Original rock formation shot in Utah

Original tree image shot on the island of Lanai in Hawaii

I flipped the tree image horizontally because I thought the placement on top of the rock formation would be better.
The palette below shows how the image has been compiled. The top layer consists of a selection from the original tree image. It was added to extend the image. The tree layer shows the mask. The third layer down shows the shadow. And the bottom layer is the original rock formation. Both of the images had previous exposure corrections.

The mask above shows the detail required to make selective cuts from an image. There is also a pinpoint measuring mark numbered "1" that picked the whitest area when I made the mask. Masks are made in several ways. For this image, I picked the most contrasty channels layer, "red," and duplicated it. I added a levels adjustment to that channel to get the needed contrast. I selected the white area and made the mask from it.

A shadow was created from the original tree image with the mask intact by using a duplicate layer that filled the tree with black. I changed the opacity of this shadow layer to overlay so the background luminosity of the rocks would show through.

The final image shows both tree and shadow with their masks intact. The shadow of the tree emulates the same direction of the sun that is falling on the rocks (see center right side of image). One of the biggest mistakes people make when putting images together is to ignore the shadow directions of the two images. It is the subtle things that make an image believable.