“I want you to imagine a tree, if you will, and how it can be an entire ecosystem in and of itself; its own little world that also connects to the greater macrocosm of its forest. Insects and other creatures live under its roots, which are fed by the nutrients in the soil. Squirrels run up and down its trunk and make homes within it, while birds build nests in its branches. Its seeds or fruits feed various animals, including humans in some cases, which then help to spread seedlings about so that new trees can grow. Leaves will fall and cover the earth, only to rot and provide the very nutrients that fed the tree’s roots in the first place.
Now expand this view to an entire forest. Botanists are only recently learning the full extent of the symbiotic relationship between trees in a given wood. Using fractal geometry they can actually use the number and size of the branches on any given tree to predict how many others there are in the forest, and how many are large or small. We have seen trees adapt to fend off certain predators or to benefit the creatures that aid in its survival, so the connection a tree has to its surrounding environment is both profound and unquestioned.
Given these remarkable observations, which can certainly be expounded upon, it is no wonder that our ancestors chose the tree as the principal symbol of life in our lore. It is both Yggdrasill, the World Ash which supports the cosmos and the substance with which the first man and woman were created. Yggdrasill then becomes the tree whose fruits are delivered “on fire” to the wombs of women, when they are to bring a child into the world (TAE XIX. 15). We shall see later on how this forms our link to the primal elements, with which all life is connected.
Yggdrasill is the ultimate convergence of these elements, for it is fed by all three fountains (representing Water, Fire, and Air/Spirit) and holds all of the worlds within its domain. We are thus told that “no one knows from what root it springs” (ibid. II. 12, V. 4), since there is no origin for the elements that have always been and always will be.
Just as a regular tree on Earth is its own miniature ecosystem, Yggdrasill represents the greatest system of them all: the living universe. As such it becomes the fourth most important symbol of our lore, behind the three Underworld fountains that provide its sustenance, and is the representative of the fourth element of Earth. This is the created or terrestrial element, whereas the other three (Fire/Water/Air-Spirit) are the divine, creative elements. Yggdrasill is identical to the ‘World Pillar’ or ‘Irminsul’ that was worshiped by the Anglo-Saxons.
The stars in the sky were said to be nails in the pillar, upon which the Web of Wyrd or ‘Destiny’ is weaved. In the temples of our Odinist/Ásatrú ancestors high-seat pillars (öndvegissúlar) were probably reminiscent of the World-Pillar, and nails were driven into them likely as prayers for a good fortune in connection to the stars and the Web.
Threads of Fate or örlögþættir were said to be weaved in the sky, with the stars as the points or ‘nails’ that would anchor these threads in connection with the divinities and their heavenly abode. It is likely that these Nails, associated with Fate and thus our Goddesses of Fate, the Norns, are thus called "The Norns' Nails."