its just rough painting using between 6 and 4 pixel brush.
i just blocked in shadow colour then kept adding the lighter leaves towards the light source.
nothing exciting really i just thought it might give u some ideas since its not too cartoony.
i guess one point i'd like to make is don't focus too much on a tree, then focus too much on a rock, then focus on something else, or u run the danger of lots of things that look good by themselves but out of place when put next to each other in game world.
my scene is random and uninspired (boring), so i'd advise get some interesting references, plan your composition, start roughing it in, and work across the scene as you build up detail so everything looks consistent.