The best source on the World Tree we have from the sagas is from the stanzas of the famous Voluspo. Where exactly it comes from or what created it is never made clear. Unlike the creation of Midgard which is described in considerable detail in the saga with the Gods making decisions by council(1) the Yggdrasil seems to emerge from the depths of time of space on its own. Some have suggested it is possible the World Tree predates Ymir and the collision of fire and ice in the Ginnungagap. As is described in Voluspo 19:
An ash I know, Yggdrasil its name,The World Tree is said to hold up all of the Nine Worlds, from Niflheim and Jotunheim in the depths to Midgard at the art rising to the towering heights of Asgard, Vanaheim, and Alfheim. Even though the inhabitants of some of these worlds are at odds with one another, with the most famous conflict being the one between the Jotnar of Jotunheim and the Aesir, this does not change that their worlds are equally supported by the same foundation. Without the World Tree these worlds would have nothing to anchor or support them. Watered by the Well of Urd, which is often associated with fate and wyrd, the Yggdrasil grows on pure possibility sustaining all the worlds on its branches.
With water white is the great tree wet;
Thence come the dews that fall in the dales,
Green by Urth's well does it ever grow.(2)
When the World Tree meets its end in Ragnarok the great fire which consumes it does not discriminate what it burns. As it says in the Voluspo:
The sun turns black, earth sinks in the sea,Nothing is spared. Jotunheim, Muspelheim, Midgard, and Asgard all burn. While grim and bleak this is understandable in an animistic, polytheistic context. Unlike Abrahamic monotheism where everything is the product, whim, and creation of a single all-powerful creator who exists above and beyond their creation the Powers of Scandinavian cosmology operate on very different rules. Like the Gods and Powers of other animistic, polytheistic cultures the Aesir, Vanir, and Jotnar exist within a greater universe which is governed by greater laws than their desires. This makes them as intimately bound to the fate, workings, and health of the universe as everything else that walks, swims, crawls, flies, grows, and breathes. If the World Tree, which is essential for holding up the Nine Worlds, were to burn then logically the worlds it supports cannot endure on their own.
The hot stars down from heaven are whirled;
Fierce grows the steam and the life-feeding flame,
Till fire leaps high about heaven itself.(3)
This kind of understanding of the universe, rather than being at odds with the theories and arguments of modern science, is very synergistic with the discoveries of ecology and environmental science. When an ecosystem is dealt great harm all living things which are a part of the threatened habitat suffer, even human beings. Beyond this the concept of greater universal laws, principles, and axioms which impact everything in the universe is one which gels perfectly with physics and broader scientific principles.
Just as the Nine Worlds are spiritually shaped by forces like wyrd and orlog we in Midgard are molded by the forces of gravity, electro-magnetism, entropy, and many others. Electro-magnetic forces, for example, bind together the universe through the charges which keep the atoms, elements, and molecules which make up all material things together. Gravity holds all of it together in an elegant dance of heavenly bodies without which life on earth as we know it would not be possible. As the Gods are bound by greater forces in the universe so to is physical bound by forces we are only just beginning to understand.
In many ways the concept of the World Tree is a very powerful one. Quite contrary to the assumption of primitive speculation the Yggdrasil is a beautiful expression of a holistic, interconnected view of the world which modern scientific discoveries have recently validated. It is a very practical approach to reality which stands in stark contrast to the external creator-god narrative or the conception of a spirit-free universe. If anything the World Tree, wyrd, orlog, and the concept of the nature of the Powers expresses a universe where all the things that live within it are free to find their own way. Nothing lives under a giant microscope, scrutinized by a judgmental yet distant tyrannical father, or in an empty, uncaring cosmos. Our universe is alive and always growing, changing, and becoming a greater expression of itself through the mutual interdependence of its constituent parts.
1. Voluspo 6, 9, 24, 25, Poetic Edda, trans. by Henry Adams Bellows
2. Voluspo 19, ibid
3. Voluspo 57, ibid