What are thinning operations?

What are thinning operations?

As part of our ongoing research into legislative approaches to forest protection, we continue reviewing common forest management practices. We have already explained the specifics of sanitary fellings and conversion fellings; next in line are thinning operations.

Thinning is a type of tending felling carried out in middle-aged stands to improve their composition and structure, increase the growth of high-quality trees, and reduce the time needed to grow mature timber. The main purpose of these fellings is to provide the best trees with more light, root space, and radial stem growth. Overall, thinnings improve the commercial value of forest stands.

Key features of thinning operations

Before thinning can be conducted, a stand must have the following characteristics:

• the required species composition;

• proper stand density (not less than 0.7 for artificial pine forests and 0.8 for other forest types);

• high stem fullness in the best trees—meaning the trunk has an approximately uniform diameter along most of its height;

• sufficient natural pruning (clean stems without branches up to a significant height);

• well-formed crowns of appropriate size.

If a stand meets all criteria, ground-and-light growth stimulation can begin. When some trees are removed, the crowns of neighboring trees gradually expand. The crown area increases, lighting improves, and the root systems occupy a larger volume of soil. After several years, the desired radial stem growth appears in the best trees. In other words, trees begin growing in thickness rather than height, accelerating the attainment of commercial maturity. Thinning supports not only the best trees (dominant layer), but also the second canopy layer and understory shrubs that shade and protect the soil surface.

Ecological purpose and benefits

Thinning operations help achieve:

• increased growth: the best trees receive more light and space, stimulating radial growth;

• improved stand composition and structure: weaker or excess trees are removed, forming more stable and resilient stands;

• higher commercial value: well-formed, uniform stems with a higher yield of quality timber reach technical maturity faster;

• better crown and stem formation: shading competitors are removed, improving stem cleaning and crown development;

• favorable conditions for the understory: the presence of understory vegetation helps shade the soil and reduce excessive evaporation.

Regulation

Recommended frequency of thinning is every 10–15 years. For other tending fellings:

• cleaning and early thinning: every 3–5 years;

• pre-commercial thinning: every 5–10 years.
  
   In mixed or structurally complex stands, intervals may be longer. Thinning is prescribed when:

• coniferous and hard-broadleaved high-forest stands reach 41+ years,

• other soft-broadleaved and low-forest species reach 31+ years.

Required documents for an electronic felling permit for thinning operations

• application for a felling permit;

• stand compartment statement;

• logging site plan;

• copy of the stand's forest inventory description.

For felling on protected areas, the following are additionally required:

• a quota for the special use of natural resources;

• a permit for the special use of natural resources within protected areas.

If thinning is conducted in stands not included in forest management planning, an inspection act must be provided. If the diameter at 1.3 m of trees to be felled is 8 cm or more, a field inventory sheet and valuation sheet must be submitted.
If the diameter is below 8 cm, an act of marking and delimiting the felling site is required. The electronic felling permit system ensures transparency, reduces corruption, and strengthens control over timber harvesting volumes.

Violations associated with thinning

• exceeding the permitted intensity: removing more than 10–20% of the stand damages the forest;

• conducting thinning more frequently than every 8–10 years;

• leaving large numbers of coppice or low-quality trees;

• felling valuable or seed-producing trees;

• creating large gaps in the canopy, especially when removing aspen or birch.

Failure to follow regulations can reduce forest quality and disrupt ecological stability. Canopy gaps increase vulnerability to pests, diseases, and fires. Excessive or improper thinning may lead to dieback, species imbalance, fragmentation of forest stands, and weakened protective functions.

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The publication is produced by NGO «ForestCom» with the support of the Askold and Dir Fund as a part of the Strong Civil Society of Ukraine –  a Driver towards Reforms and Democracy project, implemented by ISAR Ednannia, funded by Norway and Sweden. The contents of this publication are the sole responsibility of NGO «ForestCom» and can in no way be taken to reflect the views of the Government of Norway, the Government of Sweden and ISAR Ednannia.