It seems to me that just as man is an olam katan, a miniature world, the same applies to trees.
We can see in fact, that the various stages of creation all find representation in the body of a tree:
The roots, which are the most massive part by far, are entirely hidden from view and so represent the innermost level of creation which is the state of Atziluth.
The trunk comes seemingly out of nothing, shooting up out of the ground, and being the apparent essence of the tree. (If you cut off branches, the tree is still said to remain. Even if you cut off all the branches, you might think of it as a 'dead' tree but still a tree.) In this way the trunk represents the level of Beriyah, creation of something from nothing. This is the level from which all things receive their substantive nature.
The branches are clearly offshoots of the trunk, and represent further development of the tree, mirroring the 'something from something' nature of Yetzirah, formation. Similarly it is the branches that play a very large part in the appearance and shape or form of the overall tree.
Finally, the leaves represent where all the 'action' happens in a tree, the generation of energy and the breathing of carbon dioxide. The leaves parallel the final and lowest step in creation known as Asiyah, action or making.
There is a final step in the tree and that is the most evident one on Tu B'Shvat, the production of fruit. Fruit represent, to my poor understanding, the lowest sephirah, Malchuth. This is paralleled by the attribute of speech in a person. Just as the words that leave a person's mouth seem separate from him, so too the fruit seem more like a product rather than an integral part of the tree. (see today's Tanya)
There is an important lesson that we may learn about speech from the fruit of the tree. Within each fruit is a seed, the seed contains the very essence of the tree and the potential to reproduce the tree in its entirety. [The seed that created the tree to begin with can even be seen to reflect the primordial level of creation known as Adam Kadmon, a state in which everything is still unified in a potential point.] So, what does the seed in the fruit come to teach us? That within each one of our words, at its core, is the word of HaShem, the same speech that created and maintains the world in its entirety is clothed within every single word that leaves our lips.
When we eat fruit, we should raise our awareness to understand that it is HaShem who provides us life through this fruit, and know that the words that we speak share this same ability to provide life to ourselves and to those around us.