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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.
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 Central Europe. There are significant developments in this by Carlowitz 1713, Dhamel du Monceau 1750, Robert Hartig 1791, Cotta 1817, Seebach 1845, Heyer 1854, Kraft 1884, Borggreve 1891, Biolley 1901 and Schädelin 1934. 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 (Round Up) 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 done on very exposed sites where breaking the canopy through a traditional thinning operation would expose the stand to a high risk of windthrow.
Traditionally thinning has been done to create a desired balance between individual tree attributes (such as tree diameter) and per area attributes such as volume. It has been, and often still is, applied with the desire to create uniform stands. Thinning treatments are often described 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.
Thinning individual trees
In agriculture and gardening, thinning is the selective removal of flowers, fruits, shoots, and seedlings or young plants to allow adequate space for the remaining organs/plants to grow efficiently. In large-scale farming, techniques like precision seeding and transplanting can eliminate the need for thinning by starting plants at their optimum spacing. On a smaller scale, such as a home vegetable garden, thinning can be used as a way to make maximum use of space for certain crops. For example, beets, carrots, green onions and others can be planted densely, and then thinned to make room for continued growth of the plants left in the soil, and also as a harvest of baby vegetables (beet greens, baby carrots, baby onions).
- "Ecological Restoration vs. Thinning". Northern Arizona University.
- Smith et al. 1997