First thinning of conifer forests

What is thinning?

When a new farm forest is established the trees are planted at a stocking rate of at least 2,500 trees to the hectare. After a number of years they begin to compete with each other as they grow. If the forest is not thinned, it is likely that by the time the timber is ready for clearfelling the number of trees per hectare would be about 1,400, the remainder having died off due to natural competition. The average tree volume would be about 0.4 m³.

However, if a percentage of the trees are removed at various stages during the life of the trees, the remaining trees have more growing space, resulting in fewer trees but of greater quality and size. In a forest that has been thinned throughout its life there should be about 500 trees per hectare remaining at the time of clearfell with each tree having a volume of 0.8 m³. This is twice the size of trees in an unthinned plantation. This results in a more valuable  timber crop as larger trees command a much higher price as they are used to produce higher value products.

If properly carried out, thinning:

• optimises the return from your forest,
• provides periodic returns as the crop matures and
• improves the biodiversity of the forest.

Not thinning will result in a larger number of smaller sized trees, with a likely reduction in timber value.

For further details on how to go about harvesting and selling timber:

• Harvesting and Selling Timber from Conifer Forests 

Systems

The first thinning operation in conifer forests normally involves removing lines of trees to get access into the forest. Normally, one line in seven is completely removed and inferior trees are selected for cutting out of the remaining lines. This leaves the remaining trees nicely spaced and very accessible.

However, the size of the timber removed in this first thinning is very small and the quality is often quite poor. This may mean that the operation scarcely breaks even or may even cost money. It can however be looked on as an investment in the future of the forest. As a general rule, a maximum of a third of the existing number of trees and a corresponding third of the timber volume are removed in the first thinning.

Once thinning is completed, the trees are allowed to grow on for a further three to six years before it is thinned again. This second and any subsequent thinning are normally done by using the existing access lines that were created in the first thinning and removing any trees that are affecting the quality of the potential final trees.

Relevant publication

• Farm Forestry Series No. 10: First Thinning in Conifers (PDF)