I consider myself to be among the lucky few in Philadelphia that get to both live in the city and also live on a large plot of land. My home is built on a 0.6 acre lot that is flanked on three sides by dense “forests” (i.e., unmaintained growth). This forest consists of quite the diverse flora, including introduced maples, pines, black walnut, cherry, and sycamore, as well as an understory dominated by a nearly equal mix of matted wild rose briars and poison ivy.
The centerpiece of our yard, and Rob’s pride and joy, is a rather Sideshow Bob-like weeping willow tree. Last winter, the unusual amounts of heavy, wet snow broke one of its primary branches off the main trunk, such that what used to be the crown of the tree draped over our lawn for easily 5-6 months before we finally managed to remove it. The lanky trunk and wispy branches that make the willow “weep”, combined with the thin halo of leaves form a deceivingly delicate facade on an incredibly tenacious plant. This enormous, 2 foot diameter branch actually began to heal itself during the course of the 5-6 months it braced itself against the ground! By the time we chopped the branch off in the spring, as it threatened to take down the rest of the tree in its fight to survive, it was sending out new branches and had actually even tried to root a section of itself to the ground!
The aggressive roots of willows are one of their most simultaneously beloved and hated traits. Those who hate the willow tree roots complain about the nearly magical ability for the roots to detect the presence of water, even through metal pipes, and will gradually bore their way through, repeatedly clogging pipes. In fact, there is even a blog dedicated to one such incident of a swamp willow that clogged water intake pipes, and continued to clog them even after the trees were chopped down!
Those who love the root’s determined hydrophilic nature plant these trees close to rivers, streams, and lakes, to control erosion on embankments. The complex root system wraps the soil, holding it in place, while presenting a zen-like tree above ground for all to admire.
The properties of roots are highly complex. They are very weak resisting compressive and bending forces, excelling in their tensile strength. In other words, roots are strongest when you are trying to play tug-of-war with them, which is precisely why they can be hard to pull out of the ground!
It turns out that the way the roots hold the soil in, is through a process known as edaphoecotropism — quite the mouthful to simply say “stress-avoidance”. In effect, wherever there are particularly high resistive or sheer forces, roots will turn and move down the path of least resistance. In short, what this means is that wherever there is a sheer plane in soil (i.e., one layer of soil moving relative to another), when the root reaches this layer of sheer, it will turn and orient itself parallel to the direction of sheer. Interestingly, what this does is transfer the sheer forces of the soil to tension in the roots, by means of friction of the soil along the entire length of the root. This tension causes the root to lengthen and deform, further increasing the surface area exposed to friction and making the pull out even more difficult.
A potential fruitful area of continued study would be to examine the precise mechanisms of edaphoecotrophism. We already utilize ideas of tensile reinforcement in many familiar materials such as clothing and fibreglass plates. However, I suspect that if we can harness the means of edaphoecotrophism from its sensory or even mechanistic perspectives, we may have some potential innovations for the fields of robotics or artificial growth of stronger construction materials.
Want to read more on soil reinforcement? Check out Simon and Bennett. 2004. Riparian Vegetation and Fluvial Geomorphology. American Geophysical Union.