Many people don't own suitable land on which they could do such a thing (certainly I don't), and if you're talking about going and planting a tree somewhere as

Many people don't own suitable land on which they could do such a thing (certainly I don't), and if you're talking about going and planting a tree somewhere as part of a larger project run by some organization that has acquired the land for the purpose then... sure, ok, but it'll be far more efficient to pay a bunch of people to work full time planting many trees every day, rather than each one of us individually driving out to a planting site once a month to plant one tree (assuming there's even a site within a day's drive). I'm all for reforestation projects (and many other projects that could potentially have some kind of impact in reducing the rate atmospheric CO2 levels are rising), but the idea of everyone individually planting trees is not really practical. If you personally are in a situation that makes it feasible for you, then I hope you go for it and I wish you all the best.