Does "tree-borne raft" mean that the raft is made of trees (woods)?
Freddy wants a tree-house, and I always meant to build one in the copper beech. Nothing fancy – just a sort of tree-borne raft, like the one me and David Ockeridge built in his dad's orchard, when we were kids in Black Swan Green. In my daydream I'd tell Fred that the copper beech is guarding his prezzie, and I'd watch him run down, find the rope ladder, and climb up to a better, leaf-shushed world. The thought of another man building Freddy's tree-house in an ordinary green beech made me hurl my trowel through the shed window.
(Excerpt from David Mitchell's The Massive Rat.)