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How to prune for clearwood production
Producing a clearwood log requires careful
management over several years to ensure an even knotty core
and thick clearwood sheath. The most effective way to achieve
an even knotty core is to prune regularly—every year
for most species—to a predetermined stem diameter. For
example, annual pruning to a stem diameter of 8–10 centimetres
will ensure that sufficient green crown remains for growth.
This should ensure that the knots are contained within a core
of no greater than 20 centimetres.
In some regions and species detailed pruning prescriptions
based on years of research data are available. However, in
general farmers are growing different species in different
ways. Over time they may be able to refine their techniques.
Until experience or research demonstrates otherwise a useful
guide is to annually prune any branch with a stem diameter
greater than 2.5 centimetres, anywhere up to the anticipated
log length and then lop all remaining branches below the point
where the main trunk is 8–10 centimetres in diameter.
This means pruning will be done over three or four years,
beginning when the trees are about five metres tall. Removing
large branches early reduces the workload in later years as
well as the likelihood of disease entering through the stubs.
Any trees that are clearly malformed due to a twisted stem,
kinks or large branches are usually culled.
Does pruning pay?
Prioritising which trees to prune, and
when, is critical to the viability of a forest. Forest owners
need to think carefully before committing to intensive pruning.
The cost per hectare is likely to be greater than the establishment
cost, and possibly even more than the cost of the land. Three
important priorities that can increase farmers’ return
on pruning time are:
• Prune stands of lower stocking density. Because there
is less competition for site resources trees will maintain
higher diameter growth rates than in dense stands.
• Choose the more vigorous, straight trees with small
branches because they are easier to prune.
• Minimise the number of pruned trees that will need
to be culled at a later time.
The time taken to prune individual well-formed eucalypts is
conservatively estimated to be around two minutes for the
first lift, three minutes for the second and four minutes
for the final lift to 6.5metres. Skilled operators, using
a ladder and harness, can high prune at least 20 well-formed
eucalypt trees per hour in good conditions. Based on these
estimates pruning to 6.5 metres might take about 10 minutes
per tree. Depending on labour costs this shouldn’t amount
to more than $10.00. In New Zealand, experienced contractors
are expected to be able to prune 10–25 pine trees per
hour, whether working from the ground or from ladders, at
a cost of between $4.00 and $7.00. Pruning costs are higher
for heavily branched species or when pruning is done late.
Steep sites or those infested with weeds will be more expensive
again. Flat grazed plantations on cleared farmland are often
the easiest and cheapest to prune.
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Because pruning costs are directly proportional
to the number of trees, there is a great deal of pressure
on final stocking rates. There are no simple answers as to
the right final crop stocking to aim for. A major consideration
will be the degree to which competition between the trees
slows diameter increment. For the more tolerant softwoods—for
example, Pinus spp
and Araucaria—final
stockings of between 200 and 300 stems per hectare are reasonable.
For hardwoods like eucalypts and rainforest species a maximum
final crop stockings of between 100 and 150 stems per hectare
might be justifiable.
The following graph presents a simple model for estimating
near free growth stocking levels for crown-shy eucalypts and
the more tolerant pines. The model uses basal area as an indication
of the level of competition between trees. If trees contain
no heartwood then the basal area is proportional to the size
of the canopy. However, as trees grow, an increasing proportion
of their basal area is comprised of heartwood—which has
no physiological function in the tree. As a result, the basal
area corresponding to a particular level of competition in
a plantation will actually increase over time. The model used
in the graph accounts for this by relating the critical basal
area to the size of the trees. By measuring their trees regularly
farmers should be able to refine the thinning model so that
it better matches their site and the species they are growing.
If pruned trees are grown free of intense competition, the
width of the growth rings, and hence the eventual yield of
clearwood, will be greater. If the forest is not thinned diameter
growth will eventually suffer due to the increased competition
(as indicated by the area above the curve in the graph). However,
if competition is reduced too much, the volume production
per hectare will be very low and exposure may affect tree
form. Although trees grow faster on better quality sites,
the point at which competition reduces diameter increment
doesn’t vary as much as might be expected.
Concerns about low stocking densities inducing heavy branching
in the stem above the pruned section, need to be balanced
against the higher, upfront cost of pruning more trees and
the longer rotation associated with higher final stocking.
On high quality sites where trees are likely to grow very
tall (say more than 30 metres) growers may be advised to try
and prune higher (up to 8 or 10metres) because a high proportion
of the total tree volume will be above 6.5 metres.
.jpg)
Free growth lines for eucalypt and
pine based on a simple basal area model showing 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.
Where to prune—natural
target pruning
When to prune—season
and frequency of pruning
Form and
stability pruning
Stem pruning
Tools for
pruning
Documenting
pruning history
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