To what does the speaker compare herself in Line Nine of the poem, What Lips My Lips Have Kissed?

The speaker compares herself to a "lonely tree," which symbolizes the speaker's own state of isolation. She describes the tree, once full of birds, now standing empty and feeling its branches mourn for the lost presence of life. The speaker, too, mourns the loss of life and energy that once graced her past in the form of lovers in her bed.