Thinning – A Detailed Note

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THINNING

DEFINITION
Thinning is defined as a felling made in an immature stand for the purpose of improving the growth and form of the trees that remain without permanently breaking the canopy.
1.     It is a treatment of forest crops whereby the number of trees growing in a stand is reduced.
2.     Thinning consists of a series of successive felling operation for a number of times before the crop matures.
3.     The interval between two succession felling may be fixed but it is dependent on the time required for canopy closure.
4.     Thinning is carried out in a crop after it reaches the sapling stage and continued up to the beginning of the regeneration period.
5.     The thinning principles are applicable to pure and even-aged or nearly even-aged crop or even-aged groups of the trees in a crop.
6.     Thinning always increases spacing and decreases the number of trees per unit area.
7.     Most of the trees species are raised at a close spacing i.e.
                                i.     2 Meter x 2 meter
or
                              ii.     3 meter x 2 meter
or
                            iii.     3 meter x 1 meter
8.     After five or ten years depending upon species and site conditions the canopy closes or root competition is so heavy that the plants are not able to grow unless they are thinned out.
9.     They are quite spaced for the crown as well as root development.
10.  Thinning is based on the principle of natural development of crop after each thinning number of trees/hectare decreases but this is compensated by its diameter and height growth.



OBJECTIVES OF THINNING

1.     To improve the hygienic condition.
2.     To create best conditions of growth.
3.     Salvage the anticipated losses of the merchantable volume.
4.     To obtain a desirable composition of crops.
5.     Retaining seed beares.
6.     Improvement in wood quality.
7.     To obtain intermediate yield and increase net yield and financial out-turn.
8.     Decomposition of raw hummus.
9.     To reduce the risk of diseases and pests.

PURPOSE/ AIMS OF THINNING:

  • To reduce competition in root and crown
  • Discourage weed growth

726 shisham planted at 6ft × 10ft or 200 at 20ft × 20ft. But in case of 726, spacing is less, interspaces covered, which discourage growth of weeds. Later on completely trees are thinned.

  • Improve quality of wood

When trees are far apart, the crowns develop lot of branches, particularly throughout bole length which is not like. So less spacing and thinning afterwards, less spacing provides natural pruning.

  • Regulation of rate of growth

Thinning controls rotation. Closer the trees, smaller is the rate of growth longer is the rotation and vice versa. So thinning is done to increase rate of growth and reduce rotation.

Trees on ridges are exposed to hazards, so they are heavily thinned. Greater snow will be accumulated in dense crop and smaller amount in open crop. Bigger the crown, larger is the production of resin. Wider the crown, greater is seed/fruit production.

Thinning is also a determinant of quality. Wider space provided give wider annual rings.

  • To increase seed production
  • Reduction in the net cost of growing

Whatever spending is done on growing stock, thinning pays some of it back as fuel wood, etc.

  • Reducing the risk of fire and diseases
  • Encouraging the advanced growth
  • Fulfilling the market demands

Thinning is always conducted for even-aged crops.

BASIS OF THINNING

1.     Tree classification basis
2.     Statistical basis

Tree Classification Basis

Ø  Before planning the nature and intensity of thinning the preference between the intensity of thinning and quality of thinning should be decided.
Ø  The trees remaining after the thinning should be able to compensate for the loss in increment by trees removed in thinning.
Ø  There will be an optimum stocking for a given site and species.
Ø  For this individual trees are classified by height and size of crown and thinning is decided on the basis of which classes of trees are fit to be removed for maintaining the desired qualitative and quantitative nature of the thinning.
Ø  The main purpose of the thinning is to release more space for future development of trees, the freedom of crown is the guiding factor for thinning purpose.
Ø  The retention of trees per unit should be decided first and then the number of trees to be removed should be marked accordingly.
Ø  The development of crown and stem are the deciding factor for sub-classification of the dominance classes of trees for the given site and species.
Ø  Tree classification in regular forests are done as under:-

(I)  Dominant trees (D or d1)

§  It includes the trees that form the uppermost leaf canopy. They are the tallest trees that determine the top level of canopy they are also called pre-dominant trees.

(II) Co-dominant trees (CD or D2)

§  These are shorter than dominant.
§  About 5/6 of the height of dominant.

(III)  Sub-dominant trees (SD or D3)

§  They do not form part of the uppermost leaf canopy.
§  Their height is about ¾ that of the tallest trees.
§  They are also called dominated trees.

(IV) Suppressed trees

§        The height of the suppressed trees are about ½ to 5/8 of the height of the best trees.
§  Suppressed trees grown below the crowns of the other trees in the stand.
§  The leading shoots are over-topped by their neighboring trees or at least shaded on all sides by them.

(V) Wolf trees (W)

§  The wolf trees are large, coarse and misshapen trees.

(VI) Whips (Wh)

§  This class comprises bent over and badly leaning trees.
§  Whips have tall, thin stems which are unstable and may cause damage to other trees when they sway.

(VII) Dead, dying and moribund trees (m)

§  This category consists of trees that are dead or at the point of death.

(VIII) Disease trees (k)

§  It includes trees that are infected with parasites and their growth is seriously affected due to infection.

(IX) Reproduction or Regeneration (r)

§  It consists of recruits or regeneration.

(X) Overmature or veterans

                 i.          It consists of over-matured trees.
               ii.          In spite of these classifications the basis three types of stem and crown classification.
             iii.          The good, medium, bad and whips are shown by using symbols a, b, c, d respectively i.e.
                                    Good               =          a
                                    Medium          =          b
                                    Bad                  =          c
                                    Whips             =          d
             iv.          Therefore the further classification of D2 will be D2 (a), D2 (b), D2 (c), or D2 (d) or that of s will be S (a), S (b), S (c) and soon.
Ø  No thinning should be carried out on the edge or periphery of the plantation.
Ø  The crown spread may be taken as a better indication than canopy position to understand growth potentialities of a tree.
Ø  D or D1, CD or D2, SD of D3, S, W, Wh, m, k, r, v.

CLASSIFICATION ON THE BASIS OF CROWN

Ø  The crown spread may be taken as better indication than canopy position to understand growth potentialities of a tree.
Ø  For this tree may be classification on the basis of the crown with average stem spacing as shown below:-

i.    Spaced

Trees with free crown spreading having less than or equal to ¾ that of spacing.

ii.   Co-space

Trees with a free crown spread having ¾ to ½ (50-75%) that of the spacing.

iii. Sub-space

Trees with the spread of free crown less than half but more than one-fourth ¼ of the spaced.

iv. Mal-spaced

Trees with a very small spread of trees crown which is less than a quarter of the spaced.

v.   De-spaced.

Trees will no free crown with little vigor.

vi.  Dead, dying and low yielding trees

 STATISTICAL BASIS

Ø  In regular plantations the density and site quality vary considerably ad it is not possible to define crown and canopy classes.
Ø  For this the number of trees to be retained per unit area and their spacing should be numerically fixed.
Ø  After fixing the number of trees per unit area the remaining trees can be marked and removed.



 METHODS OF THINNING

Ø  For regular crop the following methods are used for thinning:-
1.     Mechanical thinning
2.     Ordinary or low thinning
3.     Crown Thinning
4.     Free thinning
5.     Advance thinning
6.     Maximum/ Numerical thinning

MECHANICAL THINNING

·       In this type of thinning the trees are removed b some thumb rule e.g. Removal or alternate rows, Removal of alternate diagonals or Removal of every second third, fourth line etc. Or where the spacing is irregular the minimum spacing is irregular the minimum spacing is maintained by using “STANDARD STICK” method.
·       This type of thinning is applied to young plantations in which canopy differentiation has not taken place. There are two types of system for mechanical thinning.

i.    Row Thinning

§  In which trees are removed in lines or rows.

(ii) Spacing Thinning

§  In which trees at fixed intervals of distance are selected by using “STICK” for retention end all others are cut.
§  This method is well suited to plantations having uniforms productivity but in case of high mortality plantation, this is difficult to apply.
§  The formula evolved for some species to carry out thinnings are:-
i.    GLOVER’S formula for Deodar D = d
ii.   WARREN’S formula for Deodar D = 1*1/2 d =3/2  d = 1.5 d
iii. HOWARD’S formula for Dalbergia sissoo D = 2 d
      where        D         =          Spacing of trees in feet and
                        d          =          Average diameter of trees inches
§  This method may be good for an area having uniform productivity but the disadvantage that many good trees which fall in diagonals or row are removed.

LOW OR ORDINARY THINNING

·       This is known as “GERMAN THINNING” or “THINNING FROM THE BLOW” and consists of the removal of inferior individuals starting from the suppressed class, then taking the dominated class and ultimately some of the dominant class.
·       It is a very common form of selective thinning in regular crops.
·       It has been devised to be in line with nature because only those trees which have been unsuccessful in the struggle for growth are removed first.

ADVANTAGES OF ORDINARY THINNING

                 i.          It is useful in areas where the demand for small timber is more and has a market for selling.
               ii.          It is most suited for light demander species e.g. chir pine, sissoo, semal etc.
             iii.          It is simple to apply and even a less trained staff can mark the trees for felling.
             iv.          This thinning practice improves the HYGENIC condition because several diseased and insect infected trees are removed.
               v.          This thinning is preferred where climber infestation is a problem but should not be carried out where there is a danger of soil erosion.
             vi.          In this method, smaller and less vigorous trees are removed and vigorous trees are retained for fast growth.
            vii.          Removal of lower crown classes helps in the Natural Regeneration of the species.

DEMERITS OF ORDINARY THINNING

                 i.          There is always a danger of exposure to the soil
               ii.          Increases the fire hazards
             iii.          The removal of lower crown classes is troublesome and expensive due to thorny bushes, undesirable trees and climber infestation is heavy.
             iv.          Te surplus of the nutrients store is utilized by lower crown classes and remains in cycling.

GRADE OF ORDINARY THINNING

·       The thinning grades are defined by the spacing class removed so that a check may be made by comparing yield table numbers for the mean diameter in question.
·       There are various grades of thinnings e.g. A,B,C,D,E, grades.

i.    A-Grade

§  Light thinning removal of dead, dying, diseased and suppressed trees i.e. Class V, IV, and III.

ii.   B-Grade

§  Moderate thinning. In addition to trees in A-grade further removal of defective dominated stems and whips. Brandy advance growth which can not be pruned or lopped may also be removed. i.e. Class, V, IV, III, II (b), I (d) and an occasional I (c).

iii. C. Grade (Heavy thinning)

§  This includes trees in grade A and B and all remaining dominated, defective, co-dominants that may be removed without making lasting gaps in canopy i.e. Class V, IV, III, II and I (b), I (c) and I (d).
§  The C grade thinning is the standard grade in vogue for yield table compilation.

iv.  D-Grade (Very Heavy Thinning)

§  This includes trees of all above grades and some good dominants so that no lasting or permanent gap develop in canopy.
§  The left over trees are with good boles and crowns, well spaced ad evenly distributed over the site for future development.
§  The removal of trees are made from the Calss V, IV, III, II, I(b), I (c), I (d) and some I (a).

v.   E-Grade (Extremely Heavy Thinning)

§  This type of thinning is adopted mainly for research purposes.
§  This is the heaviest thinning that can be made in the crop without creating permanent gaps in the canopy.
§  Many dominant stems including that of Class I (a) are also removed.
§  It is always better to make C-grade or D-grade thinning in a dense crop in which thinning has been unduly delayed.
§  The removal of trees should be Economic or Hygienic value. In addition to above classes the intermediate grades of thinning such as C/D or D/E are also carried cut.

CROWN THINNING

·       This is also known as “THINNING FROM ABOVE”.
·       This is a kind of selective thinning in which thinning is primarily directed to the dominant trees in a regular crop, the less promising ones being removed in the interest of the best available individuals: The dominated and suppressed stems are retained unless they are dead, dying or diseased.
·       The crown thinning favours the crown development of the selected potential final crop, trees; Retaining trees of lower crown classes can help in the natural pruning of the dominant trees which are to make the final crop.
·       The attention in concentrated still further on the selection of ELITES or ALPHA stems, evenly spaced over the ground, which are retained upto maturity or till the last thinning or two, and thinning operations may accordingly be directed primarily to the removal of other stem hindering their optimum development.
·       The emphasis is made on freeing the selection of most promising stems called “ELITE or ALPHA STEMS” from the competition of their less promising neighbors.
·       The number of future stems to be required at the rotation are first selected and retained evenly distributed over the area.
·       After selecting and marking the elite tree, if considered necessary, they are cut otherwise left for the protection of the site.
·       The crop after free thinning do not differ much in appearance from those subject to crown thinning, but the attention is concentrated on the trees to be retained rather than on the trees to be removed, just as in seeding felling.

FREE THINNING

This is also called HECK’S free thinning and is a modification of crown thinning.
It is also called “ELITE THINNING” or “SINGLE STEM SILVICULTURE”.

ADVANCE THINNING

·       This method was developed by CRAIB (1939) and O, CONNER for wattle and pine plantations in South Africa.
·       Advance thinning was tried for chir pine, but this method is not suitable for light demanding species under tropical conditions.
·       The rapid extension of the crown and quick closure of the canopy after felling does not take place in South Africa but rather grass and other weed growth are encouraged increasing fire hazard.

NUMERICAL THINNING

·       Where the yield table exists, it can be used as a standard of reference.
·       During thinning the number of stems per unit area, diameter height serve as a guiding for the optimum density of the crop.
·       These factors vary with the site quality.
·       In the absence of yield table, a standard of reference has to be found within the crop itself.
·       The following types of approaches are generally followed:-
i.    N/D Relationship
ii.   N/H2 Relationship
iii. TREE-AREA RATION
iv. If y = area occupied by a single tree and
              d = diameter
then that tree:
y = a0 + a1d + a2d2
 For a stand having n trees per unit per unit area the total ground area y is given by:
The constants a0, a1, a2, are evaluated from the yield or a representative number of fully stocked plots thinned to the same grade.
·       This method is also known as “FRESH THINNING” or “HIGH THINNING”.
·       The object in crown thinning is to promote the growth of promising trees and ensure a good value of the future crop.
·       It is suited for moderately shade tolerant species, i.e. Deodar etc in which the retention of the lower canopy classes presents no difficulty.
·       It is not suited to chir pine, sissoo etc.
·       It is suited for the forest where damage due to Frost, Snow, Wind etc are common because the gaps created by dominant trees are filled by the trees of lower crown.
·       The crown thinning should be carried out where there is no demand for small size timber.

     THE ADVANTAGES OF CROWN THINNING ARE

·       This method checks soil erosion and damage due to frost, snow, wind etc.



·       Shade bearing trees are also protected.
·       The lower classes help in controlling weed and shrubs growth.
·       The side branches are pruned in a better way.

     DISADVANTAGES OF CROWN THINNING ARE

·       The dominants are adversely affected.
·       The lower tree classes make difficult various operations. e.g. marking, felling, logging and extraction of the thinned material.
·       It requires experience and skill.
·       In crown thinning only two types of thinning have been recognized as follows:-

i.    Light Crown Thinning (L.C. Grade)

§  This consists in the removal of dead, dying, diseased and wolf trees with such of the defective and after them the better dominants as are necessary to leave room for further development of the best available trees evenly distributed over the area.
§  The trees belong to classes V, VI, I (d), I (c), many of I (b) and a few of I (a) but not III and II are removed.

ii.   Heavy Crown Thinning (H.C. Thinning)

§  This grade of thinning pays even more attention to favouring the selected best times by removing all the remaining I (b), which can be taken without creating permanent gaps and more of I (a) i.e. classes V, IV, I (d), I (c) most or I (b), some of I (a); but not III and II.
§  The final crop is target to achieve certain stocking e.g. 500 to 600 trees per hectare.
§  The dominated and suppressed trees are removed.
v.   BASAL AREA
·       The basal area of the tree is related to density.
·       If the basal area per acre in a fully stocked stand is plotted against average diameter for difference ages, a grid is obtained from which the percentage basal area of the stand of a given average diameter is calculated. This gives density.
·       The 100 percent stocking curve is drawn from the tree-area ratio method.



 PRECAUTIONS IN FIRST THINNING

Ø  The first thinning is very important for any crop.
Ø  The following factors should be considered at the time of first thinning.
·       The thinning should take place before adjoining trees check one another’s growth.
·       The thinning should not be postponed on the ground that the poles obtained will be non-saleable or uneconomic.
·       The age of first thinning should be taken on the basis of height or size attained.
·       The heavy thinning should not be carried on poorer sites, drier areas and steeper slopes, because the gaps may not filled up due to poor growth of trees.
·All thinnings should be done on the silvicultural lines.
·       The light demander species may require heavier thinning as compared to the shade bearer species.
·       If the mechanical thinning is not possible due to low survival rate, poor growth, and interference by other species, it is best to carry out silvicultural thinnings. Some admixture of secondary species may be retained where this is desired on silvicultural or protection grounds.

 THINNINGS IN IRREGULAR CROPS

Ø  It is difficult to classify tree in an irregular crop.
Ø  The general classifications which are used for the research purposes are THREE crown positions and THREE Crown from viz. D, d, and s, good, medium and small.
Ø  SEHT (1956) has suggested that in regular crops the degree of crown freedom and their further classification into large, medium and small crown sizes, should be taken in account.
CROWN FREEDOM
FREE
RESTRICTED
CONFINE
Crown development or crown size
Well developed or large
Average or medium
Poor or small
Ø  The crown freedom is judged irrespective of the size of the tree and its position in the stratification.
Ø  The thinning is an irregular crop is done by selection method.
Ø  The trees to be cut is that which restrict the growth of their neighbors on all sides and should into more valuable than their neighbors.
Ø  The aim should be to maintain a crop of all diameter classes in a sufficient number of maximum sustained yield.
Ø  The valuable species are retained whereas inferior trees of all diameter classes are removed.
Ø  The removal of trees is carried out in diameter classes.
Ø  It aims at continuous improvement by repeated removal of inferior stems in all diameter classes so that basal area or standing volume becomes more or less equal to theoretical ideal value.

FACTORS AFFECTING THINNING PRACTICE

Ø  The following factors may be considered before the thinning practice:-
1.   Site Factors
2.   Nature of species
3.   Age

1.   Site Factors

·       The site quality influence on thinning practice is that a relatively close canopy on the poorer sites should be maintained.
·       On low quality sites heavy openings should not be made.
·       The spacing out widely in earlier stages should not be carried out on hot, dry, slopes, on poor soils and on all sites where grass and other weed invasion is likely to occur.
·       A poor site will usually support less stems per hectare than a good one.
·       For chir pine having crop diameter of 30 cm the number of stems are 420 (Q. C. II) per hectare and 306 (Q. c. III) and Deodar the number is normally independent of site quality.

2.   Nature of Species

·       The shade-bearers are more tolerant of crown than the light demanders therefore more frequent thinnings is needed for light demanders species.
·       Light demanders usually show good response to ordinary thinning where as shade-bearers to crown thinning.
·       Chir pine and sissoo, being light demanders require heavier grades of ordinary thinning.
·       For Deodar advance thinning is suitable as the site is not likely to degrade even after expose.

3.   Age

·       The crown formation depends on age and in young ages when the crown formation is not completed mechanical thinning or stick thinning may e done with succession.
·       The light demander species require heavier grade of thinning at frequent intervals in younger crops.
·       On the other hand some species in middle age or maturity require crown thinning.



LIGHT-DEMANDERS AND SHADE-BEARERS

Ø  The terms should refer only to the ability to achieve maximum photosynthesis under low light intensities SHADE DEMANDERS or to show an increase in photosynthesis activity wt increasing light up to full intensity, LIGHT DEMANDERS.
Ø  Shade-bearers are those which are capable of regeneration and development under a more or less complete canopy of other species whilst light demanders require more light for regeneration and development. e.g.

(a) Light Demanders:

                           i.               Quercus ilex
                         ii.               Pinus roxburghii
                       iii.               Pinus wallichiana
                       iv.               Pinus geradiana
                         v.               Populus ciliate
                       vi.               Olea ferruginea
                      vii.               Dodonea viscose
                    viii.               Acacia catechu
                       ix.               Bombax caiba
                         x.               Dalbergis sissoo
                       xi.               Plantanus orientalis
                      xii.               Alnus nitida
                    xiii.               Acacia nilotica
                    xiv.               Azadarachta indica
                      xv.               Melia azadarachta
                    xvi.               Cappris aphylla
                  xvii.               Prosupis cineraria
                xviii.               Exotic poplars
                    xix.               Eucalyptus camaldulensis
                      xx.               Salvadora oleoides
                    xxi.               Tamarix aphylla
                  xxii.               Zizyphus mauritiana
                xxiii.               Avicennia marina

(b) Moderate Shade-bears

                           i.          Salix species
                         ii.          Quercus utilis
                       iii.          Picea smithiana
                       iv.          Quercus dilatata
                         v.          Cedrs deodara
                       vi.          Abies pindrow
———-
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