mythological that would make sense. Riemu being the one that truly holds gyensko (spelled wrong) together would sever as the base of world tree. (even if in yukkuri form) It could also just be an artist being all artsty. Or it could be it's some sorta of pole that a yukkuri died next to and a tree was born using it as nutrients. as for aya... it's an aya nuff said. the last one though doesn't explain the ice. ... ... ... or Yukari could be mucking about again.