Pruning -the treatment of individual
An important aspect of silviculture is
the potential to manipulate growth, form and productivity
of individual trees by direct treatment. Pruning the canopy
or roots, coppicing, the application of chemicals and hormones,
and other interventions can, if carefully timed, change the
pattern of growth and improve the value of a tree for its
intended use. Of these methods, pruning of branches for enhanced
wood quality is the most relevant to farm foresters.
Pruning for wood quality
Branches on the trunk leave knots in the
log that can reduce the timber strength, pulping quality and
appearance values. Timber clear of knots and other defects
is called clearwood and is commonly prized for its attractive
appearance and is used in furniture, flooring and joinery
work. Knots larger than 5 or 6cm in diameter can result in
timber being unsuitable for many structural grades, due to
weakness around the knot.
Pruning does not reduce the number of knots in the tree, only
their size and location. The woody stem of live branches,
other than those formed from epicormic buds, runs back inside
the trunk of the tree to the pith. Pruning simply confines
the branch knot to the core of the tree allowing the trunk
wood to grow over it. If all the branches on part of the trunk
are pruned early, when the trunk diameter is small, then a
clearwood sheath can develop over a core of branch stubs.
On milling, clearwood timber is sawn from the sheath and the
core discarded or sawn for lower grade timber.
A 20cm core in a log with an underbark diameter of 60cm means
that almost 90% of the log volume is clearwood. The central
knotty core will also contain the pith and juvenile wood that
is often discarded due to poor wood characteristics. Farmers
are advised to aim for underbark log diameter at least three
times the size of the knotty core diameter to allow large
dimension clearwood timber to be easily sawn from the log.
In some cases the knotty core of specialty timber species
may produce sawn timber with interesting character if branches
are pruned when small and live.
Tree growth & wood production
Key: DOS Diameter over stubs,
DOO Diameter over occlusions, CLW Clearwood
Simplistic representation of the silvicultural
options facing forest owners:
(a) No pruning and the use of competition
to promote self-pruning. The dead branches are commonly held
for many years after they die. The competition necessary to
induce self-pruning also suppresses diameter growth, resulting
in the need for a longer rotation.
(b) No pruning with heavy thinning to promote diameter growth.
Results in large diameter and large branches and low quality
Pruning without heavy thinning. Causes knotty core control
and a high timber volume per hectare but competition suppresses
diameter growth, resulting in the need for a longer rotation.
(c) Pruning with heavy early thinning to minimise competition.
Results in knotty core control and large diameter. Results
in maximum clearwood production in the shortest time but at
the cost of total volume per hectare and the quality of the
unpruned portion of the tree.
How to prune
for clearwood production
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