Thinning

In agricultural sciences, thinning is the removal of some plants, or parts of plants, to make room for the growth of others. 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.

Tree thinning may be practised in forestry to make a stand more profitable in an upcoming final felling, or to advance ecological goals such as increasing biodiversity or accelerating the development of desired structural attributes such as large diameter trees with long tree crowns.

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. Epicormic shooting 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. It has been, and often still is, applied with the desire to create uniform stands. 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 conditions 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) whereas other species have not for example slash pine.

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.

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 studied for use in forest conservation. The primary aim of forest thinning is to increase growth of selected trees, but ecological thinning is done to favor development of wildlife habitat (such as hollows) rather than focusing on increased timber yields. Thinning may also reduce the risk of wildfire by increasing availability of groundwater as well as reducing fuel for wildfires.