Pruning is a forest operation designed to produce knot-free timber in the pruned section of a tree. Unlike horticultural pruning which encourages plant vigour and flowering while keeping growth in check, forest pruning involves the total removal of all dead and heavily shaded branches usually up to a height of 6m.

Pruning broadleaves and conifers require different approaches. While the aim in both is to maximise the knot-free content in trees, pruning broadleaves also ensures a straight stem.

Pruning broadleaves

While conifers grow straight as forest trees, stem form in broadleaves is poorer. As a result, pruning – or shaping – broadleaves may need to begin as early as year four in trees that are forked. The removal of one of the forked stems or competing side branches will allow the tree to form a leader and grow straight. This operation can be carried out with a secateurs.

Shaping may be repeated but as the objective is to ensure a final crop of 100-200 trees/ha it is important to only select the trees with best form and growth potential and to allow for damage and possible disease during the future rotation, so at least 400 trees/ha should be selected. These can be marked using spray paint so they are clearly identifiable and as a result will not be removed during tending and thinning. It should be remembered that most broadleaved woodlands carry an initial stocking of 3,500 trees/ha so the optimum number of straight stems – less than 15% of stocking – may require minimal or no shaping.

As trees mature, pruning may be necessary to remove side branches. This can be carried out with a normal pruning saw or lopper up to 2m in height while a telescopic pruning saw will be required to prune up to 6m or higher.

Most of the final crop trees identified will require the removal of side branches if quality sawlog and possibly veneer wood is the ultimate aim. After the initial shaping, timing of pruning is important. Pruning of the final crop trees should be carried out after tending (removal of small defective stems) when species such as beech, ash, sycamore, maple, alder and oak are between 7m and 10m top height. Top height is the average height of the 100 trees/ha with the largest diameter After initial tending, thinning is carried out when top height growth reaches a further 2-4m. This allows pruning up to 6m for most broadleaves when they reach at least 12m top height.

Pruning conifers

Pruning conifers is a much more straight forward operation compared with broadleaves. Shaping is not required in the early years as conifers generally grow straight, so pruning is carried out as soon as possible after first thinning when the forest is more accessible.

Select approximately 400 final crop trees/ha evenly spread throughout the forest at an average 5mx5m spacing. These should be the straightest and healthiest trees. Generally, pruning is carried out in two ‘lifts’. The first lift is up to 3.5m and second up to 6m. As for broadleaves, pruned trees should be marked with paint so they are easily identifiable not only to the forest owner but also the harvesting operator who should avoid them when carrying out second and subsequent thinning. Is pruning worthwhile? Pruning is the one operation that can be easily carried out by the forest owner. It is an essential operation in broadleaves if the production of prime sawlog and veneer wood is the objective.

It’s not as clearcut in conifers where growth rates and form are generally more uniform. However if trees are not pruned, dead branches will form part of the wood produced annually which can weaken the timber when it is sawn. In light branching species such as Sitka and Norway spruce, this is not a major problem but in species with high added value potential in specialist markets, such as Scots pine and Douglas fir, dead knots may actually fall out during sawing and reduce their commercial potential.

At the moment, owners who prune probably don’t receive a premium for their trees but this is changing especially for hardwoods and minor conifer species where the value of straight, knot free timber is being acknowledged.

Apart from adding value to each final crop tree, pruned forests and woodlands are extremely attractive to potential growers. Plantations can be dramatically transformed into forests or woodlands as the trunks of the pruned trees stand out against their less attractive neighbours. Clearly marked final crop pruned trees safeguard them against premature extraction during thinning as they are allowed to remain until the final lucrative harvest.

Unlike harvesting, which few forest owners are either financially or technically equipped to carry out, pruning can be implemented by most growers. All you need is a pruning saw and can of spray paint to initially low prune. Later on, a high pruning specialist saw is required. “Forest owners who wish to prune up 7m should use the ARS Japanese telescopic manual pruning saw which is priced at €200 + VAT,” said Sean Lenihan of Kestrel Forestry, who supplies pruning saws and other forestry equipment ideally suited to small-scale growers, now mainly farmers.

Apart from adding value to the crop, pruning is an operation that allows the forest owner to get to know his or her forest. In a 10ha forest, this means that the owner who identifies and prunes 400 trees/ha has selected 4,000 trees as the core high performing final crop. These remain – and add value – throughout the rotation to achieve final harvest revenue of over €250,000 for conifers with much higher returns – albeit over a longer period – for hardwoods