Difference Matte effect:
Note that because of subtle differences between any two frames (due to grain, compression artifacts, tiny lighting changes), it's hard to get perfect results with this effect alone. But it can be a good starting point.
For a Difference Matte to work absolutely nothing can be moving in the scene. Difference Matte will not work with compressed original footage. I know I've tried many times. If the wind is blowing in the grass or the trees are moving the Difference Matte will not work.
if you are planning this kind of shot then the best thing to do is try to find some good areas of contrast in the shot so you can create a procedural matte. A little garbage matte and a procedural matte created by adjusting levels or color or hue can create a much better matte and does not rely on the pixels being exactly the same on every frame.
Even easier is a carefully planned shot and Roto brush.
I agree with Rick. My answer was a narrow answer to the question as it was asked, but his answer is good advice on better ways to approach the overall problem.