ecological basis for oak silviculture quercus is a dominant genus throughout north america and has...

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 oak-dominated ecosystems throughout oak’s range


• 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.


• 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


• 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.


• 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


• 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.


• 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?

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, two-aged 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: 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

Midstory Removal Practice

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

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



• 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


• 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 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


• 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


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

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

ecological basis for oak silviculture quercus is a dominant genus throughout north america and has...

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