in pruning. the selective removal of live branches to provide light or air penetration through the tree or to lighten the weight of the remaining branches.
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Thinning is a term used in agricultural sciences to mean the removal of some plants, or parts of plants, to make room for the growth of others but does not involve the cutting of the whole tree. Selective removal of parts of a plant such as branches, buds, or roots is typically known as pruning.
In forestry, thinning is the selective removal of trees, primarily undertaken to improve the growth rate or health of the remaining trees. Overcrowded trees are under competitive stress from their neighbors. Thinning may be done to increase the resistance of the stand to environmental stress such as drought, insect infestation, extreme temperature, or wildfire.