This pattern commemorates Treebeard, a tree-giant, or Ent. He is said by Gandalf to be "the oldest living thing that still walks beneath the Sun upon this Middle-earth". Living in the ancient Forest of Fangorn, to which Treebeard has given his name (Fangorn in Elvish) which lies at the southern end of the Misty Mountains.
In this detailed depiction you can see Treebeard and Beechbone hide amongst the flora and fauna of the forest, and as the roots of trees twist below the earth the quote from Treebeard reads
“The world is changing: I feel it in the water, I feel it in the earth, and I smell it in the air.”