So I will start by describing my understanding of the way trees are portrayed in the Bible, and then move to Norse and Indo-European myth generally. Finally I will move to the question of what the tree metaphor tells us in a pagan context, and what truth we find.
In the Bible, trees are always known by their fruit. There are the trees of life and knowledge in Eden, the likening of a teacher to a tree, and knowing the teacher by the fruit in the New Testament, etc. Norse myth however tends to look to the tree generally, not as an olive tree or another element of agriculture, but rather as a deep, universal, self-contained metaphor. Trees thus represent the universe, the society, and the individual, and these three things are considered to be interchangeable in terms of mythic patterns.
The Tree of Man in Norse Myth
Then came three,The first interesting parallel here is to Hesiod's Cosmogony where Zeus, the chieftain of the gods, creates the human race from ash trees. Add to this the sisters of Phaethon being transformed into amber-teared poplar trees, and you see a strong connection between humans and trees in Greek myth as well. Complete Celtic cosmogonies have not survived, nor were myths of this sort used in Roman times. On to the Indo-Iranian branch, we see the association of Kundalini coiled around the base of the tree, but the tree is the human spine, again showing this metaphor found in these other branches as well. It is almost certain that tree as human and human as tree is a motif found across the Indo-European cultural and mythological traditions.
three from the throng,
From the lair of the Gods,
And they found on the land
Of little might
Ask and Embla
Of no fate
Breath they had not
Song they knew not
Hair, nor looks
Nor good countenance
Breath gave Odhinn
Hoenir gave Song
Hair gave Lodhur
And good countenance
-- Voluspa stanzas 17-18 (my own translation)
But back to the creation of humans in Norse myth. Rather than created from the dust of the earth, as in Genesis, humans are created instead from trees which are already living but seen as lacking the unique aspects of human vitality. The gods provide these capabilities, animating and providing additional social context to the trees (but see below). Hair was a marker for social status among the Norse with slaves having very short hair while others allowed to have longer hair. Similarly countenance provides social context.
However the application of this area doesn't presuppose a total lack of social context to trees. In the 12th century Icelandic poem "Havamal," we have:
The lesson here is that just as trees shelter and support eachother, so too with human society and that any one of us that is abandoned by society will not live long. Similarly we have cases of logs being dressed as people and becoming like humans, and the importance of clothing in determining social context.
The lonesome fir-tree
Stands in the field
Bereft of bark and needle
So is the man
Who is shunned by all
Why should his life be long?
-- Havamal stanza 50 (my translation)
The overall note here is that the tree is a model for the human. In my own spiritual approach, I say: "Grow like a tree and be not afraid of shadows: Seek the darkness and then the light!"
When the seed of a tree germinates a slow process of awakening occurs, which culminates in a root beginning to push outward from the tree and move into the earth below. From the darkness of the earth below, the seed finds water and some nourishment. Slowly in this way, the seed then begins to send up leaves reaching towards the sun. It is through this process that human growth is best done.
The darkness below represents many things. It represents the tradition and the ancestors (the land of the dead usually being below the earth in pagan societies). It is a land inhabited sometimes by fearsome monsters, and so facing fears and poking around dark corners of history and the psyche are part of what I recommend. It is through the discipline developed here that one can be grounded in one's quest for the divine.
The light is of course the land of the gods. It is the part of the spiritual experience that gives those peak experiences that Maslow wrote about, and where we can be tall and imposing, carrying spiritual weight wherever we go.
The tree reminds us generally of the nature of the hero, tall, strong, and unyielding like a tree.
The Tree of Society in Norse Myth (and Platonic Parallels)
The tree is also a social metaphor and this is perhaps best seen in the Volsung Saga (a prose edition, basically, of the earlier Sigurd cycle of epic poems) where the Barnstokk hall is built around a giant tree with the tree in the middle. This is in essence a manifestation of the king but also the society. Odin thrusts a sword into the tree proclaiming that whoever can pull it out can have it. This may be the origin of the Arthurian sword in the stone story. In this story the tree nicely sums up the connection of the human and social structure.
At this point, suffice it to say that the world, the social, and the human tree are all interchangeable and interrelated. Examples will be given in the next section. A good organization is no different than a good individual, and even when we talk about "incorporation" we talk about it as creating a figurative "person." While in some ways we arguably carry this too far in other ways we do not go far enough. Our organizations would be better if we think of how to structure them in human terms rather than merely as machines.
In Republic, Plato makes the argument that society is fundamentally like a human in anatomy because we make up societies (in Letters he extends this to Godhead and therefore establishes this framework as the precursor to the Christian Trinity). Plato argues that humans are divided into three centers, a head, a heart, and a belly with different functions and that society works with the greatest justice when these functions are performed by their experts in harmony with eachother.
One other important aspect discussed by Mircea Eliade and others is the use of the societal tree to represent the center of the world. This leads us directly into the final portion of this metaphor: The World Tree.
The Tree of The World in Norse Myth and Hindu Parallels
The World Tree is noted in Voluspa immediately following the creation of humans from Ask and Embla:
I know stands an ash,
Called "Yggdrassil," [Yggdrassil means "Odin's Horse]
A tall and high tree
With white dew
Then comes the floods
that fall in the dales---
It stands evergreen
At the well of Urdh.
Then came maidens,
Much in knowing
Three for the hall
That stands under the tree:
The first was called Urdh,
The second Verdhandi--
They scores did cut--
Skuld was the third.
They lots allotted
They lives chose.
To the sons of men
They uttered destiny.
-- Voluspa stanza 19-20, my own translationThe tree is mentioned over and over possibly in this context in the mythic poems Vafthrudhnismal, Grimnismal, Havamal, and others. It is the central model of Norse myth, and we learn that its branches reach above the home of the gods, and the roots go down into unfathomable depths below. The World Tree is all pervasive and alive.
One important point though is that we can not be entirely sure whether the tree in any of these contexts really is used to represent the universe, human society, or the individual, and it is in this ambiguity that the myth takes on its deepest and most vital aspects.
Turning to Hinduism, I am reminded of one of the Upanishads where a young Brahmin in training is sent out to meditate on various natural elements including the sun, moon, sky, wind, and lightening. He is told to meditate and try to realize that "That is what you are." The message, as found in many of the other Upanishads, is that the deep truth (Brahman means "The sacred truth") in each of us is the same as in everything else, and the Upanishads teach that when we fully recognize this that we become no different than the Gods. The world tree here is similarly all pervasive and it teaches that we are in everything and everything is in us. The models do differ to some extent. While the Hindu model is more static, the Norse model suggests that the tree may serve various metaphorical aspects during our development and the unification with everything is really only at the end.
I can think of no better model of integrative living than the tree, however, because it reminds us that our organizations, ourselves, and our world are more different in scale than in nature.
 Ask = "Ash" as in ash tree. It is also found generally as a poetic title for a warrior. Embla is of uncertain meaning. Proposed meanings have been "Elm" or "Vine." "The ornamented" is another proposed etymology. Given that "ash" was also used in kennings for yew trees, which have separate male and female plants, and where the female trees show ornaments, I tend to support this etymology.
 The original word here is "Odh" which can mean song, inspiration, frenzy, or madness. The name of the god "Odhinn" or "Odin" means "Master of Odh."
 Hoenir is otherwise known for his silence.
 See also Polome, Edgar, "Essays on Germanic Religion"
 Not purely a scholarly reference but see Travers, Chris, "The Serpent and the Eagle: An Introduction to the Elder Runic Tradition"
 Peter G. Foote and David M. Wilson. The Viking Achievement.
 Borrowed in part from Assagioli, Roberto. "Psychosynthesis: A Collection of Basic Writings" Assagioli saw Dante's "Divine Comedy" as the ideal model for psychotherapy, in particular that of preparing the human spirit for mystical endeavors. My model is similar.
 Eliade, Mircea. "Shamanism: Archaic Techniques of Ecstasy"
 Note the Old Norse word here is "ask" which is the same as in "ask ok embla" in the human creation myth.
 The implication is that these are sorceresses.
 The names of the sorceress-norns are interesting. The first's name is usually used to mean "fate" but literally means "What has turned or become." Verdhandi means "What is turning or becoming" while Skuld usually simply means "debt." The debt is quite possibly related to the primordial debt in Greek myth discussed by F. M. Conrford in "From Religion to Philosophy."
 The Old Norse here is "orlog seggja" and can be more literally translated as "They spoke the primordial lots or layers." The idea of fate becoming effective on speaking is also found in the Old English poem "The Wanderer.