Very interesting and very strinking. Do you mean that the mesh was given to you and you made all the leaf, bark and stuff textures ? Is the background modelled separately? There is some weird stuff around the ivy leaves seen in silhouette. What is that?
i followed the tutorial and made the same stuff , for the models i made 'em throw Maya PaintFX and textured them , and yaa, i noticed that problem with the ivy leaves , they have alpha maps but when i rendered the occlusion pass they looked like planes . thats why