The speaker tells us why the tree is so important to her. Having mentioned how ancient, beautiful and vast it is, she adds in the third stanza:

But not because of its magnificence
Dear is the Casuarina to my soul:
Beneath it we have played; though years may roll,
O sweet companions, loved with love intense,
For your sakes, shall the tree be ever dear.

The tree reminds her of her childhood and the companions with whom she played under it. The tree is so ancient that the passage of a few decades had scarcely altered it at all, while the people who ran around it are completely changed. Because it is a physical symbol of her childhood, the speaker has often recalled it to mind wherever she is in the world. In "France or Italy," it has reminded her of home, transporting her mind through both time and space.

The poet has immortalized the tree first through writing the poem. It was first published in 1881, which means that it has already survived longer than any human being on earth, though only a fairly modest amount of time for a tree. The virtuosity of the poem and its universal themes of childhood, home, and memory are vital in ensuring that it is still read. However, there is also something more. Shakespeare's sonnets immortalize the fair youth and the dark lady in the ways discussed, but we know very little about these figures: even their names are disputed. In this poem, however, the physical presence of the tree is described in details. We have similes, metaphors, images, and colors to help the reader to understand what it looks like, along with all the wildlife that surrounds it. It is this tree, not just a tree that reminds the speaker of her country and her childhood, that is immortalized in the poem.