Carbon forestry

Carbon may be stored or sequestered in the key components of forest ecosystems and their associated carbon pools. For simplicity, these pools may be defined as above- and below-ground living biomass, dead biomass, soil carbon and the carbon associated with the wood products pool in its many forms. However, the potential for forestry activities to reduce or offset emissions of GHGs is not simply the sum of the impacts on these carbon pools. There is another, often potentially very important impact on the carbon balance due to forestry activities arising from opportunities to use wood products to reduce the consumption of fossil fuels.

Forest carbon management

Sustainably harvested wood can substitute directly for fossil fuels in the form of renewable wood fuel (or ‘bioenergy’), or indirectly as renewable wood products replacing, where appropriate, materials such as concrete and steel which involve high fossil fuel consumption in their production. As a consequence of this important dimension to the carbon balance of forestry systems, maximising the carbon benefits of an area of woodland is not always a simple question of leaving the trees to grow and maximising the on-site carbon stock. Forest carbon management requires an in-depth assessment of numerous factors including site conditions, potential productivity, vulnerability to natural events, proximity to point of use and the local practicalities of the best and most realistic options for end-use of harvested wood. Most of these considerations go beyond the commercial, social and environmental issues which currently form the basis of conventional forest management plans in the UK.

A number of options are available for maintaining or enhancing the carbon benefits of forests based on widely differing forest management systems and objectives. An evaluation of these options for a particular stand, forest or region can be based on a common set of principles, most importantly:

Maintenance or enhancement of long-term on-site carbon stocks in the woodland itself
Minimisation of disturbance to litter and soil, to avoid carbon emissions and soil degradation
Ensuring energy efficiency in woodland management operations and in the conversion of harvested trees to products
Matching timber and woodfuel production to possible end-uses to achieve optimal utilisation of wood in reducing fossil fuel consumption
Harmonising carbon management with other objectives and with practical constraints.

Options for woodland management

Three contrasting options for woodland carbon management, based on the principles described in the previous section, and an indication is given of their relative merits and potentials. These options are not prescriptive or an exhaustive list, but do indicate the general range of approaches that are available for forest carbon management. This section also shows how the above principles may be used to evaluate management options in terms of woodland carbon

Carbon reserve management

This option is characterised by minimal intervention, with a gradual long-term increase in carbon stocks. In addition to a climate change mitigation role, carbon reserve management may also have significant amenity and biodiversity benefits, particularly if native species are planted. Loss of carbon through fire, drought, floods or storm damage is minimised. For this reason it is necessary to take account of wind-hazard, flood risk and climate change predictions regarding the suitability of a particular site-species combination to achieve this objective. At the same time, the use of fossil fuel during woodland management operations should be minimised so as not to negate any enhancement of carbon stocks. Carbon reserve management is particularly well suited to forest stands with very low growth rates and poor stem quality, or in localities where there are limited opportunities for utilisation of harvested wood. The most extreme example of carbon reserve management might involve conservation of existing forest carbon stocks through avoidance of deforestation.

Carbon substitution management

This is characterised by cyclical changes in carbon density in the forest ecosystem, with maintenance of on-site carbon stocks being of secondary importance. This form of carbon management and its objectives are not far removed from the production forestry practised across most of the UK forest estate. Woody biomass is harvested as good quality stemwood for use in product displacement and renewable wood fuel (in the case of thinnings and harvest residues). Soil disturbance following thinning or clearfell is minimised to limit carbon emissions. Because of the high nutrient content of a significant proportion of harvest residues compared with timber, often only a carefully selected fraction of this material is harvested, primarily avoiding the removal of foliage. As with carbon reserve management, there may be benefits associated with the woodland in addition to its climate change mitigation role. Carbon substitution (or displacement) management is particularly well suited to even-aged forest stands with moderate to high growth rates in localities with obvious opportunities for utilisation of harvested wood. Short rotation coppice managed for bioenergy production represents the ultimate expression of carbon substitution management. Stem quality may also require consideration when options are being evaluated, because it will have an impact on the potential to convert stemwood into different products.

Selective intervention carbon management

This is similar to carbon reserve management but, in addition, there is low-level harvesting of certain trees to clearly defined specifications in order to supply high-value niche applications. It is well suited to stands containing trees of variable quality where risk of significant natural disturbance is low and which may be some distance from centres of population or industry. Examples of this type of management include occasional tree harvests in stands to meet a requirement for fuelwood in a small local community and selective felling in continuous cover forestry systems to satisfy specialist timber markets.

As an general guide, selective intervention and carbon reserve management will generally result in higher long-term carbon stocks within a given woodland ecosystem. On the other hand, only substitution and, to a lesser extent, selective intervention carbon management have the potential to lead to an ongoing increase in the carbon stocks of the forestry system beyond the one-off increase in the carbon pools associated with new woodland.