No. 7 Pruning
NEW ZEALAND FARM FORESTRY ASSOCIATION INFORMATION LEAFLET
Although in NZ we normally think of pruning in terms of producing
clearwood, there are a number of other reasons for pruning.
Stability pruning
Form pruning
Pre-emptive pruning
Access pruning
Clear pruning
Pruning height
Pruning intensity
Defect core size
Stability pruning
This is normally carried out in plantations between the ages of 1 and 3
that are liable to topple in strong winds or be damaged by snow. Trees
in this age range often have relatively weak root systems (particularly
radiata of high GF rating), and are easily toppled in wet conditions if
planted on clay soils with poor cohesion, or if the top has outgrown
the roots on moist fertile sites, or if heavy and/or steep-angled
whorls of branches promote the lodging of snow. Pruning can be carried
out with secateurs, and should aim to remove or shorten back branches
in the middle to upper crown to reduce the leverage caused by wind
resistance. Where snow damage is a problem, heavy whorls of branches
should be thinned to reduce the opportunity for snow to lodge.
Where stability pruning is likely to be necessary, it should be
employed as a preventative measure rather than a curative measure.
Where toppling occurs after age 3 or 4, this is likely to be due to
poor drainage, shallow soils, inadequate site preparation, poor
planting, poor tree stocks, or some combination of these factors, and
stability pruning at this stage is unlikely to have any lasting effect.
Form pruning
This is aimed at retaining or creating trees with a single dominant
leader and a straight stem. Forks and ramicorns should be removed as
soon as they can be identified, and any laterals likely to compete with
the leader should be removed or trimmed back. Where the leader has
already been lost or irreversibly deformed, the lateral best placed to
take over (usually on the windward side in districts with a predominant
wind) should be encouraged to take over as soon as possible, by
removing competing laterals. This operation is best
carried out in mid-summer, when retarded leaders and likely
replacements become obvious.
If form pruning is carried out early and regularly, most seedlings can
be turned into satisfactory final crop trees unless they have been
poorly planted or are genetic runts. Thus, where survival after
planting has been poor, a final crop can often still be salvaged with
timely form pruning.
Pre-emptive pruning
This is an extension of form pruning that aims to remove in advance,
those branches that are most likely to cause problems at the time of
clear pruning. In particular, it involves thinning out potentially
heavy whorls (‘basket whorls’) by removing up to half the branches as
early as possible, and removing particularly coarse branches,
especially those that are likely to be more than about 3cm in diameter
at the time of the next visit.
As with form pruning, early and regular pre-emptive pruning requires
little effort and is particularly useful in dealing with multi-branched
species with a wide range of branch sizes, such as macrocarpa and
Douglas-fir. As a general rule, straight grained timber is not produced
over branch stubs until the stem diameter is six times the diameter of
the largest branch in a pruned whorl, so removing large branches early,
can minimise the knotty core.
Access pruning
This normally involves clear pruning all stems to a height of 1.8 to 2m
to provide ready access to the stand for inspection, measurement or
thinning, to provide good visibility to allow stock mustering where
grazing is carried out, and to act as a fire control measure,
preventing ground fires from spreading to crowns and allowing good
access for fire-fighters. It is normally only done in stands which are
not to be clear-pruned, and it is often delayed until the branches to
be removed are moribund or dead.
(top)
Clear pruning
This involves pruning off all branches so that all subsequent diameter
growth of the stem produces wood free of knots. Thus it has to be a
compromise between producing the maximum amount of clear timber
(smallest defect core) and not stunting the trees by removing too much
active foliage.
In the past, fixed-lift pruning
was the norm. With this method, all
trees are pruned to the same height at the same time, say to 2.4, 4.5
and 6m, but small trees are more severely affected (over pruned) than
large ones. Where there is significant variation within a stand in the
height of trees to be pruned, a better alternative is variable-lift
pruning, where severity of pruning can be controlled in one of three
ways:
- Proportion of height. Trees
are pruned so that the length of green
crown left is a fixed proportion (say 50%) of total tree height.
Without measuring height, this is a difficult parameter to measure by
eye when standing beneath the tree.
- Length of green crown.
Trees are pruned to leave a certain minimum
length of green crown (say 4m). This tends to favour the smaller trees,
but again, the length of green crown is difficult to estimate by eye
from the ground.
- Gauge pruning. Trees are
pruned to the internode which is at or just
less than a given diameter (say 10cm). This method is relatively easy
to apply, especially if the pruners carry a simple U-shaped gauge to
measure diameter. It has the advantage that diameter is directly
correlated with cross-sectional area, which in turn is directly
correlated with the amount of foliage above that point on the stem.
Thus all trees have much the same amount of foliage left, but small
trees tend to be favoured slightly because they have less biomass to
support.
Pruning height
Depending upon growth rates and rotation ages, there is an upper limit
beyond which it is not economic to prune. As most of the value of a
tree is in the lower and fatter part of the trunk, pruning is normally
carried out on the bottom log only, which in this country is usually
taken as 5.5 to 6m. When dealing with valuable timbers (hardwoods,
veneer logs), a
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| Three Pruning Lift |
pruned log length of 3m may be acceptable, and on
exposed sites, pruning beyond 3 or 4m may not be economic. On fertile
agroforestry sites, extra high pruning above 6m may be desirable not
only to produce clear timber and minimise logs with heavy branches, but
also to minimise shading of the pasture.
(top)
Pruning intensity
The aim of clear pruning is to restrict the size of the knotty core as
far as possible without unnecessarily affecting the growth of the tree.
Healthy trees usually carry more foliage than they need, so a light
pruning has no effect on growth, and the foliage removed is
quickly replaced. Moderately heavy pruning affects diameter growth, but
severe pruning (removing >50% of the active foliage) can also
stunt height growth. Thus to have least effect on growth, pruning
should be a little and often. This has the added benefit of minimising
the size of the
knotty core (Think of pruning like humans donating blood – a little and
often does us no harm at all, but the occasional heavy loss can
seriously affect our health!).
As most logs are cut parallel to the pith, ideally the defect core
should be a perfect cylinder. In practice however, most pruning in the
past has tended to produce a defect core which is smallest near the
base of the log, and largest near the top. This is particularly so with
farm foresters, who are often keen to start low pruning as soon as
possible, but are often lax in getting the final pruning done in good
time.
On dry or exposed sites, or wherever height growth is depressed, if too
small a knotty core is aimed for, height growth will be further
restricted, and if stem crowding is not a factor, the lowest remaining
whorl of branches, which has first call on available stem moisture,
will soon become massive as the tree tries to replace its foliage. If
this whorl is removed before the branches become too large, height
growth will be even more restricted. On such sites then, either a
larger knotty core should be the aim, or size of branches should be
controlled by higher stockings.
(top)
Defect core size
The defect core in a log is the straight-sided cylinder or truncated
cone which encloses all defects, and outside which it is possible to
cut full length clear timber. Its size is determined by the radius of
the longest branch stubs, the extra radius
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| DOS/Knotty Core |
required before clearwood is
laid down beyond the stub, and the straightness of the pith. Shorter
lengths of clearwood may be produced between branches, both in a
vertical plane and in a tangential plane, but the normal aim of pruning
is to produce clearwood in lengths as long as possible.
The effective diameter of the knotty core is determined by the largest
diameter over stubs (DOS)
after any pruning operation, and this is
normally the diameter over the lowest whorl or lowest branch removed in
that pruning operation.
With variable-lift pruning then, the more often pruning is carried out,
the smaller the DOS. For radiata, the normal range to aim for is 14 to
18cm, but this can be unrealistically low on exposed sites or wherever
height growth is limited.
The extra radius to occlude branch stubs is determined by the diameter
of the stub and the species of tree. Therefore pruning branches while
they are still small will minimise the occlusion zone. The straightness
of the pith depends on how straight the tree was at the time of pruning
(not on the straightness of the harvested log).
The defect core is normally several cm wider than the knotty core for
perfectly straight trees, and considerably wider for trees which are
sinuous at the time of pruning. Therefore, to maximise clearwood
production, prune often, aim for the same DOS with each pruning lift,
keep the DOS to a minimum consistent
with maintaining an acceptable growth rate, and place high
priority on pruning only the straightest trees.
(top)
Graphics reproduced from Managing a Small Forest
for Timber – no.6 in
the Small Forest Management series, jointly produced by the Ministry of
Forestry and the NZ Forest Research
Institute.
© Crown Copyright Ministry of Forestry, May 1996.
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