I'm not sure why, but I've always been fascinated by tree roots. Perhaps it's because they are hidden from view most of the time. As a kid I used to imagine tree roots boring their way slowly through the soil, delving down into dark, damp places filled with earthworms and gopher tunnels, coiling boa-constrictor-like around rocks that have never seen the sunlight, and sipping from unseen trickles and subterranean streams far underneath our feet. The trees were the ambassadors of the world of light and warmth into the world of the deep earth. It was all very mysterious and magical to me then, and I suppose even now that I know more about how trees work, it still is.
Unless some sort of catastrophe like a flood washes the soil away or a tree is pushed over in a high wind event, we don't usually get to see what trees are actually up to down there under the dirt. That's why the Humphreys Trail on the San Francisco Peaks is so neat: the constant walking from hundreds of visitors' feet and erosion from powerful rainstorms lays the root systems under the forest floor bare for all to see. It's a bummer for the trees but we can actually see the complexity and, dare I say it, beauty, of their roots right under our feet.
From a scientific perspective, tree roots are a lot more than just simple drinking straws. Far from it. In fact, for many trees and plants the roots are actually where most of the organism's complexity lies. If you think of them in terms of complexity density, trees are actually sort of dumbbell shaped with high complexity in the branches, twigs, and leaves at the top and high complexity under the ground in the form of a network of ever-tinier branching roots. Separating these two high complexity zones is the much lower complexity bole, or trunk. So, dumbbell-shaped.
The purpose of a tree's root system is to absorb water and minerals from the surrounding soil and to transport them, in the form of sap, to the trunk where it is eventually drawn upwards through osmosis to the leaves where it is used in photosynthesis. Food, manufactured in the green leaves is then transported back down through the trunk along a different path to the roots where it is used to extend the network of roots. So, in a very real sense, trees have a vascular circulatory system of sorts: water and nutrients are pumped up from the ground to the leaves and food is transported back down from leaves to the roots.
Because there is usually very little water in the soil (by content) the tree must create extensive, chaotic networks of ever-smaller subdividing roots so as to maximize the surface area for absorption. These roots generally grow laterally and stay within the top eighteen inches of soil where there is ample air and the ground is easier to grow through. Some trees, like mature Oaks, may have root systems hundreds or even thousands of miles long if you were to painstakingly add up the length of every little root segment.
On the San Francisco Peaks, and presumably any young mountain, there is relatively little topsoil so the root systems are necessarily shallow. You can see how the roots have grown laterally in all directions in vast webworks but that the root balls exposed on downed trees only go down a couple of feet.
Aspen trees, my favorite, may grow their roots for over a hundred feet from the central trunk. This is amazing to me, as the root system would need to transport water and other nutrients along that long span of a hundred feet or more, up the bole for as much as a hundred feet on a tall, mature tree, and out to the leaf tips, adding dozens if not another hundred feet to the journey. That is a long way to be transporting something, especially if you don't have a heart with which to pump it with!
You can see the branching process in the following picture of a downed Engelmann Spruce along the Humphreys Trail.
At the tip of each growing root thread is a hard cap made of a tough bark-like material, strangely enough called the "root cap." The root pushes this cap through the soil ahead of it and constantly replaces it with new cells as it is worn away. Behind the root cap is a section called the apical meristem where the root's (and the tree's) cambium layer terminates. Cells born here help push the root cap ahead and elongate the root behind. Farther back are the root hairs, tiny microscopic filaments that do the real work of absorbing water and nutrients for the tree. There are zillions of these hairs on a typical root, all working together to find and utilize scarce resources in a difficult, highly competitive environment.
There's no way that I can do justice to this subject in just a few paragraphs, so go check out the Wikipedia article on plant roots for a bit more info. If you're more of a book person then I would also recommend Colin Tudge's The Tree: A Natural History of What Trees Are, How They Live, and Why They Matter, a most excellent tome on trees. Also, Guy Murchie's The Seven Misteries of Life: An Exploration of Science and Philosophy has a pretty good (if somewhat dated) high-level overview of how trees work and how they fit into the overall scheme of things. In any case, there is definitely more going on under our feet and in trees than we normally take into account.
