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Even-aged vs. Uneven-aged Systems

Even-aged vs. Uneven-aged Systems. Common characteristics of even-aged stands. Crown canopy is generally limited to a single layer elevated above the ground Diameters vary widely only if shade-tolerant species are present Only old stands have sawtimber sized trees

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Even-aged vs. Uneven-aged Systems

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  1. Even-aged vs. Uneven-aged Systems

  2. Common characteristics of even-aged stands • Crown canopy is generally limited to a single layer elevated above the ground • Diameters vary widely only if shade-tolerant species are present • Only old stands have sawtimber sized trees • Small trees have short live crown length when compared to total height • Largest trees often have 25-40 percent live crown, depending on stand density

  3. Common characteristics of uneven-aged stands • Crown canopy is generally comprised of multiple layers and commonly extends close to the ground • Diameters range from seedling-sapling to sawtimber sizes, regardless of species present • Trees of all diameters have a large live-crown ratio, often as high as 40 to 60 percent in managed stands • Tree heights vary with tree diameter, with short ones having small diameters and tall trees having larger diameters

  4. Even-aged vs. Uneven-aged Diameter Distributions Bell-shaped (normal distribution) Reverse J-shaped

  5. Reverse J-shaped does not always indicate a true uneven-aged stand (3+ age classes) Example from the Central Hardwood Region: Two-storied stand with oak-dominated overstory and midstory/understory canopy dominated by shade tolerants like beech and maple.

  6. Timeline of practices in an even-aged silvicultural system • During the rotation age r, treatments are applied across the entire stand to meet silvicultural objectives that are related to tree age

  7. Concurrent application of individual practices of an uneven-aged silvicultural system during a cutting cycle harvest in a balanced uneven-aged stand • Treatments are applied to subunits of the stand depending on conditions within each subunit • Each cutting cycle harvest will support similar treatments

  8. Uneven-aged Regeneration Systems • Uneven-aged regeneration systems often referred to as selection systems also called • This is not equivalent to "selective" cutting, as the term is commonly used! • “Selective" logging and "select-cut" merely mean that the harvest is not a clearcut • These terms are imprecise • They could be referring to a thinning, to a shelterwood establishment cutting, or to a high-grading cut

  9. Characteristics of Uneven-aged Systems • Selection methods produce an uneven-aged stand (with at least 3 age classes or distinct cohorts) • For regeneration, trees are harvested as individuals or in small groups • Single-tree selection method: removing individual mature trees more or less uniformly across a stand • Group selection method: removing mature trees in small groups or clusters

  10. Characteristics of Uneven-aged Systems • Maintains a continuous high forest cover • The entire stand remains under the influence of mature trees • Harvested opening widths are no more than 2 times the height of adjacent mature trees • Typically emphasizes the production of sawtimber sized trees • Pulpwood production is relatively low

  11. Characteristics of Uneven-aged Systems • Selection is particularly useful for putting an irregular stand under productive management without losing existing stocking • A selection system can be designed to obtain a sustained yield at recurring short intervals • For sustained yield in a selection system: • If the stand is balanced, each harvest should remove an amount equivalent to the growth produced since the last harvest.

  12. Characteristics of Uneven-aged Systems • Rotation length is the average time period required to obtain crop trees of a specified 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

  13. Characteristics of Uneven-aged Systems • To avoid "high-grading", each cutting should include intermediate treatments among trees other than those of the target size • For a sustained yield, the method requires frequent and accurate inventory • Best at the end of each cutting cycle • Accurate stand and stock table is needed

  14. General Procedure in Uneven-aged Systems • Harvest mature trees, either single trees or in small groups • This provides openings for regeneration of a new age class (cohort) • "Tend" the remaining cohorts to maintain approximately equal total area in each -- among these remaining sizes, "cut the worst, leave the best"

  15. Approaches to regulation in the selection method and maintaining a balanced stand with sustainable yield • Area regulation • Volume regulation • Structural regulation

  16. 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:

  17. Exercise 1 - Area Regulation: • Stand Size: 75 acres • Rotation Length: 80 years • Cutting Cycle: 10 years • Calculate the percent of the area that will be cut at each entry ___________________ • How many acres will be harvested at each entry? ____________________

  18. Exercise 1 - Area Regulation: • Stand Size: 75 acres • Rotation Length: 80 years • Cutting Cycle: 10 years • Calculate the percent of the area that will be cut at each entry ___________________ • How many acres will be harvested at each entry? ____________________ Interpretation: 12.5% of the area will be cut every 10 years with a 80 year rotation age

  19. 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

  20. Volume Regulation: An Example Volume-Guiding Diameter-limit (VGDL) approach • Determine a maximum stocking level for the stand • Estimate the annual stand volume growth rate • Set the cutting cycle length • Minimum feasible cutting cycle length is set by minimum volume acceptable for an operable cut divided by the stand's annual growth rate • Annual growth multiplied by the cutting cycle length equals the allowable cut • A guiding diameter limit is calculated so that harvesting trees in this diameter class and larger will provide the allowable cut

  21. Volume Regulation: An Example Volume-Guiding Diameter-limit (VGDL) approach • VGDL diameter limit is only intended as a guide • High-quality trees with acceptable growth may be retained above the limit, while an equal volume of lower quality trees may be cut below the limit • To avoid problems and diameter distribution imbalances, apply thinning and improvement cutting to all size classes • If the stand is understocked, remove less than the allowable cut by reducing the harvest in under-represented sizes

  22. Exercise 2 - Volume Regulation • Assume: • Maximum stocking levels: 80 ft2 ac-1 basal area, 7,000 board feet/acre • Minimum volume for operable cut is 1,600 board feet/acre/year • Annual stand volume growth rate: 400 board feet/acre/year • 5 year cutting cycle What is the allowable cut (board foot volume)?

  23. Exercise 2 - Volume Regulation • Assume: • Maximum stocking levels: 80 ft2 ac-1 basal area, 7,000 board feet/acre • Minimum volume for operable cut is 1,600 board feet/acre/year • Annual stand volume growth rate: 400 board feet/acre/year • 5 year cutting cycle What is the allowable cut (board foot volume)? Annual volume growth x cutting cycle = allowable cut 400 x 5 = 2000 board feet/acre

  24. Exercise 2 - Volume Regulation • Assume: • Maximum stocking levels: 80 ft2 ac-1 basal area, 7,000 board feet/acre • Minimum volume for operable cut is 1,600 board feet/acre • Annual stand volume growth rate: 400 board feet/acre/year • 5 year cutting cycle What is the allowable cut (board foot volume)? Annual volume growth x cutting cycle = allowable cut 400 x 5 = 2000 board feet/acre Does the allowable cut meet minimum volume for operable cut? YES 2000 board feet/acre > 1600 board feet/acre

  25. Exercise 2 - Volume Regulation In the Volume/Guiding-Diameter-Limit regulation method a guiding diameter limitis calculated so that harvesting trees in this diameter class and larger will provide the allowable cut. Using the given stand data, What is the determine the guiding diameter limit?

  26. Exercise 2 - Volume Regulation In the Volume/Guiding-Diameter-Limit regulation method a guiding diameter limitis calculated so that harvesting trees in this diameter class and larger will provide the allowable cut Allowable cut = 2000 bdft/ac Using the given stand data, What is the determine the guiding diameter limit? 21 inches dbh  = 1,992

  27. Structural regulation: use a reverse J-shaped curve of residual diameter distribution as a guide.

  28. Balance vs. Irregular (unbalanced) uneven-aged stands

  29. Structural regulation and reverse J-shaped 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 curve’s shape (steepness or flatness) and is called q (or quotient) q = where, Ni = number of trees in the ith diameter class Ni+1 = number of trees in next largest diameter class

  30. 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)

  31. Structural regulation: BDq Method The BDq Method of Regulation: • B is the target residual basal area (after harvest) • D is the maximum retained (after harvest) diameter class • Maximum diameter or largest diameter tree) • q is the ratio of numbers of stems (target-after harvest) of each DBH class to the next higher DBH class BDq Method is being researched at the Crossett Experimental Forest (Arkansas) for loblolly and shortleaf pines. Information and recommendations from their research is used as examples for the following discussion.

  32. Exercise 3 – Structural RegulationThe BDq Method • For this exercise, assumptions • Target residual basal area (after harvest) = 60 ft2 ac-1 basal area • Maximum retained (after harvest) diameter class = 21 in • q-value = 1.2 Based upon the above assumptions, how many trees per acre should be retained (Target TPA) in the largest diameter class (21 in DBH)?

  33. Exercise 3 – Structural RegulationThe BDq Method • For this exercise, assumptions • Target residual basal area (after harvest) = 60 ft2 ac-1 basal area • Maximum retained (after harvest) diameter class = 21 in • q-value = 1.2 Based upon the above assumptions, how many trees per acre should be retained (Target TPA) in the largest diameter class (21 in DBH)? The Hard Way: Nmax = Nmax = the number of trees in the largest diameter size class BA = target residual basal area di = diameter class bai = basal area of diameter class midpoint dmax = largest diameter to be retained in the stand dmin = smallest diameter class w = width of diameter class (usually 1 or 2 inches)

  34. Exercise 3 – Structural RegulationThe BDq Method • For this exercise, assumptions • Target residual basal area (after harvest) = 60 ft2 ac-1 basal area • Maximum retained (after harvest) diameter class = 21 in • q-value = 1.2 Based upon the above assumptions, how many trees per acre should be retained (Target TPA) in the largest diameter class (21 in DBH)? The Easy Way: BDq Structural Regulation Spreadsheet 

  35. Exercise 3 – Structural RegulationThe BDq Method • Complete the table • Pre-Harvest TPA is from the stand’s current inventory data. • Target residual TPA is the number of trees by diameter class in the target guiding curve for the stand • Removal TPA is the number of trees (by diameter class) that must be removed in a selection harvest to meet the target diameter curve distribution.

  36. Uneven-aged Regeneration Methods

  37. 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 and achieve other stand structural objectives

  38. Variations of the Selection Method Single Tree Selection: • Typically applied to very tolerant species • Example: spruce-fir or beech-maple forest types • Has been used in the region for other forest types • Loblolly-shortleaf pine, Crossett Experimental Forest (AR) • Oak forests in the Missouri Ozarks (Pioneer Forest) • Longleaf pine, southern Coastal Plain region

  39. Variations of the Selection Method Single Tree Selection: • Single tree selection applied in Appalachian and southern hardwood stands without intensive competitor control has generally resulted in a transition to shade tolerant species

  40. Variations of the Selection Method • Single Tree Selection in practice • Logging is difficult and costly, and may result in a high degree of damage • Aggressive control of competing tolerant species through herbicides or cutting is essential for success

  41. Variations of the Selection Method • Group Selection: removes clusters of adjacent mature trees from a predetermined proportion of the stand area • Group selection was developed to regenerate shade-intolerant and intermediate species • Group selection is easier to plan and keep the stand balanced than with single-tree (if area regulation is used) • Logging is more efficient and less damaging to residual trees than with single-tree

  42. Group Selection Method

  43. Application of group selection • Locate groups to be harvested among the oldest or largest trees in the stand • Uses area regulation to maintain balanced stand • 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 • Group selection in the Central Hardwood Region generally uses open sizes between 1 and 2 times the height of surrounding trees

  44. Application of group selection • Shape the harvested openings to fit the stand conditions or to maximize objectives/constraints considerations • rectangular openings will be more efficient for logging than circular or square ones-narrow • rectangular openings provide more sun if oriented with their long axes east-west • Complete felling of all trees in the openings is crucial to allow for good regeneration

  45. Application of group selection • 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

  46. Issues associated with group selection • Uses area regulation for structural control • Difficult (or impossible) to locate groups within a stand following second or third entry • Appropriate tool for other objectives—wildlife openings, aesthetics, salvage/sanitation

  47. 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 • In group selection, harvested opening widths are no more than 2 times the height of adjacent mature trees

  48. Potential Objectives/Benefits in Using a Selection System • Can provide frequent periodic income from the stand (3 - 10 years), with no long time gaps • Has 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

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