Trees need light, water and nutrients to grow. There is an endless competition for growth resources in the forest between individual trees. Having enough space to grow in a suitable spot is an advantage. In commercial forests, thinning is done to improve the growth of the best trees. Thinning generates wood sales income for forest owners and produces wood raw material for forest industry.

Thinning makes more resources available for trees that are selected to grow further. By cutting down the number of trees in a forest stand, the production capacity of forest soil is allocated for the remaining trees. More space also means enhanced light conditions. This allows for more effective photosynthesis, adding to biomass production. Trees react to increased space around them by speeding up stem diameter growth and putting more effort on crown development. Underground, changes also occur: trees strengthen their root systems to increase the uptake of water and nutrients.

Thinned forest stands produce higher volumes of commercial wood. Those trees that are worse off would eventually die as a result of competition. By thinning the forest at the right time, natural drainage can be turned into increased volumes of commercial wood. Thinning also promotes stem quality and value, which is important for profitability.

In even-aged forest stands, thinning is usually done by harvesting smaller and bad quality trees. The best trees in the dominant canopy layer are left to grow in optimised spatial distribution. In continuous cover forest stands, a significant number of the biggest trees are harvested to create small openings in the canopy layer to allow regeneration from the undergrowth.

In Finnish forestry, thinning is often done in two rounds during the forest rotation cycle. The first thinning is made when the forest stand is around 25 – 45 years old, depending on forest stand development, tree species and geographical location. The second thinning takes place 20 – 30 years later.

In conifer dominated stands, at least ten percent proportion of broadleaf trees is targeted over the forest rotation cycle. Growing mixed stands with native tree species promotes forest biodiversity. It also has a positive effect on forest production capacity and makes the forest ecosystem better resistant to natural damages. Rare tree species are not harvested because of their biodiversity value.

Valuable habitats, buffer zones around waters and other protected areas are left untouched. During thinning, it is already possible to leave future retention tree groups and dense thickets where game species can find shelter. Forestry planning has a long-term perspective and supporting biodiversity fits into this mindset very well.