I think a lot of this discussion is overblown. First of all, I don't know who is talking about replacing farms and pastures with trees, except in tropical countries where there's no controversy regarding forest benefits, and it's not something that I have heard any serious discussion about. And on the question of albedo, planting trees and cities and suburban areas could actually make albedo higher instead of lower. A 7% reflectivity in urban areas is less than 9% for pine forests, and 13% for deciduous trees (the ones usually planted in cities).
UPDATE: Thought I'd add that the most important global warming impact of urban trees is the amenity effect - by making cities nicer places to live, people will be more willing to stay rather than commute from the suburbs.