Science in Society Archive

Old Growth Forests Are Carbon Sinks and Must Be Protected

A forest levy and other financial mechanisms within the Kyoto Protocol are appropriate measures for protecting natural forests. Dr. Mae-Wan Ho

General misconception leaves old growth forests especially vulnerable to wonton destruction

Old-growth forests sequester carbon in live woody tissues and in slowly decomposing litter on the ground and buried in the soil, thereby acting as effective global carbon sinks. Yet, they are not protected by international treaties because of general misconception that ageing forests cease to accumulate carbon.

An international team of scientists led by Sebastian Luyssaert at the University of Antwerp in Belgium reviewed the literature and existing databases for forests between 15 and 800 years old. They found that all forests, from the youngest to the oldest, show positive net ecosystem productivity; i.e., they are growing and hence sequestering or accumulating carbon [1].

Over 30 percent of the global forest area is unmanaged primary forest and this area contains the remaining old growth forests. Half of the primary forests (covering 600 million ha) are located in boreal (near the Arctic) and temperate regions in the Northern Hemisphere. These forests alone sequester about 1.3 + 0.5 Gt of C per year. This 15 percent of the global forest area currently not considered when offsetting increasing atmospheric CO2, provides at least 10 percent of the global net ecosystem productivity. Old-growth forests accumulate carbon for centuries and contain large quantities that will move back into the atmosphere if these forests are disturbed or cut down, especially tropical forests [2, 3] (Saving and Restoring Forests Saves Far More Carbon Emissions than Biofuels, SiS 37) .

Net ecosystem productivity and age of forests

Net ecosystem productivity (NEP) is the net carbon balance of the forest ecosystem as a whole, and is the difference between CO2 uptake by photosynthetic assimilation and losses through plant and soil respiration. Tropical forests were excluded from the study because of the lack of data, and only 12 sites were found for which NEP and age estimates are available.

NEP estimates suggest that forests 200 years old and above sequester on average 2.4+0.8 tC/ha/y: 0.4 +0l1 t in stem biomass and 0.7+0.2 t in coarse woody debris, which imply that about 1.3+0.8 t are in the roots and soil organic matter. Thus, forests 200 years and older sequester 8.8 t CO2/ha/y on average. (To get from C to CO2, simply multiply by 44/12, 44 being the molecular weight of CO2 and 12, the atomic weight of carbon)

The commonly accepted and long-standing view that old growth forests are “carbon neutral”, i.e., photosynthesis is balanced by respiration, was based on a ten-year study from a single site. It is supported by the observed decline of net primary productivity (NPP, counting only the stand of trees) with age in plantations, but is not apparent at least in some eco-regions. Yet a decline in NPP is commonly assumed in ecosystem models. If the hypothesis is correct, then the expected probability of finding a sink or a source would be equal for a random ensemble of forests 200 years old. But the actual probability observed is well below one. The probability of finding a single forest to be a source of carbon at 60, 180 and 300 years of age is 0.2, 0.25, and 0.35 respectively, while the probability of finding an ensemble of ten old-growth forests that are carbon neutral is negligible, or nearly zero.

In fact, young forests rather than old-growth forests are very often conspicuous sources of CO2, because the creation of new forests frequently follows disturbance to soil and the previous vegetation, resulting in enhanced decomposition rate of coarse woody debris, litter and soil organic matter (measured as respiration) that exceeds the NPP of the regrowth.

Self-thinning theory predicts old growth forests remain carbon sinks

There is some decline in NPP of forest trees beyond 80 years, partly due to the presence or absence of management (as well as mode of management), and a reflection of differences in disturbance history between managed and unmanaged forests. Consistent with earlier studies, biomass continues to increase for centuries irrespective of whether forests are boreal or temperate. Biomass cannot accumulate forever. The data suggest a possible upper limit somewhere between 500 and 700 tC/ha or 1833 and 2567 t CO2/ha, the equivalent of 1 400 to 1 800 cubic metres of wood per hectare. These high-biomass forests were located in the Pacific Northwest USA.

When high above-ground biomass is reached, individual trees are lost because of lightning, insects, fungal attacks of heartwood by wood-decomposers, or trees becoming unstable in strong wind because the roots can no longer anchor them. If old growth forests lose individuals, there is generally new recruitment of an abundant second canopy layer waiting in the shade of the upper canopy to take over and maintain productivity. That is why a management regime that selectively cuts the oldest/biggest trees results in the greatest productivity [4] (Multiple Uses of Forests, SiS 26).

Tree mortality is rapid, but decomposition of deadwood can take decades, and natural regeneration occurs on a much shorter timescale. This ‘self-thinning’ theory predicts that the ratio between ecosystem respiration (releasing CO2) and NPP (fixing CO2) is constant and around 0.65+0.02, indicating that even old forests remain a carbon sink. There is also an inverse relationship between biomass and tree density.

Kyoto Protocol needs to take account of old growth forests particularly in the topics

Under the Kyoto Protocol, only anthropogenic effects on ecosystems are considered and the accounting for changes in carbon stock by afforestation, reforestation and deforestations is mandatory, operating from a base line of 1990. Leaving forests intact was not perceived as an anthropogenic activity, even though as pointed out recently in the Stern report on climate change [5, 6] (The Economics of Climate Change, SiS 33), the most effective measure for mitigating climate change is to halt deforestation. Old-growth forests are excluded from national carbon budgets and from the Kyoto protocol based on the now discredited hypothesis that they are carbon neutral. Over 90 percent (1.3 billion ha) of global forests area is classified by the FAO as primary forest, and this area contains the world’s remaining old growth forests.

Although the new finding that old-growth forests remain carbon sinks is based on data from boreal and temperate forests, it applies all the more so to tropical forests in south and southeast Asia, and in Latin America where demand for ‘bioenergy’ crops has accelerated deforestation in recent years. In Brazil alone, more than 15 million hectares of rainforests were cleared between June 2000 and June 2008 [7]. Cattle-ranching remains the major driver of deforestation, but soybean cultivation has expanded rapidly due to improved infrastructure in the region and rising demand for vegetable oils for food, industrial uses and biodiesel production. Since 1990, soybean cultivation in the Amazonian states has expanded at the rate of 14.1 percent per year, accelerating to 16.8 percent annually since 2000, and now covers more than 8 million hectares. While the Amazon’s largest soy crushers have had a ban on soy produced from newly deforested lands since July 2006, soy remains an indirect driver of deforestation. In areas suitable for mechanized soy cultivation, rainforests are typically cleared for cattle-ranching, then sold to soy producers two to three years later. Ranchers then move on to deforest further areas.

Measures to halt deforestation urgently needed

In view of the new evidence that old growth forests remain effective carbon sinks and deforestation results in huge carbon emissions particularly in the tropics (about 200 t CO2/ha/y [2, 3], and in recognition of the vital role of tropical rainforests in stabilising climate [8] (Why Gaia Needs Rainforests. SiS 20), measures to halt deforestation are urgently needed.

In conjunction with an immediate international moratorium on deforestation enforced by sufficiently punitive prosecution and fines, some mechanism should be created within the Kyoto Protocol to compensate for the crucial ecosystem services of tropical forests, such as a levy imposed on all nations to be paid to the forest states as financial incentive against deforestation. At the same time, a further mechanism should be created within the controversial World Bank Climate Funds [9] (World Bank Climate Funds May Undermine ClimateTalks, SiS 38) , for example, to convert existing monoculture plantations back to forests, which would sequester an estimated 180 t CO2/ha/y {2, 3], or to sustainable agroforestry [10] (Sustainable Multi-cultures for Asia & Europe, SiS 26) that still offers far more ecosystem services than monoculture plantations [4], and would discourage deforestation by small landless farmers [7].

Article first published 20/10/08


  1. Luyssaert S, Schulze E-D, Bőrner A, Knohl A, Hessenmőller D, Law BE, Ciais P and Grace J. Old-growth forests as global carbon sinks. Nature 2008, 455, 213-5.
  2. Righelato R and Spracklen DV. Carbon mitigation by biofuels or by saving and restoring forests? Science 2007, 317, 902
  3. Ho MW. Saving and restoring forests saves far more carbon emissions than biofuels. Science in Society 37, 17, 2008.
  4. Ho MW. Multiple uses of forests. Science in Society 26, 18-19, 2005.
  5. Stern Review on the Economics of Climate Change, HM Treasury, 2007,
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  7. “Future threats to the Amazon rainforests”, Rhett A. Butler, 31 July 2008,,
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