The Collaborative International Dictionary of English v.0.48:
Treebeard \Tree"beard`\, n. (Bot.) A pendulous branching lichen (Usnea barbata); -- so called from its resemblance to hair. [1913 Webster]WordNet (r) 3.0 (2006):
Usnea barbata n 1: greenish grey pendulous lichen growing on trees [syn: beard lichen, beard moss, Usnea barbata]