The great cosmic entity known as the "Tree of Life" is a form of astral construct that connects the Prime Material and Inner Planes with the Outer Planes. Though not an actual "tree" as we know them, the Tree of Life appears as a infinitely large oak tree (though it has also been known to take on the appearance of an elm or ash tree as well). The branches of the tree stretch upwards into the entry layers of the various Upper Planes while the the roots stretch downward into the entry layers of the various Lower Planes.

Planar theorists argue that the Prime Material Plane is connected to the main trunk of the Tree and that this planar entity is somehow involved in planar travel. It is likewise said that a number of celestial birds and beasts dwell within the lofty branches which also radiate a strong antipathy effect toward those of evil alignment.

Google Source: For additional ideas, consult the google entry under "Yggrisil"