Adding trees where there where once desert is not automatically good for the climate. Sand reflects sunlight fairly well, trees are dark. That is actually one of the feedback mechanisms that make climate change worse: Rising temperatures in the arctic allow forests to grow in places that were once covered in ice.
It seems to me that if you are changing significant enough areas of land that the reflectivity of the surface matters then you've changed several variables at the same time. Particularly the amount of water transpired into the atmosphere would presumably result in greater cloud cover.
