Thinning

Thinnings are used to steer the development of trees by removing some of them, which improves the growing space and other growth resources available to the remaining tree stand. An appropriate stand density maintains the vitality of the trees and ensures that a sufficient proportion of the tree’s height consists of a living crown.

First thinning and subsequent thinnings are intermediate fellings in the forest.

• First thinning is the first intermediate felling that produces marketable pulpwood and energy wood.
• Subsequent thinnings are thinnings made on a middle-aged forest stand after the first thinning. Subsequent thinnings are usually performed 1 to 2 times during the rotation period, i.e. before regeneration felling.

Thinning implementation varies by site

The correct timing, thinning method and intensity are selected flexibly in accordance with both the properties of the forest and the forest owner's objectives.

Many forest properties affect the way thinning is carried out, so it is a good idea to plan the thinning with a professional. For example, the main tree species, the growing region and the initial state of the forest all have an impact.

Timely thinning carried out with appropriate methods

• accelerates the diameter growth (thickening) of the remaining stand
• improves the quality of the tree stand
• generates timber sales revenue
• provides an opportunity to influence tree species distribution
• maintains the health and vitality of the stand
• enhances the climate resilience of the stand.

 Crown ratio reflects the vitality of the trees

Competition for growth resources, i.e. light, water and nutrients, weakens the growth potential of individual trees.

If the forest is too dense, the proportion of the live crown in relation to the height of the tree is shortened, and the stand becomes spindly. Spindly growth (etiolation) means that when competing for light, the trees prioritise vertical growth, and diameter growth remains limited.

In young forest stands, a good indicator of tree vitality is the proportion of the tree height occupied by the live crown, i.e. live crown ratio:

• in pine stands, a minimum of 40%
• in birch stands, a minimum of 50%
• in spruce stands, a minimum of 60%

The thinning method refers to the principle by which the trees to be removed are selected.

• Low thinning removes smaller and lower-quality trees, leaving the largest and most economically valuable trees to grow.
• In high thinning, in addition to small trees, some of the largest and most economically valuable trees are also removed.
• Quality thinning removes branchy main dominant trees, in connection with the first thinning of pine stands, and makes room for thin branched codominant trees, thus increasing the yield of high-quality log.
• The extended-rotation thinning model is suitable for managed conifer-dominated forests. The model prescribes lighter thinnings, and they are carried out slightly later, which results in slower diameter development of the stand.
• The thinning model for mixed-species stands provides suitable growth space for different tree species. Models have been prepared according to the site for three different mixed-species stands, spruce-broadleaf mixed stands, pine-broadleaf mixed stands and spruce-pine mixed stands.

From the various thinning methods, the forest owner can choose a method suitable for their own objectives and for the initial state of the stand to be thinned. 

Different thinning recommendations have been drawn up for both managed and unmanaged forests, in accordance with the location of the stand, the main tree species, the site, the soil type and the forest owner’s objectives. 

First thinning is the first cutting of a young forest stand after the seedling stand tending phase, in which marketable pulpwood and energy wood is harvested.

The main objective of first thinning is to improve the quality of the stand and to safeguard their diameter growth. First thinning has a significant impact on the future growth and development of the stand. Trees grow well when they receive enough space, light, nutrients and water.

Timely first thinning is important to prevent a decline in stand growth performance. Depending on the site and tree species, the first thinning is carried out when the trees are between 10 and 16 metres tall.

When selecting a suitable time for first thinning, the balance between total roundwood removals, harvesting costs and the preservation of the viability of tree crowns should be considered.

First thinning in unmanaged forests 

Unmanaged forest refers to young forests that have not been tended to in a timely manner in the seedling stand phase, and their quality is therefore not as good as that of managed forests. The objective of a first thinning restoring an unmanaged forest is to improve the forest management situation.

Thinning unmanaged forests is more expensive than thinning managed forests, due to the larger number of boles to be removed and their smaller bole volume. However, thinning is an important measure in terms of forest management, as it promotes the future development of the remaining stand.

There are also several ways of thinning unmanaged forests, depending on the initial state of the stand and the objectives of the forest owner. 

Instructions for timely tending of seedling stands 

Energy wood thinning

Energy wood is usually harvested from first thinning sites and young forest management sites. Energy wood thinning is suitable for both managed and unmanaged forests.

The harvesting of energy wood from forest stands as part of thinnings most often generates income for the forest owner to cover the costs of harvesting. The cultivation of energy wood alone is usually not economically viable.

Depending on the initial state and the objectives of the forest owner, there are also different implementation methods for energy wood harvesting in forest stands:

• The harvesting of whole trees is suitable for young forest restoration sites with plenty of small-diameter, spindly trees. The sites are usually not cleared in advance, and the trees are harvested without delimbing.
• The harvesting of delimbed stems is well suited for all thinning sites containing small-diameter wood. These sites generally have larger trees than typical whole-tree harvesting sites.
• In integrated harvesting, small-diameter wood is harvested from the same site for both processing and energy use. The profitability of harvesting requires sufficient timber grade volumes.

Further reading: 

Energy wood thinning (in Finnish, Forest management recommendations)

Thinnings include fellings after the first thinning. They are also called subsequent thinnings.

Subsequent thinning is often carried out approximately 10 to 20 years after the first thinning, at which time logs can also be harvested from the stand. The need for thinning is assessed based on the height and density of the stand.  

Spruce and birch are often thinned twice before forest regeneration, while pine trees may have three thinnings. Only one thinning is recommended in spruce forests contaminated with root rot before final felling. 

The aim of thinning is to accelerate the diameter growth (thickening) of the remaining stand by increasing the growing space of trees and concentrating growth on economically valuable and good-quality trees. 

Thinning affects tree species distribution, as well as maintains a healthy stand, reduces natural loss and generates felling income. The profitability of thinning and the further development of the stand can be influenced by thinning methods, which include low and high thinnings, as well as thinning models for extended rotation period and mixed-species stands.

Forests’ ability to adapt to climate change and its impacts can be ensured through timely measures that comply with forest management recommendations. Healthy, viable and diverse forests are able to recover well from natural disturbances and, at the same time, provide habitats for diverse species.

Most of our forests are commercial forests. For this reason, it is important to promote biodiversity and game management also in thinning by commercial forests’ nature management methods, such as saving decaying trees, leaving game thickets in addition to retention trees and favouring deciduous trees.

Climate-resilient forest management
Nature management methods in commercial forests