The skeletons of dead giants. They're visually trees - HUGE trees (20,000 feet, at their smallest,) that rise throughout all of Songhaven and border the entire land. Not much is known about their properties, other than that they're magical in some nature, keep a long-forgotten terrible evil at bay (the stuff of legends, these days) and cannot die or wilt or be cut down; they also can't be artificially torn apart in any way. Different trees do have different properties - the bark a tree from Wintervale, for example, might have a healing property, where a leaf that falls from a tree in Violl's Garden will strengthen your sword or make you impervious to being harmed. The Skeleton Trees are tended to by the Canopists on the Sky Islands.
These trees also border all of Songhaven, and the thicket extends so far out that nothing can be seen past it - it's rumored that something lies beyond this strange greenbelt, but all who attempt to traverse it or leave Songhaven's border circle back and end up where they started - no matter how straight a path they travel.
The top branches, and their leaves, are transparent - the sky is still visible through these trees, and everything else above.