There's actually a name for it.  Arborglyphs.  The definition being:  dendroglyphs, silvaglyphs, modified cultural trees, or aspen carvings are tree carvings made in the bark of aspen trees by shepherds, many of them Basque and Irish American, throughout the Western United States, according to online source.  

The photo I have posted is a lovely depiction of the way these sheepherders would pass a little time.  It's just one example taken by Ken Proper.  There are many more.  If you do an internet search, you will find all manner of carving with figurals and words.  Very endearing and expose what many of them had on their minds.   There's even a book about this called "Speaking Through the Aspens:  Basque Tree Carvings in California and Nevada".  

The USDA Forest Service even has an information page for this with a gallery of photos.  

I wonder if the sheepherder today are still doing this type of thing?  I know of one, whose message is simple and clear.