Typically, forests are primarily grown for roundwood production. However, Ireland’s forests have the potential to provide large volumes of by-product residues, such as branches and tops, collectively known as brash.
In theory, Irish forests have the potential to produce 3,980 million m3/year of residues, of which 700 million m3/year remains unused.
This is seen by many as a huge potential energy resource, with Bord na Móna, among others, aiming to tap into this.
However, brash is often used to make mats or roadways for machinery to get into and out of forests during felling, making it unusable as an energy source.
For brash material to be used for energy, it must be harvested and kept separate from brash mats, a process called specified biomass harvesting (SBH).
SBH refers to the planned and specified harvesting of tops and branches which are removed during normal felling operations for use in energy.
The tops and branches are removed during roundwood harvest and piled in situ on the forest floor adjacent to extraction racks, and left for a sufficient time to allow needle and leaf fall before collection (see Figure 1 and 2).
By harvesting this material through SBH, it is possible to generate large volumes of useful biomass while improving the sustainable forest management practices. SBH also facilitates forest regeneration, more efficient subsequent crop management, and more uniform and higher quality crops in future cycles.
The Irish BioEnergy Association (IrBEA) recently published a technical guide aimed at foresters, forest owners and environmentalists, to outline how SBH should be practised in a cost-effective and sustainable way.
Advantages
SBH material has advantages as a biomass fuel, due to significant greenhouse gas savings when used to replace fossil fuels. This is due in part to the avoidance of emissions from the decaying of these residues when left in the forest.
Furthermore, where clearfell logging residues are removed, reforestation costs can be lowered.
Regeneration is more uniform, as brash, logging debris and potentially soil are not scraped into windrows.
Windrowing has been shown to lead to uneven crop growth, as trees grow faster closer to the windrows, due to higher levels of soil nutrients and better shelter.
The guide outlines four best practices for SBH as below: