Like said, these kind of works are often a composite of a render and postpro in photoshop (or similar app). In this case, there are topics on Thea Render forum and here in sketchucation that do explain the process in some degree. From the topic at Thea Render forum you can even find some pure test renders, so you can get a idea about how much postpro work is done. Btw the birch tree model is downloadable to registred users of Thea Render.