Thinning is best described as the selective removal of stems in order to favour the growth of the remainder. It is a normal part of most forest crop rotations, and is essential if higher yielding crops are to produce the maximum number of high value logs.

In a typical Sitka spruce crop, thinning normally begins around year 18 although this varies according to the yield class of the crop, (Yield class being the scale by which crop production is measured.) Thinning is normally on a 5 year rotation so second thinning would be in year 23, third in year 28 and so on.

Types of Thinning

There are many types of thinning, indeed, whole books have been written on the subject, but this kind of detail is unnecessary here. Once again, in the typical Sitka spruce crops, a machine almost always carries out thinning.
Usually in a first thinning every 5th or 7th line of trees is removed with some trees being removed between the lines also. Typically around 50 cubic metres per hectare (m3/Ha) are removed in a first thinning.
In a second thinning a new line is removed between the original 5th or 7th line and suitable trees are again removed between the lines. Subsequent thinning are of trees selected carefully to bring out the best growth in what is left behind.

Uses of Thinning

The uses for timber removed as thinnings vary according to the size of the timber removed. So for first thinning, (material which is small in height and diameter,) the main markets are pulp (for board production or paper) and fencing posts.
In a second thinning, markets would include the box wood and pallet industries.
Third and subsequent thinnings would supply mostly small and large sawlog to the commercial sawmills.

Value of Thinnings

The value of thinnings obviously depends on the prevailing market conditions as well as other factors such as volume of material in crop, accessibility of crop for lorries and distance from market. As such putting figures on this is meaningless, however, some rules of thumb apply:

(1) First thinnings are not generally a big profit-maker. They generally make at
best a small profit and can show a loss if the markets are flat.

(2) Second and subsequent thinnings show generally, an increase in returns
corresponding to an increase in size of timber.

(3) Profits can be improved to an extent by thinning heavily in a time of good prices, However, this is not a recommended approach and could be highly detrimental to the crop. As with all forest operations, advice from a qualified forester should be sought at all times.