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Managing blue gum (Eucalyptus globulus) coppice
TreeNote No. 35 May 2002
[Reviewed May 2005]
By Robert Archibald, CALM Science Division, Department of Conservation and Land Management, Kensington; Keith Little, Scottsville, South Africa; and Richard Harper, CALM Science Division, Kensington
[Reviewed May 2005]
About 200,000 ha of blue gums will be approaching harvest in the next 10 years in Western Australia. Consideration of how to manage this land following harvest is therefore an increasingly important issue. This TreeNote describes management procedures for one post-harvest option - obtaining another rotation by coppicing. Coppicing is using the stems that naturally regenerate from the base of a harvested tree. The information that follows is largely drawn from South African experience where coppicing of eucalypts is more widely practised.
Coppice
Coppice describes the many small stems that arise from dormant buds beneath the bark of a tree stump following removal of the trunk. Blue gums coppice readily and large areas of blue gum plantations are managed overseas in short rotations as coppice. Few such plantations exist in Australia as a result of the early stage of the industry. It is generally accepted that a tree grown from coppice will produce greater yields than a seedling of the same age. This is because the coppice is growing from a large and well-established root system. Site, weather, establishment and management factors will influence the success and productivity of a coppice stand.
At an experimental coppice site in the Wellington catchment now at 12 years of age, overall production was below that recorded for the first rotation at 7.5 years. This was most likely due to lower water availability in the second rotation as a result of depletion of stored soil water by the first rotation. It could be argued that such a yield reduction might also apply to seedlings established on a second rotation site, unless there is a fallow period of several years.
Determining whether to coppice
With the rapid expansion of the blue gum industry in southern Australia, based on an 8 to 10 year rotation, large areas are nearing harvest. There are three post-harvest options for the management of these plantations:
1 Revert to farmland
Where the first rotation has performed poorly due to inappropriate site conditions (for example, low rainfall, high evaporation, shallow or saline soils) it may be appropriate to revert to farmland.
2 Replant
Where the plantation has either performed poorly due to poor stocking (less than 1000 stems/ha) or it is considered that new genetic material will deliver significant yield benefits, it may pay to replant. To warrant replanting, yield benefits will have to outweigh the costs of re-establishment.
3 Coppice
In uniform, disease-free plantations on good quality sites where there is sufficient stocking (more than 1000 stems/ha) it may be more economic to coppice.
The cost saving in coppicing over replanting may be some 20 to 30 per cent when the management guidelines below are followed. This difference arises from not only the saved costs of seedlings, planting and site preparation, but also the likely need for herbicides and possibly a starter fertiliser, when replanting. Coppicing involves the shoot reduction operation and removal of trash from the stumps, but no fertilising or weed control. The advantage of replanting will come from higher future yields from improved genetic material.
Coppicing has been used to a limited extent in Western Australia and the strategies described here are mainly developed from overseas experience. Recent work in South Africa has shown that detailed principles of rose gum (Eucalyptus grandis) coppice management also apply to a wide range of other eucalypts.
Harvesting
If coppicing is desired after harvesting, then harvesting must be done so as to maximise the survival of stools (living stumps) from which coppice growth occurs. Harvesting, being a major disturbance to the structure and function of a tree, will result in some deaths. With rose gum in South Africa, a stool death rate of about 3 to 5 per cent occurs in association with each harvest.
Harvesting season
Studies in Western Australia have shown that harvest timing affects stool survival. Cutting in summer will delay sprouting and increase the chance of the stool drying out. It may also lead to the separation of bark from the stool. The Wellington catchment research site showed that March felling was preferable to August felling, and similarly August was better than December. Coppice stems from the March and August-felled stands grew better in the early years than those from the December-felled trees. Similar results have occurred in South Africa with shining gum (E. nitens).
When determining harvesting time, the local conditions should be considered. In higher rainfall areas the timing may not be so critical. In areas prone to frosts, felling should be delayed until late winter/spring.
Minimising stump damage
The ideal stool is one with a smooth surface on a slight angle with good bark attachment. When using machinery, care should be taken not to damage the stools (see photo). The importance of stool conservation needs to be stressed to harvesters, especially in areas where the coppicing potential is low or where they are not familiar with the principles underlying coppice management. Trash can be used to protect the stools while harvesting of nearby trees takes place, but it should be removed before the coppice shoots begin to appear.
Coppice with shoot reduction to two stems, on E. globulus at the Malcolm Experimental site, Collie, Western Australia
Stools with a smooth, slightly-sloping surface shed water and so reduce the chance of fungal infection. However, this can only be achieved by use of a chainsaw when felling.
Stool height
Stools should be 10 to 20 cm high. Shoots that develop from high on the stool tend to be weaker and less wind-firm than those that develop lower. A low-cut stool will direct growth into more choice stems.
Trash handling
Spreading trash is the preferred option so it acts as a mulch. Although trash retention may cause access problems and a fire hazard, it is desirable as it retains nutrients, conserves soil moisture and minimises erosion.
Access problems may be partly overcome by directing the bulk of the trash into alternate rows or every third row when harvesting. Studies by CSIRO in replanting situations have shown that both potentially-available nitrogen levels and growth rates, are greater in the early years in areas where trash is retained. These studies are now examining practical techniques for managing trash retention on an operational scale.
Although retained trash has many benefits, it should be kept clear of stools, as trash (on or over the stool) interferes with shoot growth and can lead to poorly-formed, spindly stems.
Coppice management
Shoot reduction
Without interference, a stool will coppice from one to many stems. By pruning unwanted stems, growth can be channelled into selected stems, giving them a greater chance of survival to harvest. The ultimate aim is to maximise the wood volume of the utilisable stems.
Number of shoots to retain
It is recommended that one or two shoots per stool are retained.
While it is the practice to retain up to three stems in Portuguese blue gum plantations, observations with rose gum in South Africa suggest that the retention of more than two stems per stool leads to poorer form, greater butt sweep and lower utilisable volume. (Butt sweep is a substantial curve in the trunk of a tree near the ground.)
More work is needed to determine whether one or two stems are more desirable in Western Australia. Local information indicates that one-stemmed coppice appears to be more prone to developing further coppice shoots (called feathering).
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Bark separation due to harvesting damage (E. globulus) |
The potential for feathering was confirmed by recent measurements at the experimental site in the Wellington catchment for 12-year-old trees from coppice. It was shown that for single stemmed trees, both the proportion (46 per cent) that developed further shoots and the average volume of the 'fresh coppice' per tree was greater than for two-stemmed trees (of which only 13 per cent developed further shoots). However, no significant difference in overall volume per hectare from trees with either one or two retained stems was apparent. It follows that retaining a single stem will result in handling fewer logs at harvest. Two-stemmed trees also have the potential to hinder harvesting if the stems grow too close to one another.
Current knowledge suggests it is best to decide the number of shoots to be retained for each individual tree by considering in combination:
- the characteristics of the shoots on the stool
- the surrounding stocking of live stools
- the size of the stool
Two shoots should be retained if:
- the area is not fully stocked
- the stool is not below average diameter for that stand
- the shoots are co-dominant and showing good growth and form
- the shoots are on opposite sides of the stool.
The optimal number of stems to be retained may require further analysis in terms of the economics of harvesting. With a two-stemmed tree, the time required to harvest per unit volume is greater. Wastage is also likely to be slightly higher.
Where a stool has a single stem retained, the stem should be located on the windward side of the stool, as shoots on the leeward side of a stool are more prone to wind-shear. So, consider the direction of the strongest winds.
Selection of shoots
As discussed, shoots arising from lower on the stool tend to be more firmly attached and therefore more likely to withstand wind-shear. Shoots should be firmly attached to the stool.
Timing of shoot removal
Shoots should be removed when they are about 2 m in height.
Timing is also an issue in maximising the growth of selected stems and in preventing feathering. Work by the Southern Timber Company (SOTICO) and the Department of Conservation and Land Management suggests that when the stems are about 2 m in height (about 18 months of age after harvest in a typical area), the shears can still be used and the dominance of the selected stems limits feathering.
Reduction of shoots too early will lead to feathering and the possibility that a second pass is required. On the other hand, late reduction is likely to result in trees of poorer form and a reduction in growth of the selected stems. Correct timing of shoot removal is therefore critical.
Method of shoot removal
The shoot reduction operation is labour intensive and the most significant cost in coppice management. The choice of equipment is limited to chainsaws and/or pneumatic shears. One person will take about two to three days per hectare to remove coppice by chainsaw. Pneumatic shears reduce the time needed but they have trouble handling shoots that approach the diameter of a soft drink can (50 mm). Persons carrying out the operation need to be informed of the importance of not damaging the remaining stems. Carelessness in shoot reduction in South Africa has been found to increase the incidence of wind-throw.
In South Africa, chemical removal of unwanted shoots in the second reduction phase has been shown to be more effective than removal by machete (commonly used in South Africa). This method is only an option in the second reduction stage when the height of the unwanted stems is lower than the selected stems.
In Western Australia (2002), using pneumatic shears, the cost of the operation is about $500 per hectare. In the high rainfall zone where the number of stems per stool is usually greater, the cost could be closer to $650 per hectare.
Other issues
Fertiliser and herbicide application
The blue gum coppice study in the Wellington catchment did not detect a fertiliser response.
Eucalypt coppice is not fertilised in South Africa because little benefit is obtained from the amount of fertiliser needed to bring a response. In addition, any small growth benefit is negated by increased weed growth which then requires herbicide application. Herbicide applications are generally unnecessary in a coppice stand. As with fertilising generally, the need for fertiliser will depend on site conditions and should be based on soil and foliar analysis for the particular site. The sites most likely to respond will have low levels of soil nitrogen and a good overall water supply.
The addition of fertiliser could increase tree water stress on drier sites and this has been suggested as a possible cause of stool death in coppice stands.
Wood density
Wood density of coppiced trees is related to the wood density of the parent trees. However, the growth rate of the particular tree is a major determinant of density, with slow growth producing greater densities and vice-versa. The significance of this effect for blue gums grown for fibre requires further study. South African work has found that wood density increased as a result of leaving two shoots per stool and reducing the stems in two operations.
Third and fourth rotations by coppice
Blue gum is capable of coppicing over many rotations. In the Nilgiri Hills in India it has been successfully grown for fuel wood in rotations of 10 years for the past century. However, it is generally accepted that after the initial gains in yield from the first coppice crop, a gradual decline takes place. It should be considered that there might be no such gains if the initial stand has depleted underground water reserves. As an example, CELBI, a large blue gum plantation manager in Portugal, carries out three coppice rotations after the planted crop. Since stool density is a factor in stool mortality, perhaps plantations intended for multiple rotations should be planted at lower densities.
No shoot reduction
Given the high cost of shoot reduction, and the possibility of feathering if the timing of the operation is not optimal, a 'no management' option could be further investigated. This is being evaluated in trials by SOTICO. South African work also indicates that there can be a reduction in basal area of up to 10 per cent if shoot reduction is not carried out. The higher cost associated with the processing of a larger number of more poorly formed stems is a critical factor to consider. Another problem may be the close location of stems on the stool at harvesting time.
Acknowledgements
The authors gratefully acknowledge assistance and information provided by Peter Ritson (Department of Conservation and Land Management) and Stuart Crombie (formerly of the Department of Conservation and Land Management), Tim Grove (CSIRO), Ray Fremlin and Brendan Peet (Forest Products Commission), and Simon Hunter (SOTICO).
Further reading
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Grove, T. S., O'Connell, A. M., Mendham, D., Barrow, N. J., and Rance, S. J. (2001). Sustaining the productivity of tree crops on agricultural land in south-western Australia. Rural Industries Research and Development Corporation, RIRDC Publication No. 01/09. 69 pp.
www.rirdc.gov.au/reports/AFT/01-09.pdf -
Harper, R. J., Mauger, G., Robinson, N., McGrath, J. F., Smettem, K. R. J., Bartle, J. R., and George, R. J. (2001). Manipulating catchment water balance using plantation and farm forestry: case studies from south-western Australia. In 'Plantations, Farm Forestry and Water.' (Nambier, E. K. S., and Brown, A. G. Eds), Water and Salinity Issues in Agroforestry No. 7, RIRDC Publication No. 1/20 pp. 44-50. (Joint Venture Agroforestry Program: Canberra.)
www.rirdc.gov.au/reports/AFT/01-20.pdf
Other TreeNote titles
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Page last reviewed 30 May, 2005