A multiple win-win. In addition to the carbon-oxygen exchange, trees provide cooling shade (drive into OP from the highway and notice a 5+ degree drop in the Summer), they are attractive/decorative, when the leaves fall, the give you an opportunity to exercise that you might not have had otherwise, and leaf mulch is a great, organic fertilizer (we've been using in our vegetable garden for years.) I'm sure you can think of others.