Super interesting, also from a printmaking and photographic perspective!
The original prints were collotypes; undoubtedly, somebody out there still practices this process, but it's extremely uncommon. I've seen some modern (10-20 years old IIRC) collotypes last year. they were marvelous! Coincidentally, they also featured tree motifs.
Another interesting bit is the extensive manipulation done on the prints, similar in effect to burning & dodging. I also believe that the skies in the photos are the result of some kind of composite process - Photoshop avant la lettre.