You only have to do that for the Bigrock_1. You can use the cliff behind it, and the player will only get as far as on top of the bigrock_1. That way you can spread your trees out moe , and use less of them.
Its hard to see, but if you look closely at the bottom left screenshot of the Desert scenes, youll see in the background, it goes from front to back: slope, bigrock_1, then the cliff. The big round rock is placed at the low side of the ledge of the bigrock_1 to keep the player off of it.
I know that sounds like a pain, but if you set it up right, you can use the bigrock_1 to create a rockface with a narrow ledge that the player has to transverse, too close to the edge and you plummet to your death!
In the Universe, nothing ever stays the same.