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 plantation is not thinned it is likely that by the time the crop is ready for clearfelling the number of trees per hectare would be about 1,400, the remainder having died off due to natural competition in the crop. 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 crop the remaining trees have more growing space, resulting in fewer trees but of greater quality and size. In a plantation 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 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 crop, 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 crop value.
It is necessary to obtain a felling licence before any trees are felled in a crop.
The other major consideration is access. It may be necessary to construct a road or at least a loading area for timber. Grants may be available towards the cost of this road.
The first decision to be taken is whether or not to thin the crop. Thinning may not be an option where the site is very exposed, wet and/or unstable, has restricted access or where it is not economically viable.
To help in making this decision and in deciding if the crop is ready for thinning, inspection paths should be cut through the crop. This allows access into the crop so that a forester can assess its suitability for thinning.
This process is called 'brashing' and involves removing branches to head height between two lines of trees. Parallel paths should be 50 to 100 metres apart depending on the size of the forest. Ensure that the paths are straight. Take appropriate safety measures if using a chainsaw.
If it is decided that the crop is suitable for thinning, it is then measured and, if it is considered a viable long-term economic prospect, a schedule of thinning operations is drawn up. First thinning can take place as early as 13 to 15 years in a very productive crop of Sitka spruce while it can be as late as 22 to 24 years on an unproductive site.
The first thinning operation in conifer crops normally involves removing lines of trees to get access into the crop. Normally one line in seven is completely removed and inferior trees are selected for cutting out of the remaining lines. This leaves the crop 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 the farmer money. It can however be looked on as an investment in the future of the crop. As a general rule, a maximum of a 1/3 of the existing number of trees and a corresponding 1/3 of the crop volume are removed in the first thinning.
Once thinning is completed, the crop is allowed to grow on for a further three to six years before it is thinned again. This second and any subsequent thinning is 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 crop trees.
Before beginning to thin, it is important to establish a market for the timber. There are generally three markets for first thinnings. Pulpwood will either go to board mills or for energy production. These are low-value options. The alternative market is for stakes if the crop is suitable. This tends to be more profitable.
Planning is vital
Effective planning is crucial to optimising the return from your crop where the decision is to thin. It is advisable to plan at least two years in advance of first thinning.
- As forests become inaccessible, inspection paths should be cut. Inspection paths are essential to gain access into the crop. These paths permit the assessment of the crop by a professional forester.
- Decide whether access to the site needs to be improved. Good road access and loading bays are essential for efficient timber extraction. Construction of forest roads, where necessary, should be completed well in advance of thinning. Roading grants may be available from the Forest Service.
- A felling licence is required to carry out thinning. Apply well in advance. Application forms are available from the Forest Service or Teagasc.
- Put together a timber sales pack including species, age, volume and relevant maps. Consider the need for professional supervision.
- Check contractors for appropriate insurance. Make sure you have a harvesting contract in place with a timber buyer before beginning to thin.
- Co-operation between forest owners in the timing and co-ordination of harvesting operations is beneficial to all.
- And last but not least, follow all Health and Safety regulations during thinning operations!
John Casey, Forestry Adviser with Teagasc explains why thinning your forest is important. Filmed at a Talking Timber event in Co Cork, June 2019.
John Casey, Forestry Adviser with Teagasc explains the various steps that are required before timber harvesting takes place. Filmed at a Talking Timber event in Co Cork, June 2019.