Just like Diamond, I was wondering that as well. I had some theories, like: in Manga Studio you can have a brush with multiple different stamps like the various trees, in Adobe Illustrator you can have a set of symbols that you "spray" onto your document (with variations in size & attributes), and I think you can have an Image Hose in Corel Painter do something like this. Having gotten a *very* close look at these trees, I'm reasonably confident that they're not all the result of hand ink-work. Not that there's anything wrong with that. ;-)