A treesinger is an Ogier who has the ability to sing to trees, called "treesinging", either healing them, helping them to grow and flower, or making things from the wood without damaging the tree. Objects made in this manner are called "sung wood" and are highly prized. Few Ogier remain who are Treesingers; the Talent seems to be dying out. Stedding Tsofu has two tree singers and Stedding Shangtai has one. Items made of sung wood can have a resonance that can reveal details of the singer to any other treesinger who sings to it later, though this fades over time (TSR, Ch. 24).
Examples of Sung wood
My bed is sung wood! Why, it must be well over a thousand years old. No Treesinger has sung a piece so large in at least that long. I myself would not care to try it, and I have the talent more strongly than most, now. Well, to be truthful, there are not many of us with the talent at all, anymore. But I am among the best of those who can sing wood (The Shadow Rising, Chapter 24).