The Collaborative International Dictionary of English v.0.48:

Arborist \Ar"bor*ist\, n. [F. arboriste, fr. L. arbor tree.]
   One who makes trees his study, or who is versed in the
   knowledge of trees. --Howell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arborist
    n 1: a specialist in treating damaged trees [syn: tree
         surgeon, arborist]