When having anti-aliasing on high and then alpha to coverage on all trees and grass, only grass gets turned on with alpha to coverage and not the trees.

I put two comparison images, one with alpha to coverage on for both trees and grass, and one completely turned off.

No alpha to coverage: http://i.imgur.com/HMYaaqH.jpg

Alpha to coverage for both trees + grass: http://i.imgur.com/dk7CRUu.jpg

If you quickly flick between the two, you can clearly see only grass is affected.