Plants absorb carbon dioxide and give off oxygen.
Carbon dioxide is returned to the atmosphere when the plant parts decay or are burned. Burning of fossil fuels, such as oil and coal, also add CO2 to the atmosphere. Increased carbon dioxide may have warm the whole Earth, some scientists believe, because the gas blocks heat from returning to space and traps it about the planet. The gas acts like the glass in a greenhouse, letting sunlight in, but preventing the escape of heat. In three test plots, pipes steadily pumped in carbon dioxide, keeping the gas level among the trees at 560 parts per million.