Stem pruning



Stem-pruning aims to confine knots and associated wood defects to a relatively uniform cylindrical core up the centre of the pruned log. Because young trees are tapered, it is usually impractical to prune to the full height in one pruning event—generally referred to as a lift. So pruning usually involves several lifts. Prior to the 1980s, most pruning operations were done to a fixed height. For example, the first lift was to a height of two metres, the second to four metres, and the final lift to six metres. When applied across a plantation this resulted in small trees being over-pruned and large trees being under-pruned.

The alternative approach is variable-lift pruning. One method involves the use of a pruning calliper corresponding to the target diameter at the base of the green crown. The calliper is used to guide the pruning operation so that each tree is pruned to the same stem diameter. Pruning to a fixed stem diameter effectively mimics constant leaf area pruning. That is, where trees are pruned so that they all retain a similar leaf area and therefore growth potential. Regular pruning to a constant diameter is the only effective way to achieve a uniform knotty core.

How hard to prune

In most cases, a smaller knotty core is better. But achieving a tiny knotty core is counterproductive if the inner clearwood is downgraded because of juvenile wood or other defects. There may also be limits to how close to the core the processor is able to cut. For veneer production, the chuck diameter of the veneer cores—the core that cannot be peeled for veneer on a lathe—will be critical. In sawmilling, the limit might be the size of the boxed heart. This is the core cut out during the milling process that is generally unsuitable for milling because it contains the pith.

A maximum knotty core diameter equal to one-third of the underbark log diameter is a useful target. This would result in about 90 per cent of the log volume being clearwood. To achieve a diameter over occlusion (DOO) of no more than 20 centimetres up the entire length of the pruned stem may require that no diameter over stubs (DOS) is larger than about 17 centimetres. This allows for 1.5 centimetres all round to cover the wounds and account for sinuosity—or bends—in the stem. Form-pruning large branches—over say 2.5 centimetres—and annual pruning to a stem diameter of 8–10 centimetres would be sufficient in most cases to contain the DOS to less than 17 centimetres. For a 60 centimetre diameter log, reducing the knotty core diameter from 20 to 15 centimetres—by pruning more severely or regularly—would increase the proportion of clearwood by less than five per cent.

For P. radiata grown on fertile farm sites, a DOS of less than 19 centimetres is a good compromise between the need to restrict the DOO and concern about the effect of heavy pruning on tree growth. On low fertility sites, trees are commonly less tapered and can be pruned to a smaller DOS, while leaving plenty of green canopy. Where the sapwood of a species is unmarketable, the heartwood sheath diameter—rather than the underbark diameter—should be considered. This would be appropriate for a wide range of species including Californian redwood (Sequioa sempervirens), black walnut (Juglans nigra), and blackwood (Acacia melanoxylon). For these species, it might be worth pruning to limit the knotty core to less than 15 centimetres by using an eight centimetre gauge and spacing the trees wider to encourage larger diameter growth.

Heavy pruning can increase the risk of epicormic shoot development. Epicormic shoots on hardwoods such as eucalypts, poplars and oaks, are easily removed from the ground using a long pole saw. Softwoods, such as pines and Californian redwoods, tend to produce a multitude of fine epicormic shoots up the stems. These are more difficult to remove from the ground using a pole saw. The tree may need to be climbed so a knife or the back of the saw blade can be used to scrape them off.


Stem pruning E. nitens using an eight centimetre pruning gauge. Every branch
below the gauge is pruned every year until the full pruned height is reached.

How high to prune?

A large proportion of the market for high quality appearance grade timber uses lengths shorter than 1.5 metres. Plywood manufacturers around the world generally use lengths shorter than 2.4metres. However, despite this, pruning to less than three metres would not allow the forest grower to adopt the wider spacings necessary to promote diameter growth without degrading the greater volume of unpruned timber growing above the pruned height.

In Australia, most hardwood sawmill carriages cannot hold a log more than 6.1 metres long. In any case millers often prefer to cut their logs into shorter lengths to reduce the effect of growth stresses and taper on sawn recovery. What might be more critical is the minimum length. Some mills cannot restrain a log shorter than three metres on a line bar carriage.

Harvesting, sorting, loading and transport costs are lower per cubic metre for longer logs. Given this, it seems reasonable, for most markets, to prune to a height that allows for at least one 6.1 metre pruned log. This may mean pruning to a height of about 6.5 metres to account for the stump.

Even in a conical tree, pruning to 25 per cent of the expected total tree height at the time of harvest will still mean that about 60 per cent of the tree volume is in the pruned section. As the tree becomes more cylindrical, this proportion will improve. For example, in a 10-year-old pruned E. nitens nearly 23 metres tall, more than 60 per cent of the total tree volume was extracted in the 6.1 metres of pruned log. About five per cent of the total volume was in the 0.4 metre stump, leaving about 40 per cent of the total volume in the low value top logs left on site.On better quality sites, where trees are likely to grow taller, it may be necessary to prune higher to maintain the same proportions. High pruning is likely to be easier on these sites, because the lower branches are less likely to grow large if height growth is vigorous.

How many trees to prune?

Where tree form and vigour are very uniform it might be possible to establish the plantation at the expected final stocking and prune every tree. This can happen in clonal forests grown on uniform sites or on very well-sheltered sites where open grown trees do not suffer from exposure. This is the practice in many poplar plantations and might also be justified for other species that grow well in the open, such as Californian redwood. However, where the variability in tree performance is expected to be high—for example, Acacia melanoxylon—a selection ratio better than one in five might be worth considering. This would allow managers to select vigorous trees with good form for pruning and progressively cull the remainder.

In many cases, vigorous early growth and good tree form depends on maintaining a sufficient level of mutual shelter. A minimum initial stocking for P. radiata of 600 stems per hectare has been suggested, despite aiming for a final stocking of approximately 250 stems per hectare for pruned trees. In Australia, the risk of hot dry winds suggests that initial stocking rates for hardwood plantations on exposed sites should be more than 600 stems per hectare. Where exposure is likely to affect tree form early thinning should not reduce the basal area to less than about five square metres per hectare. This would be equivalent to retaining 600 trees per hectare with an average 10 centimetre diameter or 340 trees per hectare with a 15 centimetre diameter. The final stocking density of a eucalypt sawlog plantation might be only 120. This would allow a selection ratio of more than one in five during the pruning phase of the plantation.

If more trees were established than required, thinning would occur in concert with pruning to ensure that there were always sufficient trees for selection. For example, if 600 eucalypt trees per hectare were planted—with the expectation that at least 150 would be available for sawlog harvest—then 450 might be selected for a first pruning lift. Three hundred of these would receive a second lift, and 150 could be selected for a final lift. All trees not selected for pruning would be thinned immediately to reduce competition. This would provide around fifty trees in case of windthrow, disease or poor performance. These trees would also be available for an early harvest of small sawlogs—around 50 centimetres in diameter.


Free growth lines for eucalypt and pine—based on a simple basal area model—and a possible regime for eucalypt sawlog production. Diameter growth is compromised if plantations are growing above the line. Dots show actual plantations of eucalypts in Australia and New Zealand up to 40 years of age.

Which trees to prune?

New Zealand researchers stress the need to achieve an even final spacing of P. radiata trees to ensure full site occupancy and the efficient use of resources. They do not mention concerns about irregular wood development around the trunk or the risk of uneven spacing inducing growth stresses in the timber. In the crown-shy eucalypts, and possibly in other hardwoods, these risks are very real. If the crown is unbalanced most of the carbohydrates will flow down one side of the tree leading to an asymmetrical growth of the trunk. This suggests that establishing eucalypts at an even spacing of three or four metres will only present a dilemma when thinning and pruning are done at selection time. If the final stocking density was expected to be less than 150 stems per hectare, an even spacing would imply an average of eight metres between trees. Because eucalypts are crown shy, it is desirable to maintain a minimum distance of at least six metres between any two final crop trees. If this is accepted, then despite having established around 1000 stems per hectare, more than a third of the trees would need to be thinned on the basis of spacing alone. This reduces opportunities to thin on the basis of other important factors such as form, branching habit and vigour.

An alternative approach is to establish the same number of trees in groups of three or four trees at least six metres apart. The best trees in each group would be selected for the first one or two pruning lifts. This would reduce the stocking to around 270 stems per hectare. The final selection of the best 150 trees could be based on form and vigour alone.

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