Carbon sequestration through forestry is based on two premises. First, carbon dioxide is an atmospheric gas that circulates globally and, consequently, efforts to remove greenhouse gases from the atmosphere will be equally effective whether they are based next door to the source or on the other side of the globe. Second, green plants take carbon dioxide gas out of the atmosphere in the process of photosynthesis and use it to make sugars and other organic compounds used for growth and metabolism. Long-lived woody plants store carbon in wood and other tissues until they die and decompose at which time the carbon in their wood may be released to the atmosphere as carbon dioxide, carbon monoxide, or methane, or it may be incorporated into the soil as organic matter.
Plant tissues vary in their carbon content. Stems and fruits have more carbon per gram dry weight than do leaves, but because plants generally have some carbon-rich tissues and some carbon-poor tissues, an average concentration of 45-50 percent carbon is generally accepted (Chan 1982). Therefore, the amount of carbon stored in trees in a forest can be calculated if the amount of biomass or living plant tissue in the forest is known and a conversion factor is applied.
Carbon fixation through forestry is a function of biomass accumulation and storage. Therefore, any activity or management practice that changes the biomass in an area has an effect on its capacity to store or sequester carbon. A variety of forest management practices can be used to reduce the accumulation of greenhouse gases in the atmosphere, through different approaches. One is by actively increasing the amount or rate of accumulation of carbon (i.e., “sink” creation or enhancement). The second is by preventing or reducing the rate of release of carbon already fixed in an existing carbon “pool”. For forest plantations the first mechanism is important.
New tree planting results in the creation of new carbon sinks, i.e., carbon fixation during tree growth in afforestation, reforestation, forest rehabilitation, or agroforestry schemes. In the context of the Kyoto Protocol, these activities conform to the concept of Article 3.3. Although carbon sequestration is often discussed in the context of the establishment of new forests, carbon fixation can also be achieved by improving the growth rates of existing forests. This can be achieved through silvicultural treatments such as thinning, liberation treatments, weeding or fertilization. Since substantial amounts of carbon are stored in soils management practices that promote an increase in soil organic matter can also have a positive effect. These activities fit into the spirit of Article 3.4 of the Protocol.
When considering carbon storage, not all forests are equal. Generally, longer-lived trees with high density wood store more carbon per volume than short-lived, low density, fast-growing trees. However, this does not mean that carbon offsets which involve big, slow-growing trees are necessarily better than those involving plantations of fast-growing trees and vice versa (Moura-Costa 1996a and b).
