Thinning creates a danger of tree damage

The impact of thinning on forest biodiversity is similar to that of final felling, but is considerably less severe. In particular, the impact on soil is negligent, but above ground thinning will also increase light and warmth.

Certain studies have found that final felling, forest regeneration and especially soil cultivation may also intensify the harmful effect of acid fallout and increase the leaching out of nutrients.

The purpose of thinning is to improve the growth of the best tree specimens and to increase their size by removing the weaker specimens from the forest. It is particularly important that thinning operations do not damage the trees left growing in the forest. On the other hand, decaying trees may be beneficial for forest biodiversity.

The most typical damage is caused to the stem, such as the breaking or abrasions of the bark, and root damage caused by machinery. Spruce and birch are most prone to damage. All damage deteriorates the growth and the quality of timber. It also exposes the trees to wood-rotting fungi. As an example, a deep-going damage at the point where the base and the roots meet may cause the tree to decay, and when the decay spreads the tree will be useless for sawmills, thus reducing its value by as much as several dozen percent. However, the breaking off of a root at a distance of more than 70 centimetres from the soil surface generally does not cause the stem to decay.

Nowadays, about 2–3 percent of the trees left in the forest are damaged by thinning. For the first thinning, which is normally done about 40 years after the seedling stage, this means about 20–30 damaged trees per hectare. Trees damaged during the first thinning are generally removed in the following thinning, when the forest is about 60 years old.

Damage may be prevented by a good planning of the work sites, by selecting suitable felling methods and machinery, and by careful harvesting. There is less damage if the trees are harvested in winter, since the frozen soil will protect the roots. A dense network of roads and tracks will decrease damage to the stems, but will increase root damage and decrease the profit from forestry in many ways.

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