This is why you have to thin your trees out.  You must give preference to the trees that are capable of producing the greatest quality timber.

Of course, there are other motivations for thinning.  These motivations would depend on the type of forest, and the objectives of the landowner.  They would include improvements for wildlife, for recreation, or for aesthetics.

In any forest, there is only so much soil moisture, light and nutrition to go around.  Competition can become so intense as to reduce the overall growth rate.

The solution is forest thinning.

Forest thinning means  cutting out immature or young trees to  improve the overall yield of the whole stand.

Many thinning methods were developed over a hundred years ago during early trials of forest management in Europe.

Here is a brief outline of thinning methods:

• Low thinning;
• Crown thinning;
• Selection thinning;
• Mechanical or systematic thinning.

Low thinning, sometimes called the “German” method or “thinning from below”, involves the removal of any weak and deformed trees that are under the canopy.

This is the oldest thinning technique, and is not used much nowadays, especially in commercial forestry.  This is because the removal of lower trees provides little extra growing space for the dominants.

Crown thinning, also known as the “French” method or “thinning from above”, involves the removal of intermediate and upper level trees.  The idea here is to remove some of the overstory trees to allow light to penetrate to the lower and middle crown classes.

Trees are selected for cutting that are crowding out straighter boled co-dominant trees or are merely competing side-by-side with other trees. Selection is based on health and growth potential, with healthy and higher potential trees left uncut.

Weak or diseased trees in the lower canopy are also removed at this stage for convenience.

There is a higher value for the removed trees and much faster growth for the remaining specimens.  A skilled forester is required for this type of thinning.

Selection, or improvement thinning, involves removing trees based on size, quality, and their position in the canopy.  Many trees are removed using this method and much space is made available to high quality stems.  I

It is impractical for a machine to undertake this type of thinning.  A lot of damage can be done to the surrounding  trees.

Systematic thinning, or mechanical thinning, is based on removing trees in a predetermined pattern.  Quality is not taken into account.   There are different systems used, and most involve taking out complete lines of trees.

The most common method used today is a combination of selective and systematic.

Here, a row would be removed to improve access, normally 1 in 7.  Selected stems are then removed from each side of the rack.

This method is all about compromise.  Many quality stems will be felled when the full line is taken out.  But then there is plenty of access to select the weaker trees for removal.  This means that sufficient volumes of the better trees are left for future growth, while still permitting the contractor to fully utilize harvesting equipment.

The faster your trees are growing, the sooner you need to thin.  If you try and delay thinning to increase the volume of timber, you only increase the risk of windthrow.

The reason for this is simple.  Trees need light.  When they are tightly packed inside the forest, trees can only get access to light by growing upwards.  This fast, upward growth is made at the expense of bottom stability.  So much so that when the forest is eventually thinned, the trees haven’t been able to develop their roots systems properly…a strong wind is all it takes to pull them up and toss them over.

A good indicator that you need to thin is the length of the live crown.  This is the green portion of at the top of the tree.  You should consider thinning if the live crown length is less than a third of the overall tree height.

Not all forests can be thinned.  In some areas, such as along the west coast of Ireland, if your site is very wet and/or exposed, thinning can actually increase the risk of windthrow.

Every forest will naturally thin itself.  When left alone, a forest is a perfect eco-system, and survival of the fittest is king.  Any weak trees will be smothered, and starved of nutrition and light.  Eventually they will die, and in doing so will rot and feed the forest as a whole.  But that is for another article.

So, the main advantages to thinning are:

• It allows easy access for forest management.
• Thinning will improve the quality of the overall crop.
• Overall timber revenue is increased.
• Early source of income to the forest owner.
• Air circulates much more freely promoting stand hygene.
• Improved saleability.

Here are some of the questions you need to ask before you consider thinning.

• How well are the trees growing? Your plantation needs to be capable of supporting yield class 14 Sitka spruce. This means the potential to produce 14 cubic meters per year over the full rotation.
• Is your site at risk from windthrow?
• What method of thinning will you employ?
• By what means will you extract the timber?
• How will you sell your timber?

Forest Ireland specializes in forest inspection paths.  You need to put these access paths into your forest so you can assess the viability of thinning.

Please see our main article…Forest Access Paths