Even-aged vs. Uneven-aged Systems

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. 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

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

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

20. 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

21. 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)

22. 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.

23. 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)?

24. Uneven-aged Regeneration Methods

25. 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

26. 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

27. Variations of the Selection Method Single Tree Selection • Central and southern upland and bottomland hardwoods • Generally, without intensive competitor control, single tree selection has resulted in a transition to shade tolerant species

30. 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

31. Group Selection Method

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. Potential Objectives/Benefits in Using a Selection System • Maintains high diversity within the stand • usually provides good wildlife habitat for many, but not all species • Maintains good site protection • although frequent logging may result in increased soil damage on sensitive sites • Maintains pleasing aesthetics without time gaps

39. Potential Drawbacks/Disadvantages in using a Selection System • Involves a high level of complexity, requires higher management costs than other methods • Produces less pulpwood than other methods • Harvesting is usually more difficult and costly per unit area or product than with even-aged methods • Typically, selection results in more logging damage to potential crop trees than with even-aged methods, due to more frequent entry of equipment into the stand • Can lead to high grading if not applied carefully

40. Two-Aged (Hybrid) Silvicultural Systems

41. 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 • Referred to as: irregular shelterwoods, reserve shelterwoods, leave tree systems, and deferment methods

42. Benefits of a Two-Age System • Development of large-diameter sawtimber or veneer trees • Production of a wide range of forest products from pulp to veneer in the same stand at the same time • Ability to regenerate shade-intolerant and intermediate shade-tolerant species • Improved aesthetics compared to clearcutting • Increased structural diversity and retention of habitat components compared to clearcutting

43. Benefits of a Two-Age System • Increased initial revenue compared to other types of non-clearcut regeneration techniques • Development of old-growth structural characteristics • Maintenance of sexual reproduction in reserve trees throughout the entire rotation and the ability to “life boat” species that would otherwise be eliminated if the area was clearcut

45. 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

46. 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

47. Irregular or Reserve Shelterwood: • Leaves residual overstory for an extended period of time into new rotation – creates two-aged stand • In central hardwood region, reserve tree density is approximately 10 to 15 ft2 ac-1 of basal area • Has ecological/aesthetic vs. economic/operational tradeoffs • Characteristics of reserve trees are important

48. Irregular/Reserve Shelterwood Uncut Stand

49. Irregular/Reserve Shelterwood Uncut Stand Establishment Cut (45-60 ft2 ac-1 BA)

50. Irregular/Reserve Shelterwood Uncut Stand Establishment Cut (45-60 ft2 ac-1 BA) Reserve trees (10-15 ft2 ac-1 BA)