Ecological thinning is a silvicultural technique used in forest management that involves cutting trees to improve functions of a forest other than timber production.

Although thinning originated as a man-made forest management tool, aimed at increasing timber yields, the shift from production forests to multifunctional forests brought with it the cutting of trees to manipulate an ecosystem for various reasons, ranging from removing non-native species from a plot to removing poplars growing on a riverside beach aimed at recreational use.

Since the 1970s, leaving the thinned trees on the forest floor has become an increasingly common policy: wood can be decomposed in a more natural fashion, playing an important role in increasing biodiversity by providing habitat to various invertebrates, birds and small mammals. Many fungi (e.g. Calocera viscosa) and mosses are saproxylic or epixylic as well (e.g. Marchantiophyta) – some moss species completing their entire life-cycle on a single log.

Where trees are managed under a commercial regime, competition is reduced by removing adjacent stems that exhibit less favourable timber quality potential. When left in a natural state trees will "self-thin", but this process can be unreliable in some circumstances. Examples of this can be found in the Buxus–Ironbark forests and woodlands of Victoria (Australia) where a large proportion of trees are coppice, resultant from timber cutting in decades gone by.

Thinning decreases canopy closure and increases the penetration of solar radiation into the canopy. The photosynthetic efficiency of this energy is improved, and needle retention is prolonged, especially in the lower parts of the crown. The root system, crown length, crown diameter, and crown area all increase after thinning. Even if soil evaporation and individual tree transpiration increases after thinning, total evapo-transpiration at stand level tends to decrease; canopy water interception is reduced and throughfall increased, so that tree-water status usually improves after thinning.

Radial annual growth is an integrative index of tree physiological response to environmental variation. Working with Norway spruce, which can be expected to behave in some respects similarly to white spruce, Misson et al. showed that a reduction in stand density alters the classical climate–growth relationship. At individual tree level, thinning could be used to increase tree resistance to drought stress. Nevertheless, this effect is limited when site conditions are limiting. Misson et al. concluded that heavy thinning should be applied when forest decline is expected from drought stress. Furthermore, stands on dry sites should be thinned more heavily because such sites cannot support high-density stands.^{[citation needed]}

Aussenac, also working with Norway spruce, investigated the response to thinning. Thinning altered the temporal evolution of radial growth at all frequencies in radial growth chronologies. Previous studies had shown, not surprisingly, that thinning decreases the canopy closure and encourages solar radiation to penetrate the canopy. Solar energy then becomes more important inside the crown. The improved photosynthetic efficiency of this energy influences the retention time of needles, especially in the lower part of the crown. Furthermore, the length, diameter, and area, of the crown, and the size of the root system all increase after thinning.

These modifications at the crown level influence photosynthate production positively as long as water supply is not limiting. Even if soil evaporation and individual tree transpiration are more important after thinning, total evapo-transpiration at stand level tends to decrease. Furthermore, with the reduced importance of leaf area index (LAI), thinning reduces canopy water interception and increases throughfall. This explains why the soil moisture content increases in a thinned stand. Thus, during the vegetation period, tree-water status is usually better in a thinned stand than in a high-density stand.