I don't know of a naturally tree-structured language (setting aside XSLT, which isn't a general purpose language), though maybe Prolog would qualify.
Why not Lisp as a "naturally tree-structured language"?
Lists are constructed and destructured really as trees, less frequently as flat, single level lists. From list pattern matching down to the traditional list primitives (
car, cdr, caar, cadr, ..., cdddar, cddddr), Lisp and lists have been manipulating trees.