Thinning | Agroforestry

Thinning plantations for high quality sawlogs

How to grow sawlogs fast

Practical techniques by Rowan Reid

You need to be cruel to be kind

Having worked so hard nurturing young trees through the first few years of life it is difficult to come to terms with having to thin, or cull. But whatever your interest, be it conservation or production, the selected removal of some trees in a forest will help those remaining. In plantations thinning improves diameter growth by increasing light and moisture availability to the remaining trees. In mixed species forest it important to ensure the slower growing trees and understorey plants do not succumb to the dominance of the fast growers.

Trees essentially grow in two ways. The most obvious growth occurs on the leading shoots of the main stem and branches. The second is the cambium. The cambium is a layer of dividing cells found just below the bark on the main trunk and all branches that produces wood and bark therefore increasing diameter growth. When we manipulate the growth of a tree we do so be influencing the growth and development of these two growing points. Height growth is best where there are sufficient trees to provide mutual shelter from damaging winds and excessive radiation. However, if the competition reduces the canopy area diameter growth slows.

/edit/images_hr/GRANDIS DBH BA 2.JPG
Flooded Gum plantation (E. grandis) thinned and pruned. The Basal Area (about 17m2/ha) is about half the Mean Tree Diameter (about 35cm). In order to maintain diameter growth it will need to be thinned again in a few years but at this stage I am comfortable with the stocking rate. Note the healthy canopies which suggest good diameter growth rates. The next thinning will produce small sawlogs and will relieve the competition further to drive diameter growth on the best trees. The aim is to produce pruned logs of over 60cm in diameter - preferably approaching 80cm or more.

The Silvicultural Tools

Landowners can influence form, diameter and wood quality of their trees by manipulating competition between the trees (thinning) and by tree shaping (pruning). For a single species competition in the plantation can be estimated from the size and spacing of trees on the site.

By understanding how competition in a plantation affects the growth of individual trees and stand characteristics, managers are able to manipulate the stand and change the outcome. Photo 1 illustrates the powerful influence of inter-tree competition on tree diameter and stand volume. If the object is to maximise the volume of timber (as for pulpwood or fuelwood) then the higher the stocking rate the greater the yield. This is why pulpwood plantations are established at over 1,100 trees per hectare (3x3m spacing) and left un-thinned until they are harvested.

If, however, the aim is to produce large diameter logs suited to milling for solid timber, the trees may be planted at a lower initial stocking rate or thinned to maximise diameter growth. Although the total volume of production may be lower, the shorter time needed for the trees to reach the appropriate size, the reduced pruning costs and the higher value per tree, can offset the loss of volume.

It is common in plantation forestry to plant more trees than are expected to be harvested. This not only provides mutual shelter for the young trees and helps control tree form and branch growth but also allows for a degree of selection so that all the final crop of trees are of high quality.

To grow good trees you must cull some

To increase diameter growth on selected trees the competition from adjacent trees must be reduced as the tree grows. The difficulty for landowners is in determining how many trees to thin and when to begin. Thinning too few trees means that tree growth is not increased sufficiently. Thinning too heavily may encourage excessive branch growth and will reduce total yields. Forester use the term Basal Area per Hectare to help them determine the degree of competition and severity of thinning. The Basal Area (m2/ha) is the cross-sectional area of all the tree stems and is directly proportional to the volume of timber of the site. Basal Area can be measured a number of ways (see MEASUREMENT for details).

If all the trees were the same size:

Basal Area (m2/ha) = (Diameter (cm)/200)2 x 3.142 x Stocking (trees/ha)

Experience in eucalypt plantations suggests that if trees are to grow during the early years without excessive competition the plantation should be thinned such that the basal area is equal to half the tree diameter. For example, if the dominant trees in the plantation are 20 cm in diameter the basal area after thinning should be close to 10 m2/ha. This corresponds to a stocking of about 320 trees per hectare. Assuming an initial stocking of 1000 stems per hectare regular thinning can be used to reduce stocking rates appropriately.

If the Basal Area is over 30 m2/ha in eucalypts then diameter growth is often reduced to less than 5mm per year compared to more than 20mm that may be possible in more open stands. Measuring BA is relatively simple using optical tools but formula below can help landowners estimate Basal Area.

Basal Area is approximately equal to (DBH/200)2 *3.142* S

where DBH = average tree diameter at 1.3m (cm)

S = trees per hectare

Note this will always be an underestimation

Spacing Factor

The Spacing Factor is simply the average distance between the trees (m) divided by the average stem diameter (cm) and is a useful way of estimating Basal Area in uniform plantations. To thin a plantation to a certain Basal Area simply thin to an average spacing equal to the diameter of the retained trees multiplied by the appropriate spacing factor. The higher the Spacing Factor the lower the Competition.

For example, if the trees are 20cm in diameter and spaced on a regular 3m grid then the Basal Area is about 35 m2/ha. At this level of competition diameter growth is restricted. To thin the stand to a BA of 20 the stocking rate would need to be reduce to 625 per hectare or an average spacing of 4m. As a result diameter growth would be enhanced. Once the trees reach 25 cm in size they may require thinning again. If the aim is to reach a diameter of over 45cm then the final spacing may be as low as 156 trees per hectare.

The temptation to postpone thinning until the trees are large enough to allow for a commercial thinning for posts or even small sawlogs is common but will only slow the growth of the best trees, therefore postponing the lucrative final harvest.

This may be acceptable for landowners who are happy to wait or are unable to fund the costs of non-commercial pruning or thinning but in most areas, where the return for small wood is very low, it is advisable to thin to waste.


Download my presentation from the recent Silviculture Workshop in Melbourne CLICK

Download my practical paper on thinning and pruning CLICK

Look at my peer-reviewed paper on Thinning using the DBH:BA Ratio. CLICK