La détermination du taux de rétention du carbone

Une cause importante de l’effet de serre est du ressort de la quantité de dioxyde de carbone rejetée dans l’atmosphère. La rétention du carbone, un processus selon lequel le carbone entre dans la composition de matières solides, est considérée comme un moyen efficace pour diminuer le taux de concentration de dioxyde de carbone se trouvant dans l’atmosphère. Ce processus a été validé au Sommet de Kyoto, en 1997, à la conférence portant sur le Changement climatique.

Un problème persiste cependant puisque le taux de rétention du carbone des différentes espèces d’arbres n’a pas été déterminé. La durabilité du bois est un attribut qui augmente la valeur de rétention du carbone. Conséquemment, une espèce qui se désagrège rapidement libérera le carbone qu’elle retient avec la même rapidité.

Les arbres en croissance contribuent également à la création de matières organiques dans le sol. Le système biologique de la forêt peut retenir du dioxyde de carbone atmosphérique pendant des périodes dépassant 4000 ans. Par ailleurs, en tant que produit de la forêt, le sol est un contenant important de carbone.

On the setting of value for Carbon sequestering

Ranil Senanayake
Analog Forestry Network
August 1998

c.gif (365 bytes)arbon sequestering, or the locking up of carbon in a solid state, is becoming an increasingly important mechanism to consider present trends in the growth of carbon dioxide in the atmosphere. The increasing rate of carbon dioxide input from human activity has been identified as a possible cause for global concern. Consequences of this increase have been predicted to be changes in global climate and temperature. A major response to reduce the concentration rate is to take the carbon dioxide out of the atmosphere, a process that has been accepted as valid by the International Conference on Climate Change at the Kyoto meeting in 1997. A most cost-effective way to do this is through photosynthesis, through the activity of trees and other plants.

Trees have been widely appreciated as a useful tool in sequestering carbon from the global atmospheric pool of carbon dioxide. However, the value of a forest ecosystem in enhancing this effect, by the differential sequestering value of different tree species, has not been fully appreciated. In a growing forest, the processes of sequestering or incorporating into long term carbon cycles have two distinct pathways. One is by photosynthetic activity, which sequesters atmospheric carbon in living biomass. In this process, the effective rate of sequestering is confined to the life of the individual organism. The other is by respiration activity, which uses the energy fixed by photosynthetic activity, such as in the synthesis of humates. Here, the effective rate of sequestering is dependent on the nature of the respiring ecosystem.

The photosynthetic activity of plants takes carbon dioxide out of the atmosphere and fixes it in a solid state as organic matter. This act of sequestering carbon is what provides forest biomass. Its quality, in terms of sequestering value, has to be measured in time (a fact not recognized in the application of 'Joint Implementation'(JI) projects at present). While all plants sequester carbon, trees and woody plants are most efficient, as they produce resistant compounds such as lignin. Consider the fate of two photosynthetically-derived objects of similar biomass - a large pile of seaweed and a log lying on a beach. Both are plant products, but one (the tree) is strengthened with lignin. The same biological, chemical and physical forces will affect both. The seaweed will disappear within a few weeks, while the log may remain more or less the same for years.

An important attribute of the wood in terms of its sequestering value is its durability. Natural durability is a reflection of the wood's ability to withstand the attacks of decay organisms. Archeological finds often demonstrate wooden construction items dating back about 1000 years. In America a durability standard has been devised by using White Oak as the standard. In this method of evaluation White Oak is given a rating of 100. Wood with higher scores, such as Red Cedar (150-200) or Black Locust (150-250), is more durable. Wood with a lower score, such as Hemlock (35-55) or Birch (35-50), is less durable. The rate at which carbon can be sequestered by a forest is a product of its primary productivity. The rate of production is reported as net annual volume-growth in stemwood (cubic meters/ha/yr). Durability is also measured in terms of its density. Different tree species have timber of different densities, so that a cubic meter of softwood weighs about 0.43 t. and hardwood about 0.63 t.

Thus the current practice of giving carbon sequestering value merely to the production of biomass can be seen to de dangerously simplistic, as it does not recognize the relative value of different types of biomass in sequestering carbon.

In addition to producing photosynthetic products such as wood, a growing tree also contributes to the creation of soil organic matter. As a forest product, soil also has great value as a carbon sink; the process of biochemical distillation of photosynthetic products can keep atmospheric carbon dioxide sequestered by the biological system for periods exceeding 4000 years. About 16 percent of the long-lived fraction identified as 'old carbon' can have lifetimes from 5700 - 15,000 years. The role of soil in sequestering atmospheric carbon dioxide needs to be better recognized. An evaluation of the sequestering potential of various forest ecosystems suggests that forest soils contain a large proportion of the carbon pool. These long lived compounds are a product of the bio-chemical distillation of photosynthetic products and tie up about 20-30% of the organic matter reaching the soil from the above ground environment.

Any design that incorporates carbon sequestering as a goal will also tend towards long rotation tree crops. This is due to the fact that in long rotation species the active sequestering or growing phase is longer, and the total biomass is larger. Such design will allow many species of trees determined to be 'marginal' to be brought into culture.

Thus, for the purpose of sequestering carbon the most productive forests are those that have a long standing life as well as a high potential to develop deep organic soils. Commercial monocultures have a disadvantage in this respect as they are harvested for timber after a set period of time and develop deep organic soils very rarely. A better model is provided by a polyculture with long rotation times, such as that seen in some forms of traditional forestry where a high diversity of tree species with a good development of organic soil has been recorded. Further, as the trees used in this approach to forestry are crop species which produce large crops as the trees mature, there is a disincentive to fell the trees unless they are diseased or very old. The development of this type of forestry in some temperate and tropical regions can provide a very efficient method of sequestering carbon, which also provides social, ecological and economic benefits.

A very unfortunate mistake has been made by many in evaluating relative values of carbon sinks, that the source of carbon is unimportant. What is not recognized in the equation is the cycling time of the source of carbon. There are two major cycling systems to be considered; the biological and geological. Within the biological system, carbon is taken up in many ways though photosynthesis, or carbonate cycles each having internal cycling processes that operate at different rates. Although having differential value in terms of carbon sequestering times, these cycles are essentially those involving the standing biomass of the planet. Even when the relatively slow carbonate cycles are considered, the cycling biotic carbon operates on time frames of tens of thousands of years.

The second cycling pool of carbon is found as fossil carbon and has cycling times that are measured in millions of years. These are carbon compounds that have been fixed millions of years ago but have been removed from the biotic pool by being incorporated into the geologic cycles of the planet.

To allow this fossil carbon to enter the biotic cycle is the fundamental reason as to why there is the accelerating greenhouse effect. If the real cost of the injection of fossil carbon into the biological cycle has not been calculated, the growing of trees to compensate for the burning of fossil carbon is tantamount to 'carbon laundering'. There is no way to compare the carbon from oil and coal with the carbon from a forest. One has a space in the biotic cycle the other does not.

It must be borne in mind that the pool of carbon represented as the biotic pool is a finite cycling quantity. It is only carbon that has been naturally cycling that can be accommodated under JI. For instance, the cutting and burning of a forest or oxidation of a peat bog will add to the atmospheric pool, and this volume of carbon can be sequestrated somewhere else. All increments to the biotic pool should have a real cost attached to it. The non- biotic pool carbon is that which has been extracted from fossil sources that normally does not enter the biotic pool (fig ) and its price should reflect this relationship. This is the domain of the other part of the climate convention which deals with taxes and emission reduction strategies. As the output of Carbon Dioxide is generated by the demand for growth, not only fossil carbon uses but fossil carbon miners should be called upon to pay the real cost of burning fossil carbon and adding the resultant carbon dioxide into the biotic pool.

The current debacle in the evaluation of carbon can be traced to the work of some modern modellers. For instance, the new work by Faeth, et al. (1994) raises some very important issues in the present move towards raising the economic credibility of Carbon sequestration and validation of proposals for Joint Implementation (JI). A detailed description of projects in six different countries is presented in this study. However, the assumptions made in developing the model leave much to be desired, bringing into question the ability of such model to effectively address the problem of gaseous carbon control. For example, this work misses a crucial element in evaluating the dynamics of the carbon cycle discussed above, sources and sinks. Sources are those carbon compounds that decay into carbon dioxide and sinks are carbon compounds made from carbon dioxide. However in the design of their model Faeth et al. state: "somewhat unfortunately, modelling jargon defines a source or sink as outside the boundary of interest. For example, we don't care where people come from before they were born. In the issue of global warming, sources and sinks have a different meaning and are critically important. In this analysis, few sources or sinks are indicated because land can neither be created or destroyed in the time frame of interest."

This represents a fundamental flaw in the model. If the concept of sources and sinks are critically important, why does 'modelling jargon' ignore it? Further, the temporal state of the land area is inconsequential; it is the temporal state of the sequestered carbon compounds on that land area that is important. What is important is the volume of sequestered carbon gained upon it and the time that it can be maintained as fixed carbon. This type of incomplete modelling has allowed the phenomena of 'Carbon laundering' to arise as a response to JI.

The current rush to utilize the global decision to limit carbon output through the planting of short rotation tree crops will have to be critically examined. All payments made on JI projects to date assist the fixing of a base price for sequestered carbon. However, this price will now need to be negotiated in the light of the knowledge of residence times and the source of the carbon to be fixed under joint implementation or any other incentive scheme.

Evaluating the Carbon Sequestration Benefits of Forestry Projects in Developing Countries by Paul Faeth, Cheryl Cort and Robert Livernash, World Resources Institute/ Environmental Protection Agency 1994. 96pp