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Thinning for fatter returns

ACCELERATED growth of sawlogs in thinned stands will help fill a predicted shortfall in sawlog supply in 20-30 years’ time as the forestry industry in Tasmania switches the focus from mature and regrowth forest to forests regenerated in the 1960s and 70s.

Research undertaken by Forestry Tasmania as part of a national study funded by Forest and Wood Products Australia and coordinated by CSIRO has shown thinning produces growth benefits that allow the retained trees in native forests to reach a size suitable for sawing more quickly than trees from unthinned forests.

The study compared 25 Category 3 sawlogs from a 60 year-old stand of Eucalyptus regnans thinned as part of the Chesterman’s Thinning Trial in the Plenty Valley in 1970 with 23 comparable logs from a 72-year-old unthinned stand of E. regnans 7.5 kilometres northwest of the thinned stand.

The sawlogs were cut to 25mm thick boards at the ITC sawmill at Southwood, seasoned at the ITC drying facility in Launceston and then planed and graded for use.

Sawlogs from the thinned stand had a lower recovery of sawn timber, particularly the most valuable select grade boards, meaning the sawlogs from the thinned stand had 16 per cent lower value than sawlogs from the unthinned stand ($220/m3 compared with $263/m3).

This result was at odds with those obtained from four other samples in the national trial in which logs from the thinned stand had the same or significantly higher value than the logs from the unthinned stand.

The lower value of sawlogs from the thinned stand in Tasmania was due almost entirely to logs from the thinned stand having more kino (gum) veins than logs from the unthinned stand. The amount of kino veins in logs is not related to the thinning treatment, but rather normal variation among stands. For example in the Victorian sample, logs from thinned stands had 20 percent fewer kino veins compared with logs from the unthinned stand.

Despite the lower log value, the financial benefits of thinning the E. regnans stand are considerable. Bringing forward the harvesting age from 72 to 60 years means the sawlogs from the thinned stand were 50 per cent higher in value than sawlogs from the unthinned stand.

This study provides strong evidence that well conducted thinning operations in native regrowth forest can not only accelerate sawlog production but also return significant financial benefits.

Full reports of each of the five sawing trials included in this national study can be obtained from Forest and Wood Products Australia (www.fwpa.com.au).

Dr Tim Wardlaw
Principal Research Officer (Biology & Conservation)


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