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No. 23 Form pruning deciduous hardwoods
NEW ZEALAND FARM FORESTRY ASSOCIATION INFORMATION LEAFLET
How often have you been asked by someone “What should I plant on a plot
of land where I want to grow trees for my grand children – but I do not
want to plant radiata?” Only too often? A standard reply is “If there
was an obvious answer to that one, I would have planted it 25 years
ago”. However, even if there is no ‘wonder tree’ (other than radiata?),
there are a range of other species that are attractive to grow, and
many of us would like to maximise any possible timber potential.
Amongst these species are the deciduous broadleaves such as ash, oak,
elm, sycamore, walnut, cherry and chestnut.
The challenge
The trial
Results – height growth, shelters and
target trees
Shelters
Achieving target
tree status
Form pruning
Discussion
Conclusion
The challenge
With this in mind, the two writers sat down almost 10 years ago to
design a trial to look at early silvicultural options for improving the
timber potential of deciduous hardwood trees. We knew that in the home
ranges of these species, silvicultural regimes often begin by
recommending initial spacings of 2500 to 3000 seedlings/ha – dense
stockings aimed at forcing acceptable form and branch size. Our
thinking was that most potential New Zealand growers (probably farmers)
could neither afford this high initial cost, nor were they likely to
have the large areas of good soil and shelter needed for traditional
broadleaf plantations. Most would be more attracted (mentally,
physically and financially) to planting a few trees in scattered sites,
rather than many trees in a single plantation. However, if it was not
going to be possible to get good bole form and branch control by close
spacing (as in plantations), then some other simple, individual tree
technique was needed to improve timber quality.
The trial
So we designed a trial where the aim was to grow, as quickly as
possible, a target sapling tree which we described as “A healthy tree
with a single, straight, defect-free, stem of at least 3m length”. We
managed to assemble sufficient acceptable seedlings of nine deciduous
hardwoods – Algerian, sessile and Turkey oak (
Quercus canariensis, Q.
petraea and
Q.
cerris respectively), English ash (
Fraxinus excelsior),
Dutch-elm-disease-resistant elm hybrid (
Ulmus ‘Loebel’),
wild or Gean
cherry (
Prunus avium),
sweet chestnut (
Castanea
sativa),
Paulownia
fortunei, and
Robinia
pseudoacacia (‘Jaszkeseri’ clone). Three of our
sponsors, the North, Central and South Canterbury branches of the NZ
Farm Forestry Association, asked if some native species could be
included, so we added black beech (
Nothofagus
solandri), kahikatea
(
Dacrycarpus dacrydioides),
totara (
Podocarpus totara)
and kanuka
(
Kunzea ericoides).
And, of course, we had to include radiata pine and
macrocarpa as benchmarks. That made 15 species in total – although the
Algerian and Sessile oak turned out not to be pure species, and
included many hybrids with the common English oak,
Quercus robur.
The trial site was in the Forest Research Institutes’s nursery off
Oxford Road in
Rangiora. The site is flat, with a deep, fertile, Waimakariri
silt loam and has a mean annual rainfall of 650mm. Limited space meant
that the trial had to be divided into a north and south block. The
north block was exposed with little shelter, while the south block was
well sheltered. Planting was carried out in mid-August 1991. Trees in
all treatments were given weed control for the duration of the trial.
Six silvicultural treatments (replicated five times) were tested:
- Control – left untouched
- Form pruning – leader training and branch control
- Plastic KBC® tree shelter (1.2m) +
form pruning
- Plastic KBC® tree shelter (0.75m) +
form pruning
- Plastic Tubex® tree shelter (1.2m)
progressively raised up to 3m as
tree grew
- Coppicing plus 1.2m tree shelter (yr 2) + form pruning
It soon became clear that the most important treatment was form
pruning, which is the focus of this article. Other results,
particularly relative to the use of tree shelters, are only summarised
below and will be detailed in a later article.
(top)
Results– height
growth, shelters and target trees
Many trials seem to attract Murphy’s Law, with unforeseen problems
regularly arising. This trial was fortunate enough to have few such
problems and was a joy to attend and watch maturing. Some species grew
over 1m a year, but as the species blocks were not replicated, and the
north and south sites offered different growing conditions
(particularly relative to wind exposure), comparisons between species
could only be viewed indicatively.
Shelters
KBC
® tree shelters improved initial survival
principally by eliminating
hare and rabbit damage. Tree shelters also significantly improved early
height growth, although this advantage over control trees declined over
time and was considerably less apparent by age 5. Stability of trees
with standard
(1.2m tall) shelters was not a problem with deciduous species. However,
it was a major problem with the evergreen species, radiata and
macrocarpa, and three of the native species, kanuka, totara and black
beech. Raised shelters were tried only on wild cherry, ash and Algerian
oak, and in all cases promoted instability.
Stems soon became too weak to support their crowns and had to be given
artificial support.
Achieving
target tree status (‘straight defect-free stem of at least 3m
length’)
For most species there was at least one silvicultural treatment which
promoted the majority of trees to reach target specifications within 5
years. Some species achieved it within 3 years. The slowest were the
four native species where only 13% of the black beech reached target
tree status. The most influential factor was form pruning.
Form pruning
 |
Leader training of a
4-year-old elm, before
(left) and after (right).
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Form pruning involved
leader
training and branch control. Of these,
leader training was the most important.
Leader training
This was carried out annually from year one in spring and sometimes
again in mid-summer. It made a major difference to stem straightness,
and to the number of trees reaching target specifications within the
span of the trial.
Leader training involves removing or tipping back competing leaders to
promote a single straight, vertical leader. If carried out at the right
time in spring, when young growth has not lignified (become woody), the
work required is minimal.
It is not unusual for winter frosts and insects to kill the terminal
bud on the leader of a deciduous tree, and once growth commences, a
number of secondary buds will begin competing for leadership. If left
untouched a multileadered top usually results. However, a few seconds
of pre-emptive pruning with
a pair of secateurs when the young shoots are only 5 to 10cm long, is
often sufficient to turn a potential problem tree into one of desirable
target form.
Small scars created at this time of the year quickly heal. A second
visit in midsummer may be needed to remove later development of
competing leaders, but if the spring training has been done correctly,
repeat visits are not normally needed. As the trial’s target tree
required a straight stem only to 3m, no leader training was attempted
above this height.
(top)
Branch control
Branch
control was initiated around year 3 and directed at the larger
branches, particularly those with ascending habits. The main focus was
on the larger branches in the lower half of the tree, but attention was
also paid to steeply ascending branches in the upper half, as these
tend to be more vigorous than horizontal branches, and often end up
competing with the leader. Treatment involved either removing their
upright components or pruning off at the branch base if they
had reached a basal diameter greater than 2/3rds that of the
main stem (at junction) or, in later years, if they reached 30mm
diameter. A home-made 30mm gauge was used to determine when this
critical branch size was exceeded.
Determining whether to shorten or remove a branch before it reaches
30mm in diameter can be difficult, because if green crown surface area
is reduced too severely, diameter growth will slow up significantly.
For this reason, no more than one third of the total foliage area was
ever removed in any one
season, and if there was any doubt, shortening or tipping (removal of
the outer half of a branch) was preferred.
Branch control was carried out in late winter or early in the spring at
the same time as leader training. This was not only practically
convenient, but at this time of the growing season
occlusion of pruning scars was most rapid.
Branches were not removed ‘flush’ as with radiata pine. They were cut
off just outside the branch collar, or swollen area at the base of the
branch. This is important as, if cut flush with the stem, the healing
process is uneven and considerably slowed, and there is more
opportunity for rots to set in. No pruning pastes were used, as these
days it is considered to be an unnecessary expense if pruning is
properly carried out (Rumour has it that arboriculturists in
Christchurch and Auckland use pruning pastes only in Fendalton and
Remuera respectively!).
Discussion
When discussing this trial, the authors are often asked “Why only prune
to 3m?” This question usually comes from radiata pine growers, who
always target 6m as the pruning height. However, pruning slower-growing
species (such as deciduous broadleaved hardwoods) to 6m takes much
longer than with
radiata pine, and is also more complicated due to the spreading,
multi-leadered habit of most broadleaved species. In addition, it can
be difficult to reach a growing tip above 3m, as leader training is
carried out long before the tree itself
is strong enough to support a ladder plus pruner. Furthermore, most
hardwood timber is likely to be grown for specialist use in the likes
of furniture, or as veneer, and long log lengths are not a prerequisite
for such end uses. Buyers of
hardwood timber in the Christchurch area are more than happy with
quality logs of 3m in length. This height can be achieved relatively
easily, and once reached, the crown of the tree can then be left to
grow unhindered and in so doing will ‘fatten’ out the butt log in the
shortest time.
Pruning to greater heights is certainly not discouraged, but should
only be attempted if both desired and readily attained.
With broadleaf species, far more attention needs to be paid to form
pruning. This system, involving leader training and branch control, is
very
similar to that promoted for Tasmanian blackwood (
Acacia melanoxylon)
by Ian Brown (1993 and 1997) and
Forest Research
(1995). Brown found
that leader training took very little time or effort but led to
dramatic improvements in
blackwood form, and eliminated the need for a nurse crop.
Forest
Research advocated the improvement of timber potential by annual
form pruning and the use of a
30mm gauge for determining which branches to remove.
Although this article is promoting the growing of a straight,
defect-free butt log, the writers would like to stress that they are
well aware that many broadleaf trees are grown as much for amenity
purposes as for any commercial gain.
And a twisted stem with large lower branches may have more amenity
value to some growers than a straight stem. After all, such trees can
be ideal for the grandchildren’s tree huts.
Therefore, the message from this article is not only trying to promote
the best form for a timber tree, but also informing growers that they
can be the boss as to the shape and form of virtually any tree. Form
pruning may be a means to producing a quality timber log, but the same
principles can be applied to producing a tree for a whole range of end
uses. So, when you next plant a broadleaf tree, think about its purpose
and the shape that would best meet that end use – then use your
secateurs to help make that vision into reality.
Conclusion
The major finding of the Rangiora deciduous broadleaf silvicultural
trial – that leader training and branch control are critical to the
consistent growing of quality timber butt logs – is not only
practically useful, but entertaining as well.
It is entertaining because most tree growers love using secateurs and
pruners (unfortunately, this love for pruning is often accompanied by a
hate for thinning!) and they welcome any excuses to go outside and
exercise their arboricultural inclinations. The writers hope for just
this – that the article
encourages growers to switch more attention to their broadleaved trees,
in the form of leader training in particular. Remember, just a few
seconds work in the spring can make all the difference between a
quality log which could be worth considerable money to your
grandchildren and one which may only attract attention from your local
wood turner (but only for small sections of the tree) and the firewood
merchant.
(top)
This article by Nick Ledgard and Miles Giller, appeared
with diagrams and photographs in the August 1999 issue of
the New
Zealand Tree Grower.
