Carl Jung tells us:
“The tree is an image of spiritual development.”
The tree illustrates spiritual development, as that which exists beyond our material development. Jung says:
“You see the tree is a plant and it symbols a strange development entirely different from animal life, like the development we call spiritual.
What is the nature of this development? Jung illustrates for us the nature of spiritual development, as the nature of transformation, likened to a tree:
“As a tree extracts mineral substances from the earth, the spirit transforms the course body, or the coarseness of matter, into the subtly of organic matter. The tree represents, then, a sort of sublimation. It grows from below up into the air above, has roots in the earth as if it were part of the earth, and extends roots again into the kingdom of air; and so the spirit of development rises out of the material, animal man and grows into different regions above.
The tree rises out of the dense matter of the earth, moving up toward the kingdom of the air, toward its ruler, which is the sun. Jung continues:
“Therefor the tree has forever been a symbol of spiritual value or philosophical development, like the tree of knowledge in Paradise for instance or the philosophical tree, the Arbor Philosophorum, the tree with the immortal fruits –a Hermetic symbol– also the world tree in the Edda.” (Notes of the Seminars Given in 1934-39, p. 1071)
Reference: Nietzsche’s Zarathustra: Notes of the Seminar Given in 1934-1939, Volume 2 By Carl Gustav Jung