According to wikipedia, a silhouette is the shape of an object or person (or any living thing) of a single, plain color. Silhouettes are a very old form of expression and can be created by different means. Traditionally, they were cut out from black paper and mounted on a lightly colored background, but all sorts of combinations, variations and collages exist. I may have been preconditioned to this traditional art form because my parents have paper-cut illustrations and a book of a friend of theirs, Helmut Bögel, who is known for his humorous paper-cut illustrations of people. Silhouettes can also be drawn and of course photographed.
I hope that you enjoy some of these photographs and that they may encourage one or the other to have a conscious look at nude trees!