Source : Webster's Revised Unabridged Dictionary (1913)

Treebeard \Tree"beard`\, n. (Bot.)
   A pendulous branching lichen ({Usnea barbata}); -- so called
   from its resemblance to hair.

Source : WordNet®

Usnea barbata
     n : greenish gray pendulous lichen growing on trees [syn: {beard
         lichen}, {beard moss}]