I've never attempted anything like that so normally I wouldn't be able to help at all here, but I saw these two tutorials a few days ago and thought they were pretty good:

On the style and rendering of trees
How to colour quick trees for RPG maps

I think style number 1 from the first tutorial used as a repeating texture over the top of what you've got now might look nice, plus the end result of the second tutorial looks awesome so maybe you could do that!