Multiaged Systems Unevenaged and Irregular Silviculture Unevenaged Regeneration

Multiaged Systems Uneven-aged and Irregular Silviculture

Uneven-aged Regeneration Systems • Uneven-aged regeneration systems often referred to as selection systems – Not equivalent to "selective" cutting – “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 • 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 • Maintains structural diversity within stand • Maintains good site protection – although frequent logging may result in increased soil damage on sensitive sites • Pleasing aesthetics without time gaps • Frequent periodic income • Good flexibility – • 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 • Involves a high level of complexity, requires higher management costs than other methods • Produces less pulpwood than other methods • Harvesting more difficult and costly per unit area than with even-aged methods • Results in more logging damage to residual trees than with even-aged methods – • Due to frequent entry of harvesting equipment Leads to long-term depletion of growing stock if not applied carefully

Characteristics of Uneven-aged Systems • Uneven-aged regeneration methods provide openings for establishment and recruitment of new age classes (cohorts) • Selection methods are traditionally classified as either – – • 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 achieve other stand structural objectives

Variations of the Selection Method Single Tree Selection • More commonly applied in: – Shade tolerant species • Norway spruce, beech, silver fir (central Europe) • Sugar maple, American beech, birch (Northern hardwoods) – Restrictive sites where pronounced seasonal water limitations favor natural monocultures • Ponderosa pine • 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 • Central and southern upland bottomland hardwoods – Generally, without intensive competitor control, single tree selection has resulted in a transition to shade tolerant species

Variations of the Selection Method • Group Selection: removes clusters of adjacent trees from a predetermined proportion of the stand area – Group selection was developed to create larger openings needed to regenerate shade-intolerant and intermediate species

Application of group selection • 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 • 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 • Group selection is easier to plan and keep the stand balanced than with single-tree • Logging is more efficient and less damaging to residual trees than with single-tree

Application of group selection • Complete felling of all trees in the openings is crucial to allow for good regeneration • Control of undesirable species should be considered – • 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. – Combined area of all trees removed in each cutting cycle:

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= 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 • 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: – – – 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 ft 2 ac-1

Deferment or Leave-Tree Approach Uncut Stand Reserve trees (1015 ft 2 ac-1 BA)

Deferment or Leave-Tree Approach

Reserve Shelterwood Uncut Stand Establishment Cut* (45 -60 ft 2 ac-1 BA) Reserve trees (1015 ft 2 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: – – – • Abundance and distribution of suitable residual trees and their potential Status of advance reproduction Patterns of interfering understory vegetation Characteristics of suitable residuals – – – 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