There is a direct relationship between the size of the crown and the diameter of the tree stem or trunk. Without sufficient crown to generate food, the stem will remain thin, and growth of the tree will stagnate, resulting in weak, spindly trees. The timing of thinning will depend on the species and rate of growth, and also on available markets for the thinnings. However, thinning should not be delayed, or the trees may be so weakened that they are unable to respond when thinning eventually takes place. Thinning operations should be included in the management plan for a new woodland.
General principles
Closely spaced planting is the best and quickest way to establish trees, but thinning must be carried out to allow sufficient space for the remaining trees to thrive. In woodlands planted at 2m spacing for wildlife and amenity, thinning should start about 10 years after planting, when the trees should be 6-8m high. Early thinning will encourage diversity of form and add variety to the woodland structure. Trees planted at 3m spacings may not need thinning until about 20 years after planting. In commercial broadleaved woodland, thinning is usually started between 15 and 25 years after planting, when the trees are about 10m high.
As a general rule, for trees planted at 2m spacing, about 25% of the total number of trees are removed during each of the early thinning operations, reducing to about 15% after five or more thinning operations.
Visual assessment
Within the guidelines given above, each situation will need individual assessment. A visual check will show where crowns are overlapping and thinning is needed. Any trees, apart from understorey species, which have failed to reach the canopy and have been suppressed, will need removal. The aim should be to create a ‘ring of sky’ around each tree that is retained, into which it will spread. The woodland edge should normally be left unthinned, to create dense, branching growth which shelters the woodland from the wind.
In amenity woods, trees can be selected to give a variety of form and structure, and to break up planting lines. In commercial woodlands, selection should be for commercial value (see below).
The old forestry rule is that the stem thickness in inches (dbh or diameter at breast height) should equal the radius of the crown in feet. For example, a tree with a dbh of 8 inches should have a crown of radius 8 feet, or diameter 16 feet. Another rule-of-thumb is that at early pole stage, spacing between trees should be about one fifth of average tree height, and at late pole stage, should be about one quarter of average height.
| Stage | Age | Thinning operations |
|---|---|---|
| Establishment | 0-10 yrs | |
| Thicket | 5-20 yrs | 1st thinning. Remove 25% |
| Early pole stage | 15-50 yrs | Thin at 5-7 year intervals. Remove 25% |
| Late pole stage | 30-100+ yrs | Thin at 8-10 year intervals. Remove 15% |
| Mature | 40-150+ yrs |
Thinning operations in established woodlands and those managed for commercial production will require careful assessment, including the total volume of standing timber, and the size class distribution, which gives a measure of the percentage of trees of different ages. For further details see the Forestry Commission publications, Thinning Control and Timber Measurement – A Field Guide (p199).
Selective thinning
This involves individually selecting trees for thinning, normally removing those that are weak, diseased, forked or dead, and retaining the strongest, straightest and most healthy trees. Where growth is good throughout, the removal of viable trees will be necessary. Selective thinning in amenity woodlands should also aim to partially break up planting lines, so that the woodland develops with a more varied and natural spacing of trees.
It’s important that thinning operations are planned in advance, and marked by a competent person. Selection of trees should be made in winter, when the crown and upper stem can easily be seen. Normally trees to be retained are marked with spots of aerosol paint (available from forestry suppliers), and trees to be cut are marked with a billhook. Marks are made on both sides of the trunk so that mistakes are avoided. Alternatively, tape can be tied around the trees to be felled.
Evans (1985) gives the following criteria, in order of priority, for the selection of broadleaved trees to favour in thinning, to maximise commercial value.
- Good stem form and freedom from defect on the lower 7m (23’) of stem.
- Absence of deep forking in the crown.
- Good vigour.
- Freedom from defect in upper stem and crown.
- Low incidence of epicormic branching.
- Proximity of other selected trees. Even spacing should only be considered after other criteria are satisfied.
Line thinning
Where growth is fairly uniform and timber production is the main aim, complete rows of young trees can be removed. This is very much quicker than selective thinning. Depending on the species and growth, every third, fourth or fifth row may be removed, or in mixtures, any faster growing nurse species may be removed. In very dense plantations, especially if left too long, occasional lines or ‘racks’ will have to be removed to gain access to the wood for inspection purposes.
Extraction and use
The first thinnings are usually too small to be of commercial use, and are left where they fall. Sned the side branches and leave the trunks and brash on the woodland floor, where they will provide some deadwood habitat. Depending on access and other factors, the wood can be extracted from the woodland for firewood, green woodworking or other uses.
Later thinnings will be of value for green woodworking, fencing or other purposes, and should be extracted from the woodland as part of the thinning operation. There is a range of vehicles designed for the extraction of timber. Horse extraction may also be a possibility in woodlands with poor access or on sloping ground. See Practical Solutions Handbook (English Nature, 2001).
Thinning neglected woodland
Where thinning has been neglected, the woodland will be characterised by closely spaced, spindly trees with small crowns. Access into the wood for assessment, felling and extraction, will be difficult and time consuming. The newly thinned woodland is likely to suffer from windblow and other problems (see below), and some of the trees may be so weak that they are unable to respond. Ash has a very poor response to late thinning, and may fail to develop. Beech is intermediate in response, whereas oak normally grows away well even if thinning has been neglected.
Where thinning has been neglected, the first action may need to be to cut access tracks or tracks into the woodland, so that it can be assessed and further practical action planned. A single heavy thinning is risky. Several light thinnings, at perhaps three year intervals for 9-10 years, should restore the woodland to a better stocking rate.
Effects of heavy thinning
Infrequent, heavy thinning produces a greater volume on each occasion, which may make marketing more viable, as well as being efficient in terms of labour, access and ground disturbance. However, heavy thinning can cause windblow, epicormic branching and variation in ring width. It is usually more damaging to wildlife than frequent, light thinning.
Windblow is caused when the removal of the shelter of surrounding growth exposes the remaining trees to the wind. They are particularly vulnerable immediately after thinning, as the root systems are not adapted to cope with the amount of movement, and the trees can easily be blown over. Gradually, in the increased light and space, the root systems expand to keep the tree wind-firm. Root growth is also stimulated by the swaying action of the tree in wind. Trees on the edges of woods or at ride sides are most vulnerable, and a smaller percentage should be removed for the first 10 metres or so, than in the rest of the wood. Retain shrubs and small trees at wood edges, as this helps deflect the wind up and over the woodland, and retains shelter for wildlife within the wood.
Epicormic branches are those that arise directly from the trunk of the tree. They particularly occur on English oak, and where they persist for more than one year, form a knot in the timber which will lessen its commercial value. Epicormic branching usually occurs when increased light after heavy thinning stimulates dormant buds on the trunk. It can also arise from pruning scars. The result is a thicket of horizontal branches up the trunk, with weak epicormic branches also developing on main side branches. Where they occur on potential high-value trunks the shoots should be pruned or rubbed off each year in mid-season.
Variation in ring width is caused where heavy thinning results in a sudden rapid burst of growth. This affects the commercial value of the timber.
