Seeing the wood for the trees: Forest bioenergy and greenhouse gas emissions

Words by Carly Whittaker, Rothamsted Research

There are a number of key points of concern that are raised by Chatham House that surround the calculation of net GHG emissions from the combustion of bioenergy from forest resources:

  • They state that the GHG emissions from providing a unit of energy from solid biomass are higher than those from the equivalent energy from fossil fuels.
  • They suggest that the demand for bioenergy will cause damage to future carbon stock sequestration potential of forests by changing shorter lengths, or higher extraction of wood or residues, leading to a loss in the carbon stock of forests. These changes may not be accounted for if exporting countries do not report under the second commitment period of the Kyoto protocol.

This article is an attempt to find common ground with these concerns and provide some context to the arguments. Please note that the ‘forests’ mentioned here refer to are existing plantation forests, rather than converting old-growth forests to plantations. It is widely acknowledged that old growth forests, and all forests, are highly valuable carbon stocks. If they are managed badly then significant losses of carbon can occur. This does not rule out all forest biomass as an important and abundant bioenergy resource that can make significant contributions towards mitigating climate change.

The ‘debt’ is in the detail

An issue which is central to the carbon sustainability of bioenergy is the difference between the emissions of carbon at the point of combustion and the removal of atmospheric carbon through the past growth and assumed future growth associated with the supply chain for the feedstock [2]. Biomass combustion is certainly not carbon neutral. It is also true that the GHG emissions from burning biomass to provide a unit of energy are higher than is it for fossil fuels- however these facts are often used against biomass but are taken entirely out of context.

When harvesting operations take place in a forest, changes occurs both in terms of a loss of stored carbon in the standing trees, and from disturbance of soil [3]. As the Chatham House report states, it takes around 10-20 years for soil carbon to ‘recover’ from the disturbance in the following forest rotation. The replenishment of the stock of timber in trees depends on the rotation of that particular forest, and the following rotation. In plantation forestry this can range between 30 and 150 years, though it is worth noting that for the longer (mainly hardwood) rotation forests the majority of stem wood would be highly sought after for structural timber. It is important to note that, upon harvesting, the carbon contained in the standing trees is converted to a combination of timber products (sawn timber, pulp, even chemicals) and harvesting residues [4]. The key message here is that ‘biomass’ for energy is not the sole product of forest systems, and when examining the full life cycle assessment of bioenergy systems, the role of carbon storage in timber products is important and long-lasting. This is even more so considering modern high rates of wood recovery, reuse and recycling. Timber harvested today can have a long life as structural timber (60 years) which can then be reincarnated as chipboard or panel board and last another 50 years [3]. When accounting for this stored carbon, the ‘emissions per GJ’ statement is obviously taken out of context.

Can’t see the wood for the trees

In practice, forests consist of multiple stands that are not all felled at the same time [3]. It is broadly acknowledged that analysing carbon flows on a single-stand level is restrictive and not suitable for policy recommendations  [4]. Taking a spatial rather than single-stand approach to the calculations has large implications on the estimated ‘carbon payback’ period [5,6,7]. At the scale of a whole forest or landscape, losses of carbon stocks due to harvesting may be counterbalanced by sequestration in the remaining stands which are still growing [8]. This is where stored carbon in the landscape also becomes an important sink of carbon, despite when harvesting for timber or fuel is occurring in one compartment of the forest. This factor again contributes towards the net GHG emission balance of the forest, even if the combustion of biomass occurs.

Changing stocks

Remember that biomass is not the only product from forests; and fibre price is the most significant cost in pellet production, therefore it can only afford the lowest value forest products [9]. There is a concern, however, that increased demand for this lower value material will see more of it leaving the forest that would otherwise have occurred. If this leads to increased thinning rates, or shorter rotations then there will be a net loss in carbon stock in standing trees, litter and soil. This is when bioenergy strays into the ‘carbon debt’ zone, as this stock has to be compensated for by some other activity, such as afforestation. This needs to be considered and is important, especially considering that this practice all falls under the banner of ‘sustainable forest management’ but can still lead to lower carbon stocks. But is this actually happening? This could become a complicated story, as the main reason people thin is to improve the remaining trees, not necessarily because they want to sell the smaller stem material. So is it realistic to assume that bioenergy prices alone will increase thinning rates? Evidence shows that the financial return from pellets alone is not sufficient to drive increased harvests either in Canada or Southeast USA [9], as there is general insensitivity in lower value pulpwood due to changes in value. In fact, an expanding wood pellet industry could potentially encourage more positive management of forests, by providing a market for the lower value materials (Dale et al., in press). 

Conclusion

Despite conventional thought, money does indeed grow on trees. There is a general acceptance across the forest industry that where economic value can be derived from the forest, the forest will be more likely to be preserved as forested land. The Chatham House report does not present any new concepts or challenge to the biomass industry. It is accepted that the sector must be able to demonstrate that it is delivering GHG reductions in a sustainable way [2], though many of the methods employed assume carbon neutrality. Mechanisms for detecting changes in production, or accounting for changes in forest carbon stock still need to be refined, even if the evidence suggest that this is not an issue at present. Thankfully, from areas like North America and Europe we have sufficient data in which to base our analyses and observe changes, therefore a policy recommendation would be to focus on biomass originating from countries with less transparency on details regarding land use and forest management. Information gathering from industry and landowners will be key to understanding where decisions about forest removals are made. It is important to note that, on the academic-side, studies generally agree that bioenergy can play an important role in climate change mitigation and there is a risk of failing to meet long-term goals without it.


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