The poem The Heart of the Tree might actually have a number of themes, though the primary focus is on why trees are essential for human existence.

The usefulness of trees: This is the prominent theme in the poem as the poet describes how a tree helps in keeping this earth habitable. This is detailed in another answer here: Why trees are important for us to survive in this world.

Appreciation for natural beauty: Henry Bunner here shows his appreciation and demands the same from the readers for the beautiful aspects of nature. He wants us to love nature and its objects in order to help ourselves live better. The comparison of a tree to a ‘flag of breezes free’ and to a heavenly home for birds says it all.

Responsibility for future generations: In the entire second stanza the poet has talked about our responsibility to make this world a better living place for our future generations. “And seed and bud of days to be“, “The harvest of a coming age; / The joy that unborn eyes shall see” — the lines express this theme very well.

Duties to fellow men and nation: In the third stanza of the poem, Bunner talks about how a man can perform his duties towards his neighbours and to his nation by planting a tree. “In love of home and loyalty / And far-cast thought of civic good”, “His blessings on the neighbourhood”, “A nation’s growth from sea to sea” — these lines all convey the theme of responsibility to our fellow men.