Rainforests are the 'Earth's lungs' which produce much of our oxygen

Rainforests are Essentially Oxygen Neutral

Tropical rain forests are often called the "Earth's lungs", however there is no scientific basis for such a claim as tropical rainforests are known to be essentially oxygen neutral, with little or no net oxygen production.^[1].^[2]

"Many good reasons exist for placing deforestation near the top of our list of environmental sins, but fortunately the fate of the Earth's O2 supply does not hang in the balance. Simply put, our atmosphere is endowed with such an enormous reserve of this gas that even if we were to burn all our fossil fuel reserves, all our trees, and all the organic matter stored in soils, we would use up only a few percent of the available O2. No matter how foolishly we treat our environmental heritage, we simply don't have the capacity to put more than a small dent in our O2 supply. Furthermore, the Earth's forests do not play a dominant role in maintaining O2 reserves, because they consume just as much of this gas as they produce. In the tropics, ants, termites, bacteria, and fungi eat nearly the entire photosynthetic O2 product. Only a tiny fraction of the organic matter they produce accumulates in swamps and soils or is carried down the rivers for burial on the sea floor."^[1]

Where did our O2 come from?

"Just how O2 came into being remains a mystery. The most likely explanation is that water molecules that wandered to the outer edges of the atmosphere were knocked apart by ultraviolet rays from the Sun. The light hydrogen atoms were able to evaporate to space, while the much heavier oxygen atoms were bound to Earth by gravity and mated with the reduced sulfur and carbon exposed at the Earth's surface. Only when this conversion had been completed could O2 begin to accumulate in our atmosphere. Records kept in sediments tell us this task took at least 2.5 billion years (more than half of geologic time). The evolution of multicellular organisms, and hence of our ancestors, awaited this transition from an O2-free to an O2-bearing atmosphere. Fortunately, this buildup was large enough that the Earth, unlike Bio2, became endowed with an adequate supply of this precious gas. The size of this inventory has surely varied, but once established it has never dipped low enough to threaten the existence of those who depend on it, nor will it in the future."^[1]

"FACT: Oxygen makes up almost 21 percent of atmospheric gases. Burning all the tropical forests in the world would decrease the proportion of oxygen by four-hundredths of 1 percent. Destruction of tropical rain forests, however, would significantly increase the concentration of carbon dioxide—the most important greenhouse gas—in the atmosphere by 26 percent."^[3]

Sources:

1. Broeker, W.S., 2006 "Breathing easy, Et tu, O2" Columbia University http://www.columbia.edu/cu/21stC/issue-2.1/broecker.htm. (WALLACE S. BROECKER, Ph.D., is Newberry Professor of Geology at Columbia's Lamont-Doherty Earth Observatory and chief scientist at Biosphere 2. He received both the 1996 Blue Planet Prize and the National Medal of Science for achievements in global environmental research.)
2.  Moran, E.F., "Deforestation and Land Use in the Brazilian Amazon", Human Ecology, Vol 21, No. 1, 1993 “It took more than 15 years for the "lungs of the world" myth to be corrected. Rain forests contribute little net oxygen additions to the atmosphere through photosynthesis.”

3. Environmental Myths  by Jared Diamond Discover Magazine 03.01.1999  Retrieved 05:27, February 7, 2008

4. Rainforest. (2008, February 6). In Wikipedia, The Free Encyclopedia. Retrieved 05:27, February 7, 2008, from http://en.wikipedia.org/w/index.php?title=Rainforest&oldid=189492782

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