All plants fix carbon dioxide during photosynthesis, which is released again when the plant dies and the plant material is degraded by microorganisms. The carbon dioxide fixed is used to synthesize storage compounds such as starch and oils, and cellular struc­tural components such as cellulose and lignin. It is the structural components that are the slowest to degrade when the plant dies and these are the highest in woody plants.

It has been estimated that the amount of carbon taken up by vegetation was 3.2 Gt C/year, and 1.7 Gt C/year is lost mainly through deforestation, which gives a net increase of 1.5 Gt C/year. The carbon emissions from fossil fuels are 6.4 Gt C/year so that without any mitigation measures, some 23% of the carbon dioxide is removed. However, afforestation does not remove all the carbon dioxide produced and the atmospheric levels are still rising, but it does show the potential of biomass to seques­ter carbon dioxide. Table 3.7 indicates the potential of carbon dioxide sequestration by planting new forests, managing existing forests, managing crops, etc.

Woody plants have a carbon content of 0.54 kg carbon per kilogram of dry wood (Cook and Beyea, 2000). If tree growth is linear in the early years, the carbon dioxide removed can be calculated. The estimates for carbon dioxide sequestered by maize, switchgrass, short-rotation coppice willow and standing forest wood are shown in Table 3.8. Some of the carbon dioxide is only sequestered on a temporary basis as the switchgrass and short-rotation coppice will be burnt as a fuel, and other parts of the crops will be returned to the soil where they will be degraded. However, if the wood is used as construction material, the carbon dioxide will be locked up for considerably

longer. The carbon dioxide sequestered in forest material is less because growth is slower and forest regeneration, once harvested, is not always certain. The world is losing areas of dense forests either to building or agriculture, and reversal of this trend would help to reduce carbon dioxide levels in the short term.