enhance the
number and competitive
position of oak reproduction
Options
1. Promote oak advance reproduction by
increasing light to the forest floor
through understory and/or partial
overstory cuttings
2. Enhance oak seedling establishment
with treatments such as soil
scarification, prescribed fire, or
shelterwood preparatory cut to
increase seed production
3. Underplanting
Procedures
1. Complete or partial overstory
removal (e.g., clearcut, twoaged deferment harvest, or
shelterwood establishment
cut)
2. Site preparation and control
of non-commerical residual
stems prior to the next
growing season
Clearcut Method for Oak
• Successfully used to regenerate oak on xeric sites that
intrinsically accumulate large oak advance reproduction
• Oak advance reproduction of requisite size and spatial
distribution must be present at time of clearcut harvest if oaks
are to become a major part of the next stand
• Clearcutting applied when oak regeneration is insufficient
result in stands dominated by:
– Newly established shade intolerant species (i.e., yellow-poplar)
– Shade tolerant advanced reproduction released by harvest
Shelterwood Method for Oak
• Goal: to provide an adequate number of high-vigor oak
advance reproduction that can successfully compete when the
overstory is finally removed
• Potential application sequence
– Preparatory cut and removal cut
– Preparatory cut, establishment cut, and removal cut
– Establishment cut and removal cut
Shelterwood Preparatory Cut: Midstory Removal
• Optional depending on stand conditions and oak regeneration
potential
• Uses well-timed midstory removal to improve understory light
levels that promote oak seedling development
• Generally used when sufficient numbers of advance
reproduction are present or coincides with underplanting
• Treatments are initiated at least 10 to 20 years before final
harvest is planned
Midstory Removal Practice
Midstory Removal: Remove sub-canopy (midstory and
understory trees) with chemical or mechanical methods
• Usually, removes overtopped and intermediate crown class trees
starting with the smallest stems one is willing to treat
Results with Cherrybark Oak after 7 Growing Seasons
Control
Midstory Removal
Release of Advance Reproduction
• Timely, sufficient release of oak reproduction is critical
• Release window would be at least 5 to 10 years following
initial treatment
– Timing highly dependant on site quality and initial size of reproduction
• Release done in one or more steps depending on size and vigor
of oak reproduction
Shelterwood Method for Oak
Releasing oak reproduction (one or two stages):
•
Conducted only after satisfactory height development of
seedlings
– Minimum release height usually 3 to 5 ft
– Timing between midstory removal and release
• Removal cut following midstory removal (i.e., one stage
release)
– When the regeneration potential of the oak reproduction is adequate to
replace the stand, remove the remaining overstory trees in one cut
Shelterwood Method for Oak
• Multistage release following midstory removal
– If sufficient height development is not occurring following midstory
removal, a multistage release may be necessary
– Step 1, Establishment Cut:
• Reduce overstory to 40 to 60 % stocking
– Retain higher overstory stocking (60 to70+ %) on high productivity
stands where yellow-poplar is a problem
– Leave the best dominant and codominant oaks as uniformly spaced as
possible
– Remove unwanted species
– Monitor seedling establishment and growth
– Step 2, Removal Cut: When the regeneration potential of the oak
reproduction is adequate to replace the stand, remove the remaining
overstory trees
Shelterwood Establishment Cut
Prescribed Fire in the Central Hardwood Region
Prescribed Fire in the Central Hardwood Region
• The historic fire regime was thought to play a role in the
maintenance of oak dominated forest prior to European
settlement
• Prescribed fire has been suggested as a tool for regenerating
oak
• Potential benefits included
– Increased seedling establishment
– Reduction of competing vegetation
Prescribed Fire in the Central Hardwood Region
• Prescribed fires in late spring and summer are most lethal to
oak competitors.
– Late spring and summer burn windows are narrow and therefore timing
these burns is difficult
– Early spring burns are more commonly applied
Prescribed Fire in the Central Hardwood Region
• Generally, prescribed fire is best used in combination with
reductions in overstory stocking (e.g. shelterwood, midstory
removal) to release advance oak reproduction
• Repeated burning most effective at increase oak
competitiveness
• The use of prescribed fire in oak forests has increased over the
last four decades
• Results on oak regeneration has been highly variable
Integrating Prescribed Fire and the Shelterwood Method
Site Preparation Burn
• Fire can create conditions suitable for developing oak
reproduction size and density by reducing litter layers and
understory competition
• Multiple fires are necessary to reduce dense understories
• Do not burn if acorn crop has just fallen or if new oak
seedlings from recent crop are needed to regenerate the stand
Integrating Prescribed Fire and the Shelterwood Method
Release Burn
• Burn used to free competitive oak reproduction from
competition
– Fire will select for the oak against other wood species to vary degrees
based on root characteristics
• Burning done after midstory removal, first removal cut of
shelterwood or after final removal cut
– Fire should occur after released oaks develop a more robust root system
– Typically,
• Completed 3 to 10 years following harvest or
• Completed when height growth of oak regeneration falls behind
that of other species by more than 2 ft
Integrating Prescribed Fire and the Shelterwood Method
Release Burn
• Moderate to high-intensity fires (flame lengths > 2 ft) to ensure topkill of
understory layer
• Done in mid to late spring (April to May)
– Later in this window, more advantageous to oak regeneration
Beyond Regeneration???
What to do if oaks are regenerated but not present at
sufficient numbers in the overstory?
• Crop tree release has been suggested as a tool to enhance
overstory recruitment of oak during the stem exclusion phase
– Limited research on how well overtopped and intermediate crown class
oak saplings respond to release and whether the treatment can facilitate
overstory recruitment
– Initial size of oak reproduction, site quality (i.e., growth rates and
competitor types), and time since canopy closure are likely key factors
influencing an oak sapling’s response to release