When I visited Wat Arun last year, this banner was wrapped around one of the upper tiers of the temple. It contained messages from around the world in various languages. It functioned like the prayer boards that one sees at Shinto shrines in Japan. One can’t really see it from ground level.
It doesn’t really show up well, but the tree in the center apparently swallowed and digested another tree. i.e. It encased a tree that died and rotted.