Carbon sequestration

 

Forest ecosystems make an important contribution to the global carbon budget. This is because of their potential to sequester carbon in wood and soil but also because of their potential to release it if forests are cleared. Many countries and organisations, including the UK Government and the Forestry Commission, are cautious about promoting carbon sequestration as a means of reducing atmospheric carbon dioxide. The size of the potential gains is uncertain and the accounting procedures complicated. Moreover, there is a limit to the amount of carbon that woodland can sequester, and there is a risk that the sequestered could be released – through, for example, felling, forest fires or outbreaks of pests and diseases.

Forests and woodlands in the UK contain around 150 million tonnes of carbon, and every year they remove about 4 million tonnes of carbon from the atmosphere. These values need to be compared with total UK emissions of around 150 million tonnes of carbon (as carbon dioxide) every year – mainly due to the combustion of fossil fuels. So the forest carbon sink is offsetting less than 3% of annual carbon dioxide emissions and the accumulated carbon stock in forests represents only about one year’s worth of emissions at current rates. The rate of carbon sequestration (4 million tonnes per year) is relatively high because most of the UK’s forests are young and still growing. As our forests grow older, the rate of carbon dioxide removal will fall. However, carbon sequestered in UK forests is still an add-on to the many other benefits that can arise from forestry as long as the forestry authorities maintain a clear commitment to sustainable forest management.

How can trees and forests act as a carbon sink?

The term ‘sink’ is used to mean any process, activity or mechanism that removes a greenhouse gas from the atmosphere. Forests and other green vegetation exchange large amounts of greenhouse gases with the atmosphere. Plants capture carbon dioxide from the atmosphere through photosynthesis, releasing oxygen and part of the carbon dioxide through respiration, and retaining a reservoir of carbon in organic matter. If stocks of carbon are increased by afforestation or reforestation, or carbon stocks in croplands or forest stands are increased through changes in management practices, then additional carbon dioxide is removed from the atmosphere. For example, if an area of arable or pasture land is converted to forest, additional carbon dioxide will be removed from the atmosphere and stored in the tree biomass. The carbon stock on that land increases, creating a carbon sink. However, the newly created forest is a carbon sink only while the carbon stock continues to increase. Eventually an upper limit is reached where losses through respiration, death and disturbances such as fire, storms, pests or diseases or due to harvesting and other forestry operations equal the carbon gain from photosynthesis. Harvested wood is converted into wood products and this stock of carbon will also increase (act as a sink) until the decay and destruction of old products matches the addition of new products. Thus a forest and the products derived from it have a finite capacity to remove carbon dioxide from the atmosphere, and do not act as a perpetual carbon sink. By substituting for fossil fuels, however, land used for biomass and bioenergy production can potentially continue to provide emissions reductions indefinitely.

If a forest area is harvested and not replanted, or is permanently lost due to natural events like fire or disease, then the carbon reservoir that has been created is lost. In contrast, the benefits provided by bioenergy substituting for fossil fuels are irreversible, even if the bioenergy scheme only operates for a fixed period. Frequently a distinction is made concerning the so-called ‘permanence’ of measures based either on carbon sinks or on replacement of fossil fuel with bioenergy.

Does tree harvesting cancel out the carbon sink?

Forests and woodlands managed for commercial wood production through periodic harvesting generally have lower carbon stocks than stands that are not harvested, but this harvesting should not be confused with deforestation. Deforestation implies a change in land cover from forest to non-forest land, whereas sustainable wood production involves cyclical harvesting and growing. A newly created forest managed for wood production can act as a carbon sink just as surely as a newly created forest reserve, although there may be differences in the level of the ultimate carbon stock and the time horizon over which it is attained.