Forest Thinning

Thinning in forestry is the selective removal of trees, primarily undertaken to improve the growth rate  or health of the remaining trees. This may be done to make the stand more profitable in an upcoming final felling or to achieve ecological goals such as increasing biodiversity or accelerating the development of desired structural attributes such as large diameter trees with long tree crowns.

Thinning has most been developed as a science in Centeral Europe. There are significant developments in this. These methods have been applied outside of Europe to many forests around the world, based on this basis - 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 or extreme temperature.

A thinning in which the trees removed have little or no economic value is called a pre-commercial thinning. Ecological thinning is a variant of this being trialed for use in forest conservation in Australia.  Chemical thinning is a form of non-commercial thinning in which the trees are killed while they stand by injecting a chemical such as glyphosate into a cut made in the stem. This reduces the number of live stems remaining, providing a benefit to those that remain and may be undertaken where the cost of a traditional thin is high. It can also be don on very exposed sites where breaking the canopy through a traditional thinning operation would expose the stand to a high risk windthrow.

Traditionally thinning has been done to create a desired balance between individual tree attributes and per area attributes such as volume. It has been, and often still is, applied with he desire to create uniform stands. Thinning treatments are often descried in terms of number of trees per area to remain or average spacing between trees. The intent is to create and manage uniform stands.

Another type of thinning is called variable density thinning. In this type of thinning, the intent is to manage various portions of the stand in different ways to create structural and spatial heterogeneity. The intent is often to increase biodiversity or wildlife habitat. In variable density thinning, some portions of the stand may not be entered. These unentered areas, sometimes called reserves, leave islands, or skips (as they are skipped over) help retain a large range of tree diameters, serve as a future source of competition-related mortality, and may preserve snags, down wood, and understory plants. Other portions of a stand could be heavily thinned or gaps or openings could be created. These areas accelerate the growth rates of trees in the open areas or on their perimeter and help retain or develop long crowns with live branches. Another portion of the stand, sometimes referred to as the matrix, is thinned to result in residual trees densities which area in between the other extremes. Over the whole area, a wide variety of trees with different diameters and species are retained.