Understanding Uneven-aged Silviculture Systems

1. Multiaged SystemsUneven-aged and Irregular Silviculture

2. 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

3. Characteristics of Uneven-aged Systems • Selection methods produce an uneven-aged stand (with at least 3 age classes or distinct cohorts)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. Regulating Selection Systems

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

21. 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:

22. 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

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

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

25. 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

26. 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)

27. Silvicultural Approaches to Creating Irregular Stand Structures

28. 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

29. 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

30. 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

31. Creating Irregular Stand Structures

32. Creating Irregular Stand Structures

33. Creating Irregular Stand Structures

34. Two-Aged Silviculture

35. 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

36. 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 shade-tolerant species • Improved aesthetics compared to clearcutting

38. Benefits of a Two-Aged System • Increased initial revenue compared to other types of non-clearcut 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

39. 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

40. 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

42. 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

43. 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 ft2 ac-1

44. Deferment or Leave-Tree Approach Uncut Stand Reserve trees (10-15 ft2 ac-1 BA)

45. Deferment or Leave-Tree Approach

46. Reserve Shelterwood Uncut Stand Establishment Cut* (45-60 ft2 ac-1 BA) Reserve trees (10-15 ft2 ac-1 BA) *Similar to the uniform shelterwood (even-aged), an optional preparatory cut may proceed the establishment cut

47. Variable Retention Harvesting

48. 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

49. 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

50. Variable Retention Harvesting Dispersed Retention