In this novel, the Australian bush, and more broadly nature, symbolizes religion or spirituality, both in untamed form and when cultivated into a garden by people like Stan and Amy. The bush is likened to a natural cathedral. It is a source of the life force.
Stan and Amy also symbolize life in a mythic way beyond their individuality: they are like the first humans, Adam and Eve, settling and farming a new world infused with spiritual presence, and they are often referred to not by their names but symbolically as "the man" and "the woman." They are part of the tree of life of the book's title. We find at the end of the novel that this life force is within Stan, symbolized by his spit. As he says, pointing to his "gob of spittle," "That is God."
God or the spiritual can be touched or experienced in the natural world, such as when Stan witnesses a storm. During a storm, Stan experiences an appropriate humility, as he understands that humans are specks in the cosmos. (It is interesting that humans both contain God within and are specks.) The powerful lightning Stan sees "had, it seemed, the power to open souls." In a flash of lightning, Stan feels his soul has been touched: "the flesh had slipped from his bones." Trees, growing and spreading, also become a symbol of this life force. At the end of the novel, as Stan sits in his overgrown garden, from "this heart the trees radiated," a symbol of growth and life.
In contrast, Sydney, and the city in general, represents a place of spiritual vacuum. Although Thelma is by no means a bad character and does search for spirituality, her desire to lead a cultured life in Sydney symbolizes a streak of coldness and sterility in her that she shares with her husband. The city destroys the primal spirit represented by nature.