It is the reduction of trees in an immature stand for the purpose of improving the growth and form of the trees that remain without permanently breaking the canopy of future trees.

Objectives:

• To distribute growth potential of a site amongst the trees retained.
• To increase the net yield of timber and money from a stand
• To obtain earlier returns from capital invested in a stand
• To utilizethe thinned material
• To shorten the rotation
• To produce a different smaller size of timber which can meet a different object of management
• To maintain hygienic conditions in the stand
• To allow intermediate harvests of small and large woods before the final harvest satisfying the needs of the users
• To favor natural regeneration in a stand though not of prime intention
• To control species composition in a stand by removing undesirable species during the operation

When?

• The time for thinning a stand is determined when the crowns of the adjacent trees are close together and there is no further space into which individual trees can grow
• Removing of unwanted species carries out when there is a need to change species composition in a forest stand
• When users demand of fuel wood and small wood has to be fulfilled
• Thinning is done any time when dead and diseased trees are sighted.

How ?

• The number of plants, inferior in quality, is reduced in one or several operations
• Remove all dead and diseased trees at first instance. They can be removed at any time
• Trees that are dying (75% dead) may be retained temporarily if removal of crops exposes the soil

Limitation

No numerical basis showing the levels of optimum basal area for the types of forest prevalent in the country has been found.

Major kinds of thinnings

1. Mechanical thinning (also called stick thinning): A thinning in which the trees to be cut are selected by some rule of thumb, e.g., trees in alternate diagonals or rows, alternate trees in alternate rows or every second, third, etc., line or a minimum spacing gauged by a standard stick.
2. Ordinary thinning (also called low thining or German thinning or thinning from below): The method of thinning in common use that consists in the removal of inferior individuals of a crop, starting from the suppressed, then taking the dominated and lastly some of the dominants.
3. Crown thinning (High thinning or French thinning or thinning from above): A method 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 if they are not dead, dying and diseased.
4. Free thinning (devised by Heck): A method of thinning in which attention is concentrated on evenly spaced selected stems (called elites or alpha stems) which are retained until maturity or til the last thinning or two, thinning being directed to the removal of other stems hindering their optimum development.
5. Maximum thinning: It is the modification of Heck’s free thinning developed by Gehrhardt. It is defined as a method of thinning which ‘ aims at putting as high a proportion possible of the total potential increment of the area on the retained stems; from an early stage the number of such stems limited to the minimum that can fully utilize the growing tree. It is heaviest form of free thinning so that there are no trees other than elites. As the thinning is extremely heavy, it may result in deterioration of site due to exposure, infestation of the area with shrub growth, production of knotty timber.
6. Advance thinning (also Craib’s thinning): A thinning done in a regular crop in anticipation of suppression. It was developed by Craib and O’Connor for wattle and pine plantation in south Africa. Thinning is done after the trees have been adversely affected by the competition of their neighbors. It should be done before competition actually sets in.
7. Selection thinning: A method of thinning directed to obtain and/or maintain selection composition in a crop, with all diameter classes adequately represented.’ It is carried out in all canopy classes removing the trees of the following characteristics:
   • Dead, dying and diseased trees
   • Inferior trees which restrict the development of their neighbors from all sides
   • Trees which are less valuable than their neighbors
   • Trees which are of no special importance as regards desirable crop mixture
   • Inferior trees standing over groups of well-grown advance growth which may also be thinned