Many of these tree planting schemes are a bit dubious; can you really be sure that they are planting new trees that wouldn't have grown in that area anyway, and will they still be around in 40 years time to make sure the trees grow to maturity and are not cut down and burned?
The only tree planting scheme that I really trust is the Green Belt Movement: http://www.greenbeltmovement.org/
started over 40 years ago by Professor Wangari Maathai.