A thinning formula

Judicious thinning of your conifer forest will result in bigger trees and a more valuable crop

Afarmer I was talking to recently was very pleased with himself. “Rather than trees going in, it is good to see timber coming out,” he said. “Putting money in my pocket.”

He was telling me about the thinning he was carrying out. Thinning can be one of the most important operations.

Carrying out a thinning in a conifer forest for the first time will set it up very nicely for the coming years.

A good road into the forest has been created, more growing space has been provided for the remaining trees and biodiversity will improve.

It will also give you a pretty good idea of the timber quality and which trees need to be removed next time.

When a new farm forest is established, 2,500 trees are planted per hectare. These trees will increasingly compete with each other as they grow.

Gradually removing the poorerquality trees over time will provide more growing space for the remaining better-quality trees. This will create a forest of fewer trees but of greater quality and size and of greater biodiversity value.

Eventually, about 500 carefully selected trees per hectare will be left, with each tree having a volume of 0.7-0.8 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.

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

Thinning a forest isn’t always the right thing to do, especially where forests are established on very exposed, wet or unstable soils.

This also holds true where access is difficult, the distance to the sawmill is very long or where thinning has been left very late.

I prefer to thin early, especially in the west of Ireland. This will help to create a more stable forest.

Such advice is not very popular as it makes the thinning operation commercially less attractive for both the farmer and the contractor!

First thinning of Sitka spruce should be carried out when trees are between 10 and 12 metres in height. Usually, trees are then 14-20 years of age.

It is essential that inspection paths are cut through the forest. This allows access into and through the forest so that the suitability for thinning and tree quality can be assessed.

There is a short, handy video on www.teagasc.ie/forestry explaining this process. Take appropriate safety measures if using a chainsaw.

Allow two years to plan thinning operations

Effective and timely planning is crucial because between one thing and another, it can easily take two years or more before actual thinning will take place.

In particular, building a forest road, applying for a roading grant and arranging a felling licence can be a very lengthy process.

As the loading of timber is illegal on public roads, even small forests will require road construction to provide timber loading areas.

Construction should be completed one to two years in advance so the road can settle before harvesting operations take place. Roading grants may be available from the Forestry Division of the Department of Agriculture.

Since February, the Department acts as the single consent authority for applications for forest road works licences, where the forest road provides access to a public road (other than a national road) or where substantial widening of an existing entrance is required.

A Felling Licence is required to carry out thinning and can remain valid for up to ten years. Apply well in advance to the Department.

Application forms as well as worked-out examples are available from www.teagasc.ie/forestry

Normally one line in seven is

Planning:

When carrying out a second thinning, it is important to establish thinning control plots completely removed and inferior trees are selected for cutting out of the remaining lines in a first thinning operation.

This should leave the trees nicely spaced and very accessible. As a general rule, about a third of the trees are removed in a first thinning.

A first thinning is about setting up access and removing inferior trees while allowing the valuable trees to grow on.

Once the first thinning is completed, the trees grow on for three to five years when the second thinning is carried out.

Second thinning is very different because all the trees that will be removed need to be individually selected.

Walk down the racks in your forest and look up at the crowns of the trees.

When the branches are starting to touch across the rack, then a second thinning is due.

By measuring diameters, heights and tree stocking density, you can calculate which trees need to be removed. It will also allow you to avoid either cutting too many trees or too few.

The secret to get your second thinning right is to lay down and measure plots while supervising harvesting operations.

More more information, register for our Virtual Talking Timber event taking place on Tuesday, July 14 (see panel).

First thinning of Sitka spruce should be carried out when trees are between 10 and 12 metres in height

Steven Meyen is a Teagasc forestry development officer based in Donegal