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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 eventgenerally 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 coresthe core that
cannot be peeled for veneer on a lathewill 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 sinuosityor bendsin the stem.
Form-pruning large branchesover say 2.5 centimetresand
annual pruning to a stem diameter of 810 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 centimetresby
pruning more severely or regularlywould 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 diameterrather
than the underbark diametershould 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 highfor example,
Acacia melanoxylona
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 plantedwith the expectation that
at least 150 would be available for sawlog harvestthen
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 sawlogsaround 50 centimetres in diameter.
Free growth lines for eucalypt and
pinebased on a simple basal area modeland 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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