Cycling: Trees and Carbon
Sequestering in Trees
fossil fuels as an energy source, we are taking stored carbon
and putting it back into the atmosphere at a rate that is greater
than it is being taken out. This means that the amount
of carbon dioxide in the atmosphere is increasing, and will
continue to do so until the difference in these two rates disappears.
One way to bring this about would be to greatly curtail the
rate at which burn fossil fuels. Many people do not like
this idea, as it would mean a significant change in our current
lifestyle. Another proposed method would be to speed up
the rate at which carbon is removed from the atmosphere.
One way of doing this would be to plant more trees.
trees convert carbon dioxide and water into sugar molecules
and oxygen through a series of oxidation and reduction reactions.
The overall equation for the photosynthetic process may be
+ 6 H2O + sunlight ---> C6H12O6
+ 6 O2
Some of this sugar is
stored, while most of it gets used by the tree for other purposes
such as energy and structure. For instance, a great
deal of the sugar is linked together to form cellulose which
provides the structure for the tree.
If we look at this sugar from
a mass standpoint, we see that a large fraction of it is due to
the carbon. The fact that carbon has an atomic mass of 12,
hydrogen has an atomic mass of 1, and oxygen has an atomic mass
of 16 means that 72/180 = 40% of the mass of the sugar molecule
comes from carbon. Taking into account the other types of
molecules that are found in a tree (proteins, lipids, etc.), we
find that about 45% of the dry mass (not including the water) of
a tree comes from carbon. In other words, a 100 kilogram log
of a tree that has been completely dried contains about 45 kilograms
of stored carbon.
While each kilogram of dried
tree is storing .45 kilograms of carbon, it is removing more than
a kilogram of carbon dioxide from the atmosphere. This is
because each carbon dioxide molecule contains two oxygen atoms.
Using the data from above, this means that each carbon dioxide molecule
has an atomic mass of 12 + 2(16) = 44, of which only 12 are due
to the carbon. Therefore, for each atom of carbon stored in
a tree, 44 atomic mass units of carbon dioxide is removed from the
atmosphere. This means that each kilogram of dried tree corresponds
(1 kg of dried tree)x(.45 kg
of C/1 kg of dried tree)x(44 amu of CO2/12 AMU of C)
= 1.65 kg of CO2
This large of an amount gives
the idea of using trees to remove carbon from the atmosphere a lot
of validity. However, it should also be pointed out that this
equation works in reverse. When a tree is burned or allowed
to decay completely, the carbon in the tree is put back into the
atmosphere as carbon dioxide. Worldwide, we are actually losing
forest, and this relationship shows why we should be concerned.
So how effective can vegetation
be at removing carbon dioxide from the atmosphere? The National
Public Radio segment below describes a study from 1999 that examined
the relationship between tree growth and carbon dioxide levels.
Carbon dioxide and Forests
Date: May 13, 1999
Summary: NPR's Richard Harris reports on a new study
showing that the world's greenery absorbs a significant amount
of the carbon dioxide in air pollution. The study, which appears
in Science magazine, also finds that pine trees grow faster
when exposed to higher levels of carbon dioxide (http://www.npr.org).
In this week's activity, we
are going to estimate how much carbon is sequestered in an acre
of forest land. In order to do this, all that we need to know,
given the information above, is how much dried wood is in an acre
of forest. To get this information, we will first need to
know something about how organisms grow.