Forests—in particular, the tropical forest—are often called the lungs of the planet. It is true that plants produce oxygen during photosynthesis, but they and the organisms living in their ecosystems also consume about the same amount of oxygen during respiration. Oxygen makes up 21 percent of Earth's atmosphere but annual production of oxygen by plants is only about .05 percent of the atmospheric amount. There is so much atmospheric oxygen that completely destroying all vegetation would have only a minor effect on atmospheric oxygen levels. Doubling the amount of vegetation would increase atmospheric oxygen by only .5 percent. So it is not true that plants are responsible for our global oxygen supply, at least in the short term. Other processes relating to the weathering of rocks and oceanic circulation operating at the timescale of tens of thousands of years are principally responsible for regulating oxygen levels. Plants are, however, very important in the cycling of carbon dioxide.
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