In addition to maintaining growth, pre-commercial thinning is required to safeguard the quality of the growing stock, as it helps create more growth space for better trees. In conifer stands, this means removing broadleaved trees hampering the development of coniferous trees, as well as poor-quality trees, trees growing too fast and conifers growing too close to each other.
Pre-commercial thinning should be carried out when spruce trees reach a height of 3–4 metres, pine trees a height of 4–7 metres, and silver birch trees a height of 4–5 metres. The stand is usually 10–15 years old at this stage.
In pre-commercial thinning, a mix of broadleaved trees is left in the forest, mainly silver birch grown from seed that are as tall as conifers, taking the total density into account. Other broadleaved trees of value to biodiversity are also saved in suitable places so that no tree species growing in the stand is completely removed.