The Collaborative International Dictionary of English v.0.48:

Arboreal \Ar*bo"re*al\, a.
   1. Of or pertaining to a tree, or to trees; of nature of
      trees. --Cowley.
      [1913 Webster]

   2. Attached to, found in or upon, or frequenting, woods or
      trees; as, arboreal animals.
      [1913 Webster]

            Woodpeckers are eminently arboreal.   --Darwin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

arboreal
    adj 1: of or relating to or formed by trees; "an arborous roof"
           [syn: arborical, arboreal, arborary, arborous]
    2: inhabiting or frequenting trees; "arboreal apes" [syn:
       arboreal, arboreous, tree-living] [ant: nonarboreal]
    3: resembling a tree in form and branching structure;
       "arborescent coral found off the coast of Bermuda";
       "dendriform sponges" [syn: arboreal, arboreous,
       arborescent, arboresque, arboriform, dendriform,
       dendroid, dendroidal, treelike, tree-shaped]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "arboreal":
   V-shaped, Y-shaped, arborary, arboreous, arborescent, arboresque,
   arborical, arboriform, biforked, bifurcate, bifurcated, branched,
   branching, branchlike, bushlike, bushy, citrous, coniferous,
   crotched, deciduous, dendriform, dendritic, dendroid, evergreen,
   forked, forking, forklike, furcate, hardwood, nondeciduous, piny,
   pronged, ramous, scrubbly, scrubby, scrublike, shrubby, shrublike,
   softwood, tree-shaped, treelike, tridentlike, trifurcate,
   trifurcated