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Thinning for Sawlog Production

Note Number: AG0775
Published: December 2001
Updated: September 2009

This Agriculture Note describes thinning for sawlog production.

What is thinning?

Thinning of a plantation is the removal of a proportion of its trees. Thinning is a critical part of management to produce sawlogs (see Agriculture Note AG0818: Growing trees for sawlogs), especially in stands being pruned for clearwood production (Note AG0773: Pruning for clearwood production).

In most sawlog plantations, trees are planted fairly densely to encourage early height growth and good form. Only a fraction of them can grow to sawlog size, due to competition and limited resources (e.g. rainfall, soil depth). Hence, sawlog plantations are usually thinned twice or more in a rotation.

Thinning is a means for:

• Managing competition among trees for water, nutrients and light; some trees are sacrificed in order to allocate resources to other trees, improving their growth and health, and increasing log diameters (Figure 1).
• Tree selection; small, poorly formed or unpruned trees can be removed while the best trees are retained as the final sawlog crop (Figure 2).
• In some cases (commercial thinning), providing early financial returns.

Considerations

Growers need to determine when to thin, and how many trees to keep.

The main factors influencing these decisions are:

• current plantation status,
  - tree diameter
  - current stocking or basal area
  - tree form and pruning
  - health,
• availability of markets for thinnings,
• availability of labour and equipment.

Different approaches

Approaches vary among plantations (see examples in Table 1) and there are several methods that can be used to indicate need for thinning.

In many small pruned eucalypt stands in areas of low to medium rainfall, it may be best to simply thin (progressively) to final stocking as early as is possible without overly negative effects on form.

Growers often use simple prescriptions, which can be applied with some degree of flexibility.

Table 1. Differing approaches to thinning in two plantations

	Example 1	Example 2
Plantation details
Size	Small	Large
Species	Eucalypts	Radiata pine
Ownership	Farmer	Corporate
Who does the work?	Farmer	Specialised teams
Pruning for clearwood	Pruned	Unpruned
Distance from markets and processors	Distant	Relatively close
Access to harvesting machinery	Relatively poor	Relatively good
Transport costs	High	Relatively low
Harvest	Small number of large-diameter1, high-quality, clearwood butt logs at end rotation	High volume, mix of products (pulpwood, posts, knotty sawlogs including top logs) across rotation
Rotation length	25-30 years	35 years
Approach to thinning
Aims	To simply increase production of valuable clearwood (non-commercial thinning, thinning to waste)	To harvest products (commercial thinning) and promote final sawlog value
Timing	Early2 (remove non-crop trees as soon as they are identified and not pruned)	Later (timed to maximise profit from a mix of products across the rotation)
Products harvested	None3 (slash remains on ground returning some nutrients)	Optimal mixture of pulpwood, posts, small sawlogs
Pruning	Crop trees are pruned close to the time of thinning4	Unpruned
1 Wider logs allow greater recovery of sawn timber (less waste) and produce wider, more valuable sawn products. Farm foresters usually aim to grow logs at least 50 cm in diameter. 2 Early thinning can also reduce tension wood and growth stresses in eucalypt sawlogs. 3 Thinnings could be used on-farm for rough timber, firewood, etc. Some farmers plan a commercial second thinning, if viable. 4 Thinning reduces canopy cover so lower branches are exposed to more light and may quickly grow large if not pruned.

Simple thinning program for a farm forest of pruned eucalypts (an example)

First thinning

Cull all non-crop trees when crop trees (400-500 potential sawlogs/ha) have been identified (clear pruning has commenced) and their mean diameter at breast height is about 10 cm. Retaining 400-500 trees/ha at this stage has little effect on growth of final crop trees and has several benefits:

• shade and shelter, thus suppression of branches in the lower crown, and easier pruning,
• insurance against later problems such as wind throw,
• opportunity for commercial second thinning.

Further thinning

Option 1 (non-commercial): Cull all further non-crop trees when as they are identified as inferior and cease to provide beneficial shelter, leaving 150 final sawlog trees/ha.

Option 2 (commercial): Cull all further non-crop trees when pruning (to 6.5 m) of 150 final sawlog trees/ha is finished (identify a market for thinnings in advance).

Diameter–basal area ratio

The ratio of tree diameter (D, cm) to basal area (BA, m²/ha) can be a helpful tool for understanding competition as the forest develops and determining when and how heavily to thin. D and BA are also easy to assess. One strategy for thinning pruned eucalypt stands is to maintain D:BA of between 2 and 3. Pines, which are more tolerant of competition, may be thinned so that D:BA remains above 1.

Which trees to keep

Potential sawlog crop trees should have:

• Good height and diameter growth (not suppressed compared to neighbours)
• Straight basal stem of appropriate length (usually at least 6.5 m, a grower standard for log length)
• Good health, a dense crown and no signs of damage, decay, borers, wounds or disease
• Light even branching; minimal forks, large branches or steeply angled branches
• Fairly even spacing throughout the stand (minimal competition, optimal use of the site, and symmetrical log growth)

How many final sawlog crop trees to keep

About 150 trees/ha is often claimed to be a good final stocking for a eucalypt sawlog plantation, but 100 to 300 trees/ha may be suitable, depending on site, species, management and rotation length. Species such as pines can tolerate a somewhat higher final stocking than eucalypts.

Achieving the right stocking and spacing

At first thinning growers usually select a proportion of the trees planted. For example, if the planted stocking was 1000 trees/ha, to achieve 400 trees/ha you would choose to keep 2 in every 5 (400 in 1000) trees planted (including missing trees) and cull the rest.

At later thinnings, it can be easier to work to a required average spacing along the rows. For example, to achieve a stocking of 150 trees/ha where rows are 4 m apart, the grower needs to retain one sawlog tree in every 16.7 m row segment, on average.

It is convenient to work along double rows, selecting and marking trees before the thinning operation begins. Some people prefer to paint trees to be kept while others mark trees to be culled. Either way, the method must be clear to all workers. If the crop trees are already pruned they need not be marked (pruning before thinning also leaves more space for falling culls).

Doing the thinning

Trees can be culled or removed using chainsaws, heavy machines, chemicals or ring-barking.

Chainsaws have the advantages of immediately removing competition, and allowing trees to be fallen in the desired direction (e.g., into alternate rows to maintain access). However falling trees can damage retained trees, and cut stumps will coppice vigorously, so coppice must later be managed. Chainsaw work also entails safety concerns and requires experienced operators. Protective equipment including ear muffs, helmet, visor, safety boots and cut-resistant pants must be used.

Alternatively, available excavator-based machines can simply push trees over and sweep them aside, removing the whole stem and root ball. This method shares the advantages of chainsaw use, while also removing the need for coppice control and improving safety. Forest harvesters may also be used, particularly for commercial thinning, but they do not remove stumps. Disadvantages are that such machines and experienced operators may not be readily available, particularly for work in small or isolated farm forests.

Chemical thinning can be relatively fast light work compared to using chainsaws, and it leaves no material on the ground and kills the tree stump. Disadvantages are that trees take time to die, a poor kill or flashback (poisoning of crop trees through roots) is possible, and dead trees are left standing. Ring-barking may be used for a similar result.

Wind

Thinning of a plantation exposes remaining trees to more wind. Strong wind may have a negative influence on form; in early thinnings, particularly of exposed plantations, it may be wise not to thin the edge row in order to maintain shelter. Windthrow is possible after thinning, particularly after heavy late thinning, and can mean the loss of valuable crop trees.

Further references

Bird PR (2000) Farm Forestry in Southern Australia: a focus on clearwood production of specialty timbers, Agriculture Victoria, Hamilton, Victoria.

Reid R (2009) ‘Growing high-quality sawlogs’, in I Nuberg, B George, R Reid (eds), Agroforestry for natural resource management, CSIRO Publishing, Collingwood, pp. 161-182.

Reid R (2002) ‘The principles and practice of pruning’, Australian Forest Grower, Winter 2002, vol. 25 (2), Special Liftout No. 60.

Reid R, Stephen P (2001) The farmer’s forest: multipurpose forestry for Australian farmers, RIRDC Publication No. R01/33, Australian Master Tree Growers Program.

Washusen R (2008) Sawmilling of eucalypts in Australia, Australian Forestry, vol. 71 (2), pp. 79-81.

Contact/Services available from DPI

See the DPI website (www.dpi.vic.gov.au), under “Forestry”, for more information including the Notes series.

Customer Service Centre, telephone 136 186.

Acknowledgements

This agnote was developed by Desmond Stackpole, Centre for Forest Tree Technology. December 2001.

It was reviewed by  Tim Jackson, Farm Services Victoria, September 2009.