[MPWG] Fw: The Overstory #217--Pruning of timber trees

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The Overstory #217--Pruning of timber trees











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Pruned trunks of Acrocarpus fraxinifolius and Toona ciliata for clearwood 
sawn timber production.


The Overstory #217
Pruning of timber trees 
By Rowan Reid
April 13, 2009






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The Overstory #217
Pruning of timber trees
By Rowan Reid
Introduction
The main purpose of pruning is to enhance timber value by increasing the 
proportion of clearwood. Knot-free limber commonly attracts a premium 
price in appearance grade markets for pine, eucalypt, leak, Douglas fir 
and many other species. Although not often specified, knot-free timber is 
also preferable in the structural timber market where large or loose knots 
affect limber strength.
For pruning to be effective, knots or detects resulting from them must 
cause significant product downgrade. Sawing trials of unpruned trees by 
CSIRO (E. globulus) and Queensland DPI (E. cloeziana) confirm that knots 
are the worst grade-limiting defect affecting both appearance and 
structural grade sawn timber from eucalypt plantations. Waugh and Yang 
(1994) of CSIRO actually concluded that, because of knots, "there appeared 
to be little commercial opportunity for appearance products" from eucalypt 
plantations in Tasmania.
Sawing trials involving pruned (and spaced) trees suggest that pruning can 
dramatically enhance sawlog quality in most species including eucalypts 
(when not prone to excessive kino gum) and softwoods (when not predisposed 
to large resin pockets). Milling of pruned 13 year old E. globulus in 
Western Australia and 10 year old E. nitens in Victoria demonstrated that 
it was possible to achieve reasonable recoveries of clearwood sawn timber 
from quite young trees ignoring kino, pruned E. fastigata logs produced 
30% clears compared to less than 6% for the unpruned top logs from the 
same trees in a New Zealand milling trial.
The potential for pruning to enhance clearwood production in Pinus radiata 
is well-established. For other softwood species grown in Australia the 
evidence in favour of pruning is also strong: for example, 17 year old 
pruned and thinned Cupressus macrocarpa had a recovery of high-grade 
timber from the pruned butt log of 72% compared to just 13% for the 
unpruned second log. International research also supports pruning of 
deciduous hardwoods like poplar and oak. Once pruned, the next most 
significant factor affecting the recovery of clear high-grade timber in 
almost all studies was log diameter.
Pruning aims to confine branch-related defects to a 'knotty-core' within 
the log. The shape and dimensions of this core will impact the recovery of 
clearwood in the form of sawn boards or veneer. The nature of occlusion 
over a pruned branch stub can lead to an extension of defects beyond the 
end of the branch stub. These defects include the inclusion of bark, 
resin, gum, stains or irregular grain. A study involving Douglas-fir (
Pseudotsuga menziesii) reported that the Diameter Over Occlusions (DOO) 
can be significantly larger than the Diameter Over stubs (DOS) if the 
pruned branches are large or if pruning cuts are not smooth. Based on many 
years of research it has been suggested that in New Zealand the DOO in 
Pinus radiata will be about 3cm larger than the DOS if pruned as 
recommended.
It is often assumed that forest growers who prune will be paid a premium 
price for their logs. Pruning of hoop pine (Araucaria cunninghamii) in 
Queensland has been traditionally driven by a steep price gradient that 
sees the royalty on a cubic metre of log increase almost in direct 
proportion to the log diameter with a premium of around 50% for pruned 
logs. The price gradient for P. radiata across the country is similar, but 
due to the limited supply, the premium for prune logs is less clear. In 
New South Wales, premiums of as much as 100% are suggested. In New 
Zealand, despite a drop in pine log prices off their highs of the late 
1990s, the premium for pruned logs is still around 60% on the domestic 
market and close to 100% for export logs. Unfortunately, the nature of any 
price gradients for diameter or premium for unpruned plantation logs of 
eucalypts in Australia can only be predicted from the results of sawmill 
studies. However, based on native forest prices for veneer logs a premium 
of at least 30% for pruning alone seems realistic at this stage.
There is a risk that substitutes, such as medium density fiberboard (MDF), 
or intensive processing methods (such as finger jointing) will reduce the 
demand, and hence the premium, paid for clearwood logs. Finger jointing in 
pine costs about $170/cubic metre of sawn dried timber. If related back to 
the log, this suggests that the processor could afford to pay an 
additional $40 or so for pruned logs (assuming a sawn recovery of 50% or 
which 50% was knot-free). Finger jointing results in a 'manufactured' 
appearance and MDF is simply not a 'solid wood product'.

The silvicultural options facing forest owners
(a) No pruning and the used of competition to promote "self-pruning." The 
dead branches are commonly held on the tree for many years after they die. 
The competition necessary to induce self-pruning also suppresses diameter 
growth - hence the need for a longer rotation.
(b) No pruning with heavy thinning to promote diameter growth. Results in 
large diameter and large branches - hence low quality timber.
(c) Pruning without heavy thinning. Results in knotty core control and a 
high timber volume per hectare, but competition suppresses diameter growth 
- hence the need for a longer rotation.
(d) Pruning with heavy early thinning to minimise competition. Results in 
knotty core control, large diameter - 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.

Pruning can halve your rotation time
If diameter growth can be enhanced, the rotation time required to produce 
a high quality saw or veneer log can be reduced. In unpruned plantations 
it is knot size and distribution that makes it impossible to take 
advantage of the rapid diameter growth rates that are possible when trees 
are open grown. Pruning allows forest owners to space their trees in a way 
that stimulates diameter growth up to the point at which other problems, 
such as tree form, wood quality or low volume production per hectare, 
become a concern.
In the northern hemisphere, where growth rates are very slow, the results 
can be dramatic. One Canadian report goes as far as to say: "pruning is 
the only way to produce clear wood in rotations of less than 100 years". 
In England, the rotation age for oak sawlogs can be reduced by as much as 
60 years (from 150 to 90 years) using quite conservative thinning regimes 
and pruning - without having a significant effect on wood quality.
Many authors report similarly dramatic reductions in rotations for 
plantations of eucalypts, including for E. nitens in Tasmania, E. 
fastigata in New Zealand, many eastern state temperate species in Western 
Australia and a range of sub-tropical eucalypts in Queensland. Richard 
Moore has achieved average diameters of 53 cm (E. divers color) and 57 cm 
(E. globulus) within 19 years following early pruning and thinning to 150 
st/ha. Pruned E. fastigata grown in N.Z. at a stocking of just 76 st/ha 
grew to a mean diameter of over 65 cm in 29 years. When these and other 
research results are compared with publicly available data from unpruned 
eucalypt plantations held at stockings sufficient to control early branch 
development the difference is stark.
Achieving a large diameter is more critical in eucalypt sawlogs than it is 
for pine. As well as increasing the width of the clearwood zone, diameter 
growth dramatically reduces the impact of growth stresses, and allows 
quarter-sawing techniques (often essential to reduce drying degrade) to be 
used effectively. A minimum diameter overbark of 60 cm is often 
recommended. Because of the intolerance of eucalypts to competition this 
will require much lower final stocking rates. This has a bearing on the 
number of trees requiring pruning.
Controlling competition between trees (thinning) is an important aspect of 
pruning. Although a high initial tree stocking can improve growth and form 
by providing 'mutual shelter', at some point the competition will begin to 
dramatically reduce diameter growth and therefore the benefits of pruning. 
If some of the trees have been left unpruned in anticipation of a future 
commercial thinning, these 'followers' can end up outcompeting the pruned 
trees suppressing their growth even further. This concern has led some 
researchers to recommend limiting the severity of pruning to ensure it 
does not affect growth rates. In less tolerant species, like most 
eucalypts, severe competition can occur within conventional plantations 
(established at over 800 st/ha) within the first 4 or 5 years of growth 
when the basal area may be as low as 10m2/ha. This suggests the prospects 
of a commercial thinning for pulp or small logs part way through the 
rotation are remote.

Pruning may make it viable to harvest
Although automated harvesting equipment removes branches in a single 
motion, delimbing remains one of the most significant costs associated 
with manual harvesting. In fact, manual harvesting may be cost competitive 
against automated harvesting when log diameters are large, the trees well 
spaced and butt logs are pruned. This is important since scale of 
production is a critical factor in attracting automated harvesting 
equipment and may represent an obstacle to the viability of plantations 
less than about 20 hectares.
Another argument in favour of pruning is the increased flexibility it 
offers in light of uncertain future timber markets. If the species is 
appropriate, large diameter pruned logs are suitable for almost all timber 
product options from veneer down to woodchips. They are also suitable for 
milling across a wide range of processing methods. For example, small 
diameter eucalypt logs require specialized line bar carriage systems to 
counteract growth stresses during milling whereas large logs are suitable 
for most types of mills including small portable mills. This alone may be 
a critical marketing advantage for the forest grower in that it increases 
the number of potential buyers and may allow them to undertake their own 
value adding.
The common alternative to pruning is maintaining high initial stocking 
rates to control branch development then undertaking a commercial thinning 
operation to release the stand. Many, if not most, private plantation 
owners have had problems finding a market for thinnings and suitable 
contractors to do the work resulting in the stands being left unthinned 
for far longer than desired. Direct regimes, involving pruning and 
thinning, have far more harvest age flexibility: if markets cannot be 
found at any particular time growers can wait without placing the 
plantation under intense stress or risking stand stability.

Pruning enhances non-timber values
Pruned forests, because they can be widely spaced, provide the opportunity 
to incorporate other values into the design, such as a native understorey 
for biodiversity or low shelter; pasture for grazing; or even a second 
commercial tree crop such as bush foods. It may also be possible to begin 
the next timber rotation prior to the harvest of the first. For example, 
in north Queensland farmers are successfully under a pruned canopy of wide 
spaced E. grandis. The same can be done with pine under eucalypts in the 
southern states.
Pruning to reduce the fire risk was once common practice in Australian 
pine plantations. Pruning removes the "ladder" of fuel required to 
maintain a travelling canopy fire. If fuel levels on the ground can be 
kept down during the fire season by grazing or other methods, then the 
risk of tree damage may be minimal. Because wind speeds drive the rate of 
spread of a fire, and hence its intensity, closely grazed pruned 
plantations can actually form an effective firebreak.

The negatives of pruning
Pruning is an expensive, time-consuming and labour demanding job that adds 
to the already heavy up-front costs associated with plantation forestry. 
It is a job that must be done 'on-time' to the extent that missing just 
one year may result in the plantation being worth less than had it never 
been pruned at all. Ensuring forests are pruned on time, every time has 
created real problems for industrial and small forest owners alike.
High pruning may also increase the risk of wind throw due to the increased 
exposure and the greater development of heartwood in the stem. Increased 
light at ground level can exacerbate weed growth increasing the fire 
hazard, encouraging noxious weeds and making plantations difficult to 
access. In addition, there is the risk of decay or disease resulting from 
pruning and the uncertainty as to whether there will be a premium for 
pruned logs come harvest time. All this comes on top of the many 
environmental and market risks of any form of commercial tree growing.
These problems simply highlight the need to take care to ensure that the 
silvicultural regimes adopted match the particular site, grower and market 
opportunities. There are no short cuts - if growers are to maximise the 
benefits that pruning can offer, they must understand how pruning affects 
tree growth and wood quality, be aware of the various pruning methods and 
strategies available, and be able to make well-informed decisions about 
when and how to prune.

REFERENCES
Please see the original article for extensive references and in-text 
citations. 

ORIGINAL SOURCE
This article was excerpted with the kind permission of the author from:
Reid, Rowan. 2002. The Principles and Practice of Pruning. Australian 
Forest Grower, Special Liftout No. 60. Vol. 25, No. 2. <
http://www.mtg.unimelb.edu.au/publications/AFGLiftout%20no%202.pdf>

ABOUT THE AUTHOR
Rowan Reid (B. For. Sci., M. For. Sci.) is a Senior Lecturer in 
Agroforestry and Farm Forestry at the University of Melbourne and the 
developer of the Australian Master TreeGrower Program (MTG). More than 80 
MTG programs have been conducted across Australia involving more than 1600 
farmers. In 2000 the program was awarded the $10000 Eureka Prize for 
excellence in environmental education. Rowan is also a tree grower himself 
and has recently made furniture out of 16-year-old eucalypt trees he 
planted and managed on his Otway Ranges farm. He can be reached at: Rowan 
Reid, Senior Lecturer, Agroforestry & Farm Forestry, Department of Forest 
and Ecosystem Science, The University of Melbourne, Victoria 3010. Email: 
rfr at unimelb.edu.au The Australian Master TreeGrower web site: <
http://www.mtg.unimelb.edu.au>.

WEB LINKS
The Australian Master TreeGrower (MTG) Program is an educational program 
for landholders interested in the development of farm forestry: <
http://www.mtg.unimelb.edu.au/>
An Australian Master TreeGrower publication, Design Principles for Farm 
Forestry: A guide to assist farmers to decide to place trees and farm 
plantations on farms: <
http://www.mtg.unimelb.edu.au/publications/design.htm>
University of Hawai'i's College of Tropical Agriculture and Human 
Resources' Hawai'i Forestry Extension has extensive resources for farm 
forestry: <http://www.ctahr.hawaii.edu/forestry>
Mississippi State University Extension Service offers "Forest Management 
Alternatives For Private Landowners" and other extension materials for 
prospective farm foresters at: <http://msucares.com/forestry/index.html>
Choosing Timber Species for Pacific Island Agroforestry discusses seven 
steps for choosing timber species that meet the project goals, product 
requirements, and environmental conditions for farm forestry: <
http://agroforestry.net/afg/book.html>
Economics of Farm Forestry: Financial Evaluation for Landowners introduces 
strategies for determining the financial returns of small-scale forestry 
and farm forestry: <http://agroforestry.net/afg/book.html>
Financial Analysis for Tree Farming in Hawaii, is a guide for tree farmers 
in calculating economic costs and benefits of timber plantations. The 
pamphlet explains how to compare current costs of establishing plantations 
with anticipated future timber harvests: <
http://www2.ctahr.hawaii.edu/forestry/links.html#econ>
Silvopastoral publication by the USDA National Agroforestry Center
<http://www.unl.edu/nac>.
University of Missouri Center for Agroforestry silvopasture
introduction: <http://www.centerforagroforestry.org/practices/sp.asp>.

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The Overstory #48--Farm Forestry
<http://www.agroforestry.net/overstory/overstory48.html>

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