Forestry

Shaping up: pruning for posterity

By Donal Magner

04 February 06 : January and February are ideal months to check young broadleaves especially for form or straightness as it is easier to identify trees that need shaping when they are not in leaf. Unlike conifers, broadleaves are prone to forking, so shaping will encourage apical dominance or good leader growth by removing side branches that are competing with the leading shoot.

Numbers of well-formed trees needed

In broadleaved forests the number of trees per hectare will reduce over the rotation from an initial stocking of up to 6,600 to as low as 50 final crop trees, due to tending, thinning and natural wastage. Insect, pest and disease damage may also contribute to a reduction in numbers. Before deciding to prune, the grower or forester should first select quality trees that are likely to form the basis of a good crop - these may or may not need to be pruned.

The advice often provided is to select only 'final crop trees'. This should be treated with caution because the number of "final crop trees'' can vary depending on species, growth rates and end uses. For example, a grower (or his or her successors) with veneer quality oak may decide to retain the crop until it is over 160 years old when the number of trees could be as low as 50 per hectare. On the other hand a fast growing oak wood (yield class 8) could be at "final crop'' stage at 65 to 80 years when it could have over 200 trees per hectare with an average size tree of 1.4 cubic metres at age. If, for example, the average rotation length of a high yielding oak crop was fixed at 130 years it could be producing trees from thinnings averaging over 1.0 cubic metre 30 to 40 years before rotation end. Some of these thinnings could make their way into furniture and flooring markets if they are well formed so it would be important that these potential commercial trees are selected as early as year two.

The advice to select only "final crop trees'' fails to take into account possible future damage to trees and also pre-empts future silvicultural practices and future markets. Instead, select sufficient trees that are likely to form a crop during the final stages of rotation in addition to 'final crop trees'. This approach allows for greater flexibility in future management.

Numbers to shape

Growers should aim to have 700 straight stems per hectare but between 300 and 400 will be sufficient if evenly distributed at around five metre spacing (Table 1).

In the case of ash, where there may be a market for hurleys and sycamore which is prone to squirrel damage, the grower may select more than this (Table 1). The selected trees can be marked with paint so that future harvesters will avoid these during the early tending and thinning stages. Once selected the owner can then decide if trees need to be pruned.

In recent years, pruning has been viewed as the panacea for producing quality plantations. Well-planted crops in suitable sites using the correct species may require little or no shaping. Indeed there are plenty examples of over pruning. The advice to growers with mixed quality plantations is to prune judiciously and to owners of high quality plantations: leave well enough alone. On the other hand no amount of pruning will convert a poor plantation into a good one.

So it is not always necessary to prune. For example, a well stocked plantation of oak with 15% good straight stems provides almost 1000 potential final crop trees far in excess of the number required.

Table 1 : Information on broadleaves including initial stocking, final crop trees, number to be shaped (per hectare (ha)) and rotation lengths

Species	Spacing (metres)	Initial stocking per ha	No. final crop trees per ha (1)	No. of stems shaped (% of original stocking	Rotation length (years) (5)
Oak	2.0 x 0.75	6,600	90-110	300-400 (4-6%)	75-140
Beech	2.0 x 0.75	6,600	100 -120	300-400 (4-6%)	90-120
Ash	2.0 x 1.50	3,300	90-110	300-800 (9-24%) (2)	55-70
Sycamore	2.0 x 1.50	3,300	90-110	300-500 (9-15%) (3)	60-90
Other broadleaves	2.0 x 1.50	3,300	100-200	300-400 (9-12%) (4)	& 62;30 (6)

(1) Number of final crop trees will vary depending on species and rate of growth. A grower lucky enough to have veneer quality oak may retain the crop until it is over 160 years old when the number of trees per hectare would be as low as 50.

(2) Worth selecting extra ash stems for hurley making market

(3) Due to risk of grey squirrel damage worthwhile selecting extra trees.

(4) Number selected will depend on quality of final crop and potential markets. For example, Spanish chestnut and Norway maple may be worth shaping while lower branches in birch are quickly suppressed and die, and don't need pruning

(5) Number of final crop trees will vary depending on growth rates and end use potential. Many broadleaved woods (yield class 6) will have reached maximum yield at 75 years but growers with high quality veneer trees such as oak, ash, beech, sycamore and cherry, will retain their woods long after maximum yield - in the case of oak up to 160 years.

(6) Wide range of rotation lengths (years) e.g. birch (40-60); cherry (60-70); and Spanish chestnut (60-90).

When to prune

Most broadleaves can be formatively pruned during mid winter (Table 2). The operation can also be carried out in summer but pruning in spring should be avoided as trees are flushing and their resistance to infection may be low. It should be carried out preferably during years 2 to 4 when trees are between 1.0 and 2.5 m in height. The operation should be completed when trees are about 3.0 m.

Table 2 : When to shape – and not shape – selected broadleaves

Species	Best period for shaping	Second best period	When not to shape
Oak	Mid winter	June - July	Spring
Beech	June - August	Mid winter	Spring
Ash	June - July	Mid winter	Spring
Sycamore	June - July	Mid winter	Spring
Cherry	June - August	Mid winter	Spring

How and what to prune

Trees should be pruned with a good quality secateurs. Some growers disinfect the secateurs regularly with an alcohol swab during the operation especially when pruning ash. While growers may prune up to 6 metres to ensure a clean knot-free bole, most trees shed their branches naturally over time so this may be unnecessary.

Only select trees that will benefit from pruning. For example, remove one leader in forked trees. It is unnecessary to prune very light branches. If shaping is carried out in time, pruned side branches should rarely be over 3cm in diameter in thickness and never remove more than one third of the foliage.

Grant aid

It is expected that formative shaping is carried out before year four so the cost of the operation is included in the afforestation second instalment grant payment. A once-off payment of up to €254 per hectare is provided in some instances where additional shaping is required. This should be carried out no more than three years after the first shaping. This operation will normally occur at around age six, but in well-stocked plantations with sufficient well-formed stems it should be unnecessary.

Posterity

The main objective in formative shaping is to favour the development of one straight or 'apically-dominant' leading shoot. If nature has obliged with enough of these, leave well enough alone. Pruning broadleaves may be for posterity but in good quality plantations, posterity might be better served by keeping the secateurs for the roses.