Re: How much free oxygen does the rainforest give off in one year?

	This is a very tricky question. With energy from light, green 
plants transform carbon dioxide (CO2) and water(H2O) into carbohydrates 
(CH2) that plants use for growth, and into oxygen (O2), which is a 
byproduct of photosynthesis.  A general equation for photosynthesis is:

	CO2 + H2O --> CH2O + O2

So plants emit one molecule of oxygen for every molecule of carbon dioxide 
they assimilate through photosynthesis.

	The tricky part is that plants also respire, that is, they use 
carbohydrates (and other plant compounds) to build new cells and to 
maintain old cells.  The equation for respiration is the opposite of that 
for photosynthesis.

	In a mature ecosystem, such as a forest, net annual growth is zero. 
 In other words, the ecosystem consumes as much oxygen through respiration 
as the plants produce through photosynthesis.  This means that the quick 
answer to your question is "none."

	In describing these processes, we normally refer to the carbon 
balance rather than the oxygen balance, because it is actually the bonds 
between carbon molecules that have then energy that fuels biological 
systems.  There is also far less carbon dioxide in the atmosphere than 
oxygen.  Our atmosphere contains about 20% oxygen, but only 0.04% carbon 
dioxide (0.04% = 400 ppm).  Thus, if photosynthesis and respiration are out 
of balance, it will have a much larger relative effect on the carbon 
dioxide concentration than on the oxygen concentration.

	Changes in the carbon dioxide composition of our atmosphere arise 
from two main causes.  People directly add to the atmosphere that through 
burning fossil fuels.  We also affect gases in the atmosphere by changing 
the nature of the landscape, or land uses, such as through harvesting of 
forests. When forest lands are cleared by burning, then there is a double 
effect of releasing the carbon from plant biomass into the atmosphere and a 
reduction in capacity for assimilating carbon dioxide through 
photosynthesis.

	On a global basis, terrestrial plants fix about 50 x 10(exp)15 g of 
carbon.  Marine plants fix a similar amount.  This sums to a total of 100 x 
10(exp)15 g carbon from an atmosphere that holds only 700 x 10(exp)15 g 
carbon.   Living biomass, mostly wood in forests, contains about 500 x 
10(exp)15 g carbon above ground and about 1500 x 10(exp)15 g carbon in the 
soil.  

        If all biological carbon were combusted with oxygen, the 
atmospheric carbon dioxide level would nearly triple, whereas the 
atmospheric oxygen level would drop less than 0.1%.

	For a complete discussion of the Global Carbon Cycle, see J. 
Goudriaan (1995) pp. 207-217  In. Climate Change and Rice (Peng et al., 
eds.) Springer-Verlag.

	For some good news on the capacity of U.S. forests to sequester 
(store) carbon, take a look at the US EPA web site on global warming: 
www.epa.gov/globalwarming