Although the book addresses many different issues—poverty, alcoholism, lying, etc.—its main theme is the need for tenacity: the determination to rise above difficult circumstances.[4] Although there are naturalistic elements in the book, it is not fundamentally naturalistic. The Nolans are financially restricted by poverty yet find ways to enjoy life and satisfy their needs and wants. For example, Francie can become intoxicated just by looking at flowers. Like the Tree of Heaven, Brooklyn's inhabitants fight for the sun and air necessary to their survival.
Idealism and pragmatism are weighed and both found necessary to survival in Brooklyn. Johnny lies about his family's address in order to enable Francie to attend a better school, presenting Francie with opportunities that might not have been available to her otherwise. Sissy helps Johnny recover from alcoholic withdrawals by appealing to his libido, helping Katie and Johnny to stay together despite Johnny's disease. Katie explains love and sexuality to Francie from two somewhat clashing points of view: as a mother and as a woman. The book revises traditional notions of right and wrong and suggests pointedly that extreme poverty changes the criteria on which such notions, and those who embrace them, should be judged.[5]
The novel’s commentary on poverty and the effects that it has is also integral to the overarching themes of the book. For example, outwardly Johnny seems to confirm stereotypical assumptions about the poor with his addiction and inability to provide for his family. However, when his character and intentions are taken into account along with his addiction and choices, it raises questions about agency in poverty and the extent to which gender roles should inform the reader's opinion of a character’s moral standing.[6] Along with Johnny, almost every character in the novel brings a different aspect of poverty to light making A Tree Grows in Brooklyn an intricate yet bluntly honest snapshot of the poor in Brooklyn during the early 1900s.
Other issues the book addresses include:
- Humanity vs. its environment
- Education
- Coming of age/loss of innocence
- Family/ family relationships
- Exploitation
- Love
- The American Dream
