Once again, the vast majority of the oxygen we breathe is actually produced underwater, mostly by bacteria. Still, trees are essential to the Earth's ecosystems, which we are a part of, so... Yeah, too bad we have evolved into a whole species of ignorant d-bags.
http://earthsky.org/earth/how-much-do-oceans-add-to-worlds-oxygen
http://www.blueplanetbiomes.org/rainforest.htm