Characteristics of the Selection Method：选择方法的特点.ppt

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1、Even-aged vs.Uneven-aged Systems,Common characteristics of even-aged stands,Crown canopy is generally limited to a single layer elevated above the groundDiameters vary widely only if shade-tolerant species are presentOnly old stands have sawtimber sized treesSmall trees have short live crown length

2、when compared to total heightLargest trees often have 25-40 percent live crown,depending on stand density,Common characteristics of uneven-aged stands,Crown canopy is generally comprised of multiple layers and commonly extends close to the groundDiameters range from seedling-sapling to sawtimber siz

3、es,regardless of species presentTrees of all diameters have a large live-crown ratio,often as high as 40 to 60 percent in managed standsTree heights vary with tree diameter,with short ones having small diameters and tall trees having larger diameters,Even-aged vs.Uneven-aged Diameter Distributions,R

4、everse J-shaped,Bell-shaped(normal distribution),Reverse J-shaped does not always indicate a true uneven-aged stand(3+age classes),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

5、 are related to tree age,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 su

6、pport similar treatments,Uneven-aged Regeneration Systems,Uneven-aged regeneration systems often referred to as selection systems,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

7、 as individuals or in small groupsSingle-tree selection method:removing individual mature trees more or less uniformly across a standGroup selection method:removing mature trees in small groups or clusters,Characteristics of Uneven-aged Systems,Maintains a continuous high forest coverTypically empha

8、sizes the production of sawtimber sized trees,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 interval

9、s For sustained yield in a selection system:,Characteristics of Uneven-aged Systems,Rotation length is the average time period required to obtain crop trees of a specified target sizeThe period between harvests(in years)is the length of the cutting cycle,Characteristics of Uneven-aged Systems,To avo

10、id high-grading,each cutting should include intermediate treatments among trees other than those of the target sizeFor a sustained yield,the method requires frequent and accurate inventory,General Procedure in Uneven-aged Systems,Harvest mature trees,either single trees or in small groupsThis provid

11、es 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,Approaches to regulation in the selection method and maintaining a balanced stand with sustainable yield,Area

12、regulation:this is the simplest,and is fairly easy with a group selection system,but it is difficult with the single-tree approach.,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:

13、use a reverse J-shaped curve of residual diameter distribution as a guide.,Balance vs.Irregular(unbalanced)uneven-aged stands,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 defin

14、es the curves shape(steepness or flatness)and is called q(or quotient)q=where,Ni=number of trees in the ith diameter classNi+1=number of trees in next largest diameter class,Structural regulation and reverse J-shaped curve,Influence of q on Target Diameter Distribution,A smaller q value more large t

15、rees and fewer smaller trees A larger q leaves fewer large trees,more smaller tree(i.e.less sawtimber),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 classMaximum diameter or largest dia

16、meter 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 t

17、he following discussion.,Uneven-aged Regeneration Methods,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,Variations of the

18、 Selection Method,Single Tree SelectionMore commonly applied in:Shade tolerant speciesRestrictive sites where pronounced seasonal water limitations favor natural monoculturesHas been used for other forest types,Variations of the Selection Method,Single Tree SelectionCentral and southern upland and b

19、ottomland hardwoods,Variations of the Selection Method,Group Selection:removes clusters of adjacent mature trees from a predetermined proportion of the stand areaGroup selection was developed to regenerate shade-intolerant and intermediate speciesGroup selection is easier to plan and keep the stand

20、balanced than with single-tree(if area regulation is used)Logging is more efficient and less damaging to residual trees than with single-tree,Group Selection Method,Application of group selection,Locate groups to be harvested among the oldest or largest trees in the standUses area regulation to main

21、tain balanced standOpenings must be wide enough to allow good regeneration establishment Group selection in the Central Hardwood Region generally uses open sizes between 1 and 2 times the height of surrounding trees,Application of group selection,Shape the harvested openings to fit the stand conditi

22、ons or to maximize objectives/constraints considerationsComplete felling of all trees in the openings is crucial to allow for good regeneration,Application of group selection,Control of undesirable species should be consideredTend the remaining uncut stand areas between group openings,Issues associa

23、ted with group selection,Uses area regulation for structural controlDifficult(or impossible)to locate groups within a stand following second or third entryAppropriate tool for other objectiveswildlife openings,aesthetics,salvage/sanitation,Issues associated with group selection,Group selection is of

24、ten confused with patch clearcuttingIf groups are managed as an individual“stand”and tracked through time as such,you are using even-aged silviculture at a small spatial scaleIn group selection,harvested opening widths are no more than 2 times the height of adjacent mature trees,Potential Objectives

25、/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,Potentia

26、l Objectives/Benefits in Using a Selection System,Maintains high diversity within the stand Maintains good site protection Maintains pleasing aesthetics without time gaps,Potential Drawbacks/Disadvantages in using a Selection System,Involves a high level of complexity,requires higher management cost

27、s than other methodsProduces less pulpwood than other methods Harvesting is usually more difficult and costly per unit area or product than with even-aged methodsTypically,selection results in more logging damage to potential crop trees than with even-aged methods,due to more frequent entry of equip

28、ment into the stand Can lead to high grading if not applied carefully,Two-Aged(Hybrid)Silvicultural Systems,Two-Aged Silviculture,Two-aged management is a hybrid between even-aged management and uneven-aged managementRegeneration is accomplished(in general)two times over a standard rotation.Two age

29、classesReferred to as:irregular shelterwoods,reserve shelterwoods,leave tree systems,and deferment methods,Benefits of a Two-Age System,Development of large-diameter sawtimber or veneer treesProduction of a wide range of forest products from pulp to veneer in the same stand at the same timeAbility t

30、o regenerate shade-intolerant and intermediate shade-tolerant speciesImproved aesthetics compared to clearcuttingIncreased structural diversity and retention of habitat components compared to clearcutting,Benefits of a Two-Age System,Increased initial revenue compared to other types of non-clearcut

31、regeneration techniquesDevelopment of old-growth structural characteristicsMaintenance 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,Constraints/Undesirable Features of Two-Aged

32、System,Reducing older age classes to low densities and wide spacing increase the danger of blowdownResidual trees may be prone to epicormic branchingLack of appropriate long-lived species to maintain the system,Constraints/Undesirable Features of Two-Aged System,Forest fragmentation and habitat effe

33、cts similar to clearcuttingReduction in initial revenues compared to clearcuttingLimited development of shade-tolerant speciesDamage to new age-class trees if a portion of reserve trees are removed prior to the end of the second rotation length,Irregular or Reserve Shelterwood:Leaves residual overst

34、ory for an extended period of time into new rotation creates two-aged standHas ecological/aesthetic vs.economic/operational tradeoffsCharacteristics of reserve trees are important,Irregular/Reserve Shelterwood,Uncut Stand,Reserve trees(10-15 ft2 ac-1 BA),Establishment Cut(45-60 ft2 ac-1 BA),Two-age

35、stand development,Application of Irregular or Reserve Shelterwood,Two-aged system typically initiated using a shelterwood cut,Reserve Tree Criteria,Other Partial Cuttings,Other Partial Cuttings,Timber harvesting vs.SilvicultureTimber harvesting extracts a productSilviculture involves a determined ef

36、fort 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 cutting treatmentsCommercial clearcut

37、ting:High-grading:Diameter-limit cuttings:,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 intervalsDoes not ensure adequate regeneration in terms of number,species,or distributionIgnores silvical requirements of desired species with respect to regeneration and long-term growthRemoves acceptable growing stock and leaves defective and unhealthy trees,