It is a question that we at Energy Innovations do get asked fairly regularly.
Currently the UK woodlands are underutilised and many are poorly managed. In the East of England alone, The Forestry Commission claim that we have as much as 70,000 hectares of undermanaged woodland which could provide renewable fuel. Therefore, the introduction of the RHI and consequently the increased appeal for installing biomass boilers and burning woodchip is actually a positive step – land owners are starting to use their land more effectively. And better quality woodland equals improved biodiversity benefits.
Pre-historic England was largely covered in woodland but by the end of the first millennium much had been cleared to satisfy the needs of the population. By the end of the 19th Century this had reduced to just 5% of woodland in the country. At the beginning of the 21st Century the figure increased to 8.4% but this is still small in international terms. The plan is for continued expansion of our woodland areas.
We are however, currently seeing a peak in timber availability in the UK as in the 60s, 70s and 80s lots of trees were planted; they are now coming to maturity and are being felled.
Trees are often planted quite close together to encourage upward growth and restrict the number of side branches which will improve timber quality, therefore reducing the number of knots in the wood. As part of the silvicultural management process the smaller, suppressed trees are removed so that the remaining trees grow better. Many of these smaller trees are used in the manufacture of wood-based panel products or as fuel.
When you see a giant log pile ready for chipping it is worth remembering that this often represents a collection of top portions of multiple trees, not the complete tree. The main bulk of the tree will have gone to the sawmill for use in construction, fencing, furniture etc.
Some people might argue that wood chip is better used for the manufacture of paper or wood-based products, however, in certain geographic areas of the country there are no such mills, therefore the cost to transport the wood from the felling location to the mill might negate all carbon efficiency and definitely all financial incentives. The best way environmentally, is to fell, chip and burn in a localised area. The food industry has food miles, the same principle should be applied to wood chip.
The forested area of the UK is increasing and most felled trees are replaced through restocking or natural regeneration. Old trees are important in storing carbon but it is generally understood that these young plantations absorb more CO2 and produce oxygen at a faster rate. For those of you who have forgotten your science lessons, this is via photosynthesis, which is the process of converting light energy to react water with carbon dioxide to produce sugars which enable tree growth. Oxygen is released into the atmosphere as a by-product. Additionally, in certain urban areas, specific tree species are planted because they are known for their ability to absorb and retain pollutants from the air. The carbon is released at the end of the trees life either through decomposition or burning.
Our need for timber in the UK outweighs the amount we can currently produce commercially but this is not because of our fuel requirement demands. Softwood species can grow quickly in the UK but the resulting timber is often not strong enough to meet the highest structural requirements. We produce 11 million cubic metres but we import 42 million cubic metres, much of which comes from Scandinavia where growing conditions favour higher strength timber.
So, in answer to the question ‘how sustainable are our UK woodlands for fuel consumption’? They are. But they have largely been underutilised and are already supplemented by massive importation of wood and wood products from other countries.
Perhaps the question should be ‘is the requirement for woodchip detrimental to our UK woodlands’? In which case the answer is no. Wood chip is mainly derived from thinnings, small diameter roundwood, as a sawmill co-product or as arboricultural arisings (to quote the technical terms) and will always be produced. Most of the timber currently used for the production of biomass has little commercial value or alternative markets in the regional context.
However, if you are considering installing a biomass boiler, then to ensure the optimum carbon efficiencies, think regional, and buy your wood chip locally.[i]
 Even slow growing species on poor sites will typically be growing at a rate of around 6m³ (1m³ is approximately equal to one tonne) of timber per hectare (100m x 100m) with faster growing softwoods up to 20m³
[i] Thanks to Matthew Allen, Allen Forestry for his input with this article and providing some of the photos.