Saturday 17 March 2018

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Several people contacted me following my previous article on preparing your conifer forest for thinning. Some asked me to explain how the Teagasc Thinning Ready Reckoner works.

One person, who clearly knew what he was talking about, couldn't understand how comparing the number of trees with the average tree diameter gave you all necessary thinning information.

Well, he's right. This Ready Reckoner won't tell you exact volumes but it is rather designed to indicate when the Sitka spruce trees are due for thinning.

It is a very useful and simple tool as the 'traffic light' system allows you to estimate when trees will be ready for thinning: red means the trees are not ready for thinning yet, amber indicates that the trees are nearly ready for thinning and you need to start preparing, while green means get on your bike as it is time to thin now.

That's what makes the Thinning Ready Reckoner such a handy tool as it can easily take two years to line up all your ducks and get ready for thinning.

So, how does it work?

First of all, establish a square plot with the four sides measuring exactly 10m by 10m and the sides of the square parallel to the tree rows.

Remove the branches of all trees within the plot up to head height.

Then count the number of all live trees within this 10m by 10m plot. Multiply this number of trees by 100. This gives you an estimate of the number of trees per hectare. This is the stocking density.

For instance, if you count 24 trees within the plot then the stocking density per hectare is 2,400 (24 x 100).

Now measure the diameter at exactly 1.3m above the ground of all the trees in the plot and write the results down. This is called the diameter at breast height (DBH) and is easiest measured using a specialised DBH tape.

Such tapes can be bought for a few euros from www.kestrelforestry.ie or www.irishforestryproducts.ie.

Once the diameter of all the trees within the plot have been measured, take the sum of all these figures and divide by the number of trees measured.

This will give you the average DBH. A typical average DBH of trees close to thinning is likely to be in the range of 14 to 17cm.

Now that you know both the stocking density per hectare and the average DBH, you can use the Ready Reckoner below to see if your Sitka spruce forest may be ready for thinning.

But remember, this is only a useful guide indicating readiness for thinning.

Teagasc regularly runs timber measurement courses if you're interested in learning how to use the Thinning Ready Reckoner.

Steven Meyen is a forestry advisor with Teagasc

steven.meyen @teagasc.ie

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