For the leaves material, I started by creating an opacity map in Illustatrator, then inside Unreal Engine I created a material that allowed me to change the base color, underside color, emissive colors, billboard size and a couple of other things. 

For the UVs, every single face is in the same place in the UV space. I made it this way so that i could use the same opacity map in every face.

This gave the leaves a fluffy look and made them more interesting. Just like the grass, the leaves also move slightly, which helps making them more unique and natural. I decided to use a light green as a base color with a light orange as highlight to give a more heavenly look to the trees.