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In agricultural sciences, thinning is the removal of some , or parts of plants, to make room for the growth of others. Selective removal of parts of a plant such as , , or 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 forest stand to environmental stress such as drought, insect infestation, extreme temperature, or wildfire.
Early thinning, e.g. after 20 years, rather than late thinning, e.g. after 50 years, has different effects on the trees thinned. An early thinning would encourage trees to develop wider crowns, increase trunk diameter and be more stable against threats like snow breakage or windthrow. Too much thinning too early in their lifespan could cause a site to overgrow with shrubs and prevent an understory from regenerating efficiently. Alternatively, thinning an area later would mean that trees will grow tall and slender; although the trees would gain value incrementally with time, they may be less marketable as thinness reduces said value. is a risk when thinning is carried out tardily, which can lead to tree branchiness and the presence of knots in the resultant timber harvest, again reducing value.
Traditionally, thinning has been performed to create a desired balance between individual tree attributes (such as tree diameter) and area-relative attributes such as volume.Smith et al. 1997 It has been, and often still is, applied with the desire to create plantation. As a result, thinning treatments are often described in terms of number of trees per area to remain or average spacing between trees. It is also necessary when too many trees were initially planted or survived through the seedling phase. Planting less and thinning less saves money in commercial forestry; thinning is carried out in a balanced manner, dependent on site qualities as well as planting régimes.
There is no certain outcome from thinning nor standard timing for it. Individual Site index and responses of individual or mixed species on sites may vary considerably and thus responses to thinnings likewise are inconsistent. Norway spruce on a suitable site have responded well in terms of growth increment, to late thinnings (after 50 years)Responses from thinning a 50 year old Norway spruce plantation in central New York, SUNY Coll. Enviro. Sci. & Forestry Sch. Tech. Publication No. 1 (ESF 85-002) Drew, AP 1985 whereas other species have not for example slash pine.Thinning practices in short-rotation stands. In A.B. Crow (Ed.), Advancement in management of Southern pines pg 50-60 10th Ann Forerst Symposium Baton Rouge LA. State UniPress 1961
Due to such variability it is better to talk about a thinning régime rather than one particular method of thinning taking place in a stand.
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.
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.
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