Maxxx, if the tree was part of the background plate, occluding an object in motion behind it would be no easy task, and certainly isn't efficient. It works for simple composites, but you'd have to create a dummy object in the scene that matches the tree's movements exactly (imagine all the leaves and branches moving in the wind), then apply a matte material to that object in order to occlude the dino when he passes behind it. It would be easier to erase the foreground tree entirely from the background plate, and replace it with a CG tree. Another way might be to mask out the tree in a video fx editor, and place the dino layer behind it in post. Time consuming and painstaking, either way. As for lighting, I agree IBL is probably the best way to match up the overall ambience, combined with carefully placed manual lighting rig to enhance the effect. It doesn't have to be high dynamic range, but at the very least you'd need to be able to take a series of spherical photos on location to create your 360-degree light map from the scene lighting. Camera equipment for such a task is very expensive, but you can fake that part by using the "X-mas ornament" technique, which should give you around 250 degrees of environment. Then you can fill in what you need with manual lights. As Maxxx suggested, always render out in passes so you can tinker with the colors and get a more accurate composit in post.