I hope I am putting this thread in the right board.
I recently modeled a tree in a .blend file, and placed it on top of a plane. It does not appear in the render. It, the plane, camera and lighting source are in the same layer.
What is the problem?
Upload a Blend file. There are really too many possible answers to your question.
Outliner, near eye icon - photocamera not greyed out? Reason for some 70% 
Not exactly sure but the problem might be that you have added the tree in Object Mode. Try adding it in Edit mode by selecting the already existing plane model.