Thanks for your suggests, I really appreciate them. I've been working on this for one week in my free time, but I want to achieve the best result in the time I have.

About stencil-blurred shadows, I'm using the minimum blur value, but as trees use transparency channel they can't cast a shadow, but enemies, rocks... are casting shadows. About using just decal shadows, I don't get the point why to do that?

About SSAO, I don't know if I have to do something special for transparent parts, if yes, that would be a lot of work and I don't have enough time.

After playing with HDR and Fog I achieved this result, what do you think? (right click->open in new tab to see at full size)