A metaphor compares two things without using like or as. In the case of this sentence, nothing is being compared. If you think about the trees, you'll realize that they don't hear! The sentence, "In the deep forest, the old trees can hear," would be personification. This is because something that is not human is given a human trait. In this case, the tree is given the ability to hear, something that a normal tree does not have.
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