How much fibre is left in-field after harvesting?
Fibre losses from harvesting are a mystery no longer for South African softwood clearfell systems. A study published by Ackerman and Pulkki (2012) has examined the issue with startling results.
by Pierre Ackerman and Elizabeth (Lise) Gleasure
|Volume loss from stumps accounts for less than 1% of the total volume loss.||Stem damage from rough handling.|
The merchantable volume recovery rate from this study was approximately 92%, meaning roughly 8% of merchantable fibre, and therefore value, is being lost. This 8% was also found to translate to a loss in board products of R393 million per year or a monetary log value loss of R165 million on an annual harvest of 4.9m3 – quite a substantial amount. To put it into perspective, to replace the fibre left in-field would require an additional 1 278 hectares per year to be harvested! So much for sustainable forest management.
In this study, fibre loss was broken down into seven categories. The largest losses were a result of top volume loss, contributing roughly 2.6% of total fibre losses or approximately 130 000m3 per year. Primarily a result of stem breakage, top volume loss can potentially be reduced through using directional felling methods.
The second largest category was excessive trimming and removal of merchantable wood volume loss and accounted for approximately 2.0% of total volume loss or roughly 101 000m3 per year. This is perhaps the most easily avoidable loss as it is predominantly due to log scaling errors. Proper supervision, particularly focused on the amount of merchantable wood left in field, would help mitigate this loss.
Although potentially an unavoidable source of fibre loss, log trimming allowance contributes 1.62% to total fibre volume losses or approximately 80 500m3 per year. The 10cm log allowance required by mills cannot be directly changed; however, if the above mentioned excess trimming can be avoided and scaling quality raised, mills may be willing to reduce this requirement and therefore aide in full fibre recovery. Furthermore, the incorrect application, ie. either over scaling or underscaling, this allowance amounts to 0.6% of total fibre loss or 30 000m3 per year.
Volume losses from stumps accounts for less than 1% of total volume loss or roughly 38 000m3 per year; yet, stump loss is perhaps the simplest to reduce. Stumps were, on average, found to be cut 7cm higher than acceptable. This difference is particularly acute with motor-manual (chainsaw- based) harvesting systems.
Small changes in operations and operator tech- nique can help reduce these losses. Fibre loss is a worldwide trend and this study reveals similar patterns here in South Africa. The study can be found in Southern Forests: a Journal of Forest Science Vol. 74, Issue 2.
For more information, contact Pierre Ackerman at firstname.lastname@example.org.
Ackerman, P and Pulkki, R. 2012. Fibre volume losses of eight softwood clearfell harvesting systems in South Africa. Southern Forests: a Journal of Forest Science 74(2): 133-149. doi: 10.2989/20702620.2012.701412.
|Stems damaged during felling and left in-field.||Excessive cutting... wasted effort and loss of fibre.|
Published in October 2013