Even so, its all pretty academic. The old green mantra of "the amazon rain forest is the lungs of the planet" is nonsense. The amount of CO2 absorbed by trees is a fraction of what is taken in by phytoplankton in the oceans.
Q—Do trees give off Carbon Dioxide?
A—At night they do. During the day, they give off oxygen.
All normal plants, in the process of photosynthesis, take up carbon dioxide by day and release oxygen. This switches over in the dark, and they then produce carbon dioxide through respiration in the same way as animals. While they are growing, they take up more carbon dioxide overall than they give out, which gets turned into plant tissues (especially cellulose and, in the case of trees, lignin).
http://www.answers.com/Q/Do_trees_give_off_Carbon_Dioxide