To give you some advices on achieving background / foreground distinction, you can use atmospheric perspective. In a nutshell, the further, the less saturated, the lighter.
Or go the other way, light foreground, dark background. The only thing that matters is contrast. Never should you fail to tell if something is on the same plan than the character, or if it is in the back.
As far as walking on top of trees is concerned, a good way to show the player where he could walk is, again, contrast. Look at this DK country screen shot, it summarize the dark background / light foreground, as well as showing a clear path. Ground is very light, trees you can walk on are also lighter. It's all about contrast. Close your eyes barely entirely until you only see the lighter part of the image. What did you notice ? then do the same on your tile set. What stands out ?