The World Tree is linked with physical trees in Botany. Symbolically the world tree is also associated with the notions of 2 trees (Tree of Life and Tree of Knowledge) and 3 trees, which adds a Tree of Wisdom
As Edna Kenton explained in the Book of Earths, 1928:
According again to the races of men, the type of the World Tree varied. The Date-palm was the sacred Asherah of the Assyrians. To the Greeks, and to the Norsemen, the cosmic Ash was the World Tree. But also to the Greeks, and to the Germans, the Oak was the life-giver and the life-sustainer. And the Greeks made the vine the sacred tree of Dionysos. Persian legends centre about the haoma tree, and the Egyptians had a mythical golden gem-bearing tree of the heavens, where the Sky goddess Nut had her abode. The Japanese believed that a great metal-pine grew far to the north at the centre of the world, and the Russians have a legend of an Iron Tree whose root is the power of God, and whose head sustains the three worlds--the heavenly ocean of air, the Earth, and Hell with its burning fire. To the branches of the Jambu or Rose Apple tree, the Hindu dead clung and climbed to immortality. India has also her incredible banyan tree [also called the Fig tree], declared to be more like man than man himself.
Cited from Website: Trees of the World.
This topic is dicussed in lectures
04: World Tree
14: World Trees