Whole tree harvesting ups output

William Merivale

May 01 2012 04:47 AM

Depending on its age and size, the stem of a tree accounts for about 50-60pc of its total volume.

The branches, stump and root system make up the balance. Consequently, in conventional harvesting, a significant proportion of the potentially useable fibre is left in the forest. All is not lost, however, because a proportion of this dead wood is essential for the ecological health of the woodland and should always be retained.

However, techniques have been developed to maximise the amount of the tree that can be utilised at harvest. Referred to as whole tree harvesting, it is not a new concept, especially in Scandinavia, where slash (branch) bundling and stump removal techniques are well advanced. While some research into the practice was conducted here in the early 1980s, it hasn't yet caught on in a significant way.

This is partly because of the different circumstances here compared to Scandinavia, not least our much higher rainfall and mild winters. While Scandinavian countries have the luxury of being able to harvest for several months of the year on frozen ground, we are dependent on the slash to create a 'brash mat' to minimise soil damage and compaction, and recovery of the material is often impossible, except on suitably dry sites. Moreover, markets for biomass are much more advanced throughout the continent than in Ireland.

While growers might be concerned about the potential loss of vital nutrients from the site, the fact that 70-80pc of these are contained in the needles, which return to the soil provided the slash is left for a number of months after harvest, should reduce concerns on suitable sites.

Another important downside to stump removal is the potential loss of soil carbon -- typically soils can contain anywhere from 250-1,000t/ha of carbon and a proportion of this is lost when the soil is disturbed (the aeration accelerates decomposition of organic matter leading to the rapid release of CO2).

The ability of forests to sequester and store carbon is one of the crucial weapons in the fight to combat climate change, so again considerable care is necessary to ensure that whole tree harvesting doesn't compromise this.

However, Ireland is confronted with an energy crisis of epic proportions so it makes sense to ensure that a valuable resource that can make a significant contribution towards our energy requirements should be managed to its full potential.

Donal Whelan, director of the Irish Timber Growers Association, estimates that as much as 3,000ha of the total 8,000ha of annual clearfell would be suitable for whole tree harvesting. This has the potential to produce 570,000t of chip in slash and stump harvest, rather than leaving it to rot in the forest. However, not every site will be suitable for the operation.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Cork. Email: william@cjandco.net