Kirschbaum, M.U.F. (2001). The role of forests in the global carbon cycle. In: Criteria And Indicators For Sustainable Forest Management (Raison, R.J., Brown, AG. and Flinn, D.W., eds.), IUFRO 7 Research Series, CAB International Publishing, Wallingford, UK, pp. 311-339.
Abstract. Forests play an important role in the global carbon cycle, with estimated carbon stocks in forests (990 GtC) being of comparable magnitude to that contained in the atmosphere (750 GtC). Of the carbon contained in forests, about 2/3 reside in soil organic matter and only 1/3 in vegetation. In the tropical region, 13 Mha of forests are converted to other land uses every year, leading to an estimated carbon release of 1.6 GtC yr‑1. At the same time, forest cover is estimated to be increasing at higher latitudes by about 1.2 Mha yr-1, thereby sequestering 0.7 GtC yr-1. Forest harvesting for fuelwood and wood and paper products is estimated to lead to an annual release of about 1 GtC yr‑1, with about half of that used for fuelwood. After allowing for losses during production and decay of previously produced wood products, it is estimated that the global pool of carbon in wood and paper products is increasing by about 140 MtC yr-1.
Forests can be managed to maximise carbon stocks by preserving existing forest cover, especially where forests have a high standing biomass, and by establishing new forests on currently non-forested land. It is also possible to modify forest management in order to maximise site carbon storage. However, there is a limit to the potential contribution that on-site carbon storage can make to global carbon cycles because the stocks of carbon stored on any area of land are obviously finite. However, forests can make an on-going and sustainable contribution to reducing net CO2 emissions through the substitution of wood for fossil fuels, either in energy generation, or by substituting wood for other materials that require larger CO2 emissions in their manufacture. Bio-energy currently supplies about 14% of the world’s primary energy needs, and it has been estimated that bio-energy has as much potential to contribute to the world’s energy supply in the near future (by 2025) as all other forms of renewable energy combined.
While these pools and fluxes constitute significant components of the global carbon cycle, there are no easily measurable direct indicators by which to judge whether specific forests are net sources or sinks of carbon. Net carbon flux is essentially determined by the difference between carbon uptake in growth and losses in harvesting, fire and decomposition of dead organic matter. Precise measurements of forest growth are difficult because changes are typically only small percentages of standing biomass, because growth is greatly affected by year to year variability in climate and biotic factors, and because many forests are highly heterogeneous. Measurements are even more difficult for the assessment of changes in soil organic matter. Even when trends in carbon stocks have been established, it remains uncertain whether they reflect typical conditions, and whether these trends will persist with variation in climatic and disturbance regimes. To assess whether there are changes in any region’s carbon stocks, it is, therefore, warranted to use a combination of methods, including growth modelling, remote sensing, ground-based measurements, including harvesting statistics, and atmospheric trace gas studies.