MÔN QUẢN LÝ THẢM THỰC VẬT RỪNG: UNEVEN AGE MANAGEMENT (LEC 15)

Uneven aged system: a sequence of silvicultural treatments designed to maintain and regenerate a stand with three or more age classes Thinning, regeneration and harvesting usually occur simultaneously (time), but may vary across the stand (space). The harvested trees are essentially replaced by growth on the younger trees left in the stand.

Uneven-aged vs Even aged

A forest stand or forest type in which relatively small (10-20 year) age differences exist between individual trees

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A stand with trees of three or more distinct

age classes, either intimately mixed or in

small groups

Uneven-aged Silvicultural Systems

•Uneven aged system: a sequence of silvicultural treatments designed to maintain and regenerate

a stand with three or more age classes

•Thinning, regeneration and harvesting usually occur simultaneously (time), but may vary across

the stand (space) The harvested trees are essentially replaced by growth on the younger trees left

in the stand

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Uneven-aged Silvicultural Systems

• Normally used to harvest, regenerate and tend forest cover types that will regenerate and grow

under partial shade

• Regenerate under partial canopies following minor disturbances like felling trees as

individuals, groups, and in small patches

• Uneven-aged systems refer to the general category of selection silvicultural systems

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Balanced uneven-aged

• Balanced stands have equivalent

amount of growing space occupied by

each age class

• Reverse-J age or size class distribution

• Age classes are intermixed

• In theory, one could annually cut the

current annual increment indefinitely

without degrading forest

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• Requires skilled labor

• Logging damage to residual trees and

regeneration

• Difficult to regenerate shade intolerant species

• Need to protect regeneration throughout cutting

cycle

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Selection systems

Three basic characteristics, which largely determine the silvicultiral consequences of selection systems are:

• Minimum diameter

• Length of cutting cycle

• The tree selection decision (to retain or cut)

ConflictLand owner

(concessionaires)

Forester(researchers)

As smaller as posible (45-50 cm for sawnwood)

As larger as posible (long term goals and sustainability)

Minimum diameter

Experiense has shown that the system is more safely if

the MD is not fixed at the lowest level

Harvestable volume in relation to minimum harvestable diameter

(East Mindanao, Philippines)

Minimum diameter

Havest volumne is decrease when DM is increase

Minimum diameter

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In Vietnam, DM is fixed by MARD basing on specific regions and wood groups

From Thanh Hoa to the North

Minimum diameter in Vietnam

From Nghe An to Hue

Minimum diameter in Vietnam

From Da Nang to the South

•For pine: Dmin = 40 cm;

•For Difterocapaceas in dry deciduous forest : Dmin = 35 cm

Minimum diameter in Vietnam

Length of cutting cycle

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• “Rotation” … number of years for trees to reach “D max”

• “Rotation”/ cutting cycle length = proportion of stand regenerated

– For example: 100 year “rotation” and 20 year cutting cycle means that 20% of the stand must be regenerated each cutting cycling

• There are 2 differences point of view to determine the length of cutting cycle

– Favor long cutting cycle

– Favor short cutting cycle

Arguments favor long cutting cycle

• Harvest volume increases and economic increment accumulates

• The best residual trees become mature and bear seed supporting to regeneration

• Larger number of cut trees and opens up the larger canopy, often supports the growth of saplings

• Long cutting cycle lead to concentration of felling areas and controls are easy

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Arguments favor short cutting cycle

• Short cutting cycle expects sooner economic return

• Small number of cut trees and opens up the small canopy leads to minor disturbances

• Short cutting cycle will scatter the felling operations over a wide areas reduces damage to regeneration

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The tree selection decision

With the uneven-aged silvicultural systems, the tree selection decision (to retain or cut) considers 3 main factors:

• Species desirability

• Tree quality

• Desired age and size class distribution

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Selection cutting systems

• Single-tree Selection

• Group Selection

• Patch Selection

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Single-tree Selection

The single tree selection system is defined as an uneven-aged silvicultural system in which new age classes are created by the removal of individual trees of all size classes, more or less uniformly throughout the stand

• This system favors the regeneration and maintenance of shade tolerant species

• Trees are selected for removal from all size classes (to achieve desired residual density levels)

• The goal is to achieve an optimum distribution of size and age classes so each contains a sufficient number

of quality trees to replace those harvested in the next larger size class

22Single-tree Selection

23Single-tree Selection

The group selection system is defined as a silvicultural system that removes trees in defined groups to create stand openings with a width less than two times the height of adjacent mature trees, and that manages the area as an uneven-aged stand

24Group Selection

• Trees are periodically removed in groups to create conditions favorable for the regeneration and establishment of new age classes (cohorts)

• The largest canopy opening is approximately two-times tree height The overstory provides a seed source, and modifes understory conditions to create a favorable environment for the production, competition, and growth of certain species

• This system favors regeneration and maintenance of shade tolerant and mid-tolerant species; shading effects will vary across the canopy opening

• The distribution of regeneration openings may be regular, or irregular depending on variations in stand condition, such as the age, size, vigor, quality, composition, and health of groups of trees

25Group Selection

• This method of stand-level cut determination is termed area regulation The total area to be harvested from

a stand determines basing on

– The length of the cutting cycle,

– The planned rotation age of the groups

– The percentage of the unit available for long-term management

•  Example: in an even-aged stand that has a rotation age of 100 years, 92% of the stand is available for group selection management and the cutting cycle is 25 years

• A rotation of 100 years with a cutting cycle of 25 years will support four age classes (25 years, 50 years, 75 years and 100 years old)

• The percentage of the available area that can be harvested at each stand entry is 23% of the stand area per cutting cycle( 92% divided by 4)

26Group Selection

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• Strip selection, a modification of group selection, is a system to manage uneven-aged stands essentially composed of many small even-aged patches

• Narrow strips, of a width less than two times the height of the adjacent mature trees, are progressively cut generally in linear strips distributed throughout the stand

28Strip Selection

Strip Selection

• The basic principles influencing strip orientation include minimizing damaging effects on the residual stand (e.g., wind), and optimizing shading

• In areas where drought and excessive daytime heat would likely affect regeneration adversely, strips could

be oriented east-west to maximize shading of cut strips; in this case, northern strips would be cut first

• Site preparation and follow-up release may be needed to establish desired regeneration

Patch Selection

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Selective cutting is not the same as a selection system

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Requirements for success

• Moderately to very shade tolerant species

• Long-lived species that grow well even at older ages and respond to thinning

• Regular (maybe periodic) seed crops

• Seed that germinates in undisturbed soil and shady environment

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