The Forest Carbon Stocks

Globally, forests cover about 4 billion ha or 31 percent of the world’s land surface (compared to a pre-industrial area of 5.9 billion ha). Most forests occur in the tropics and in large areas of the northern hemisphere in Canada, the US, Europe, Siberia, and China. A recent global survey has estimated that there are 3.04 trillion trees with a diameter of more than 10 cm at breast height or the equivalent of 420 trees for every person on the planet.

On a global scale, tropical forests contain the largest carbon stock (547.8 million tons in tropical and subtropical forests). There are also differences within tropical areas, with mangrove forests and swamp forests containing particularly high levels of biomass2 in their vegetation cover and soils. Forests also provide subsistence and income for more than 1.6 billion people, including approximately 60 million indigenous people. Those who rely on forests for their livelihoods are among the poorest people on the planet, and they are disproportionately female.

As forests contain substantial stores of carbon, their degradation and or conversion to other land cover causes the release of some of the carbon stored within them. Forest degradation can be defined as human activities that reduce the carbon stocks and other ecosystem functions of a forest, but that fall short of deforestation, for example, selective logging. The level of emissions depends on the amount of carbon originally stored in the forest, the extent to which the vegetation cover and soil structure are damaged or destroyed, as well as what happens to the land afterward. Particularly high emissions will result if the vegetation is completely destroyed and then the area is burned afterward, as is carried out during slash and burn agriculture in some parts of the developing world.

The extent of forest destruction is very high in some areas. For example, a recently published study on deforestation in Borneo shows that deforestation has reduced the once high forest cover on Borneo (75.7 percent) by one third. The highest deforestation rates are observed in tropical rain forest regions. The US and Europe have reversed the trend and are now increasing their forest cover. This highlights an important issue, that although the destruction of forests causes the release of carbon dioxide, their restoration can act as a sink for atmospheric carbon. As mentioned previously, the net contribution of land-use change to global emissions is about 10 percent of the total (0.9 gigatons of carbon (GtC3) per year), which is the contribution calculated by combining both emissions due to deforestation and the sequestration of carbon due to forest recovery.

Forests are not only potential sources of carbon emissions to the atmosphere; they can also act as carbon sinks, sequestering carbon. Forests sequester carbon both as they grow when they are being restored and as part of the terrestrial carbon sink. More than 2 billion worldwide may offer opportunities for restoration. In areas that were deforested but are not currently densely populated or cultivated it may be possible to undertake some form of restoration, ranging from complete reforestation of closed canopy cover to more mosaic restoration that includes restored forest areas interspersed with other land uses including agroforestry, small scale agriculture, and settlements. Such restoration sequesters carbon, with the level of sequestration depending on the extent of recovery of plant biomass and soil carbon.

The links between forests and the carbon cycle mean that actions that affect the forest sector can have a large impact on greenhouse gas emissions and so on climate change. The total amount of CO2 entering the atmosphere can be reduced by decreasing emissions from both deforestation and forest degradation. Maintaining standing forests can preserve their role as a terrestrial carbon sink and restoring forests can increase the sequestration of carbon thereby decreasing the overall levels of CO2 in the atmosphere. If all deforestation and forest degradation were halted, and the whole area suitable for ‘wide-scale restoration’ restored, emissions could be reduced by an estimated 9 gigatons of CO2 per year by 2030. How much of this potential is realized depends on national goals and policies, economic factors, and socio-cultural and institutional barriers that slow the speed of change.