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12 Apr2009

Overstory #217 - Pruning of timber trees

Written by Rowan Reid.

copyright Craig ElevitchPruned trunks of Acrocarpus fraxinifolius and Toona ciliata for clearwood sawn timber production.

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.

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@unimelb.edu.au

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