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Biogeochemical Cycling: Trees and Carbon

Carbon Sequestering in Trees
Carbon Reservoirs

In burning 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.  

During photosynthesis, 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 expressed as

6 CO2 + 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 to 

(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.

 

Listen

Topic: 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). (4:00)
Link: http://www.npr.org/ramfiles/atc/19990513.atc.13.ram

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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.