Often considered the center of the earth, the cosmic tree is said to protrude form the top of the cosmic mountain or from the 'navel of the world.'
The roots are said to extend to the underworld while the branches and boughs reach to the heavens, although sometimes, as in Indian and Kabbalistic traditions, the orientation is inverted. The great world rivers are likened to its roots or are said to flow under its roots. Great springs of the water of life are also connected with its roots.
In shamanistic traditions, the cosmic tree almost invariably appears as a vital link in the communication between humans and the gods/spirits.
Trees are often depicted as the abodes of gods and ancestral spirits, and the cosmic tree itself is sometimes viewed as the habitation or personification of deity.
Sitting atop the cosmic mountain, one climbs the cosmic tree to reach the heavenly abode. The mountain and tree form the axis that connects the three worlds -- the underworld, the earth and the heavens.
As the tree of life, the cosmic tree is linked with the cycles of nature and regenerations symbolized by the vegetal seasons. It's fruits, sap and the liquids near its roots are linked with immortality.
The inverted tree is said to represent the relationship of the Sun and the earth and also the anatomy of the human body with the head representing the root of the body. Life comes from the Heavens down to the Earth.
In Buddhist tradition the tree is associated with enlightenment. All humans reach the stage of Buddhahood while meditating under a particular tree. As enlightenened beings, the buddhas are free from the cycle of life and death through nirvana.
On the other hand, the tree of knowledge in the Bible is associated with the loss of immortality.
Stephen Oppenheimer summarizes the creative aspects of the cosmic tree in his book Eden in the East:
From parthogenesis and budding like a yeast to self-insemination, this tree, usually female, could do it all. Species was no barrier; a hawk could have human children in her branches, and could even fertilise snakes in her roots. In the oldest Eden, the tree was both goddess and creatrix of humankind.
For more information and links on the cosmic tree see the following site:
The cosmic tree -- rising from the navel of the earth on the cosmic mountain, its roots extending to the lower worlds, and its branches to the heavens.
Paul Kekai Manansala