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.
