I think it is a partially a question of scale. If close up view battle map (only showing 25 yards or so) then spacing the trees out in some spots but bunches in others work. If the scale is much bigger than that it feels to me that you lose individual trees and it is more of the clumping effect.
I use Gozzy's for quick close up quick maps. http://www.gozzys.com/ Nice random generator for a quick one off map.
I would prefer to have the trees foliage be a bit more transparent though to give a feeling of height.