Do you really not remember the famous scene?
"You fool, no man can kill me."
"I am no man!"
Stab
Head exploding.

So I didn't get the reference and had too google this, I'm pasting it here for everyone.
"Tolkien loved this line in Shakespeare: 'Macbeth shall never vanquished be until Great Birnam Wood to high Dunsinane Hill shall come against him.' He loved the thought of marching trees. However, Shakespeare had other ideas. He just makes an army cut down tree branches so they look like a marching wood."

"Tolkien thought this was rubbish and invented Ents. And the march of the Huorns on the orcs at Helm’s Deep."