This is a modern example of doubly (or multiply) -exposed photos, when the same negative film frame was exposed multiple times with different scenes. This used to be done in-camera, or in the darkroom by stacking multiple negatives and exposing them onto the same photo paper.

With Photoshop, you want to use the Screen Blend mode. It's easiest to use source images with large contrast between the subject and the background; a white or very light background is ideal, with no distracting elements. Using the Dodge tool, eliminate the undesired background from the portrait. Then, blend the non-portrait photo(s) (in this case, trees) with the portrait.

Example tutorials:

• Digital Camera World double exposure tutorial – this is a nice, simple example, very similar to the image you linked. This one is in color, and it makes a nice effect.

• iPhone Photography School's interview with Brandon Kidwell – this interview showcases lots of great shots by him, completely shot and processed on an iPhone (!). There are a couple double-exposure shots where he describes his composition and technique.

There are countless tutorials for this type of effect. Search Google for "photo double exposure blend tutorial" (or something similar). You will find more tutorials than you can go through.