As I sit on the porch soaking up the morning sun, I often find myself gazing at the base of the two red oak trees on our lawn.
I transplanted both of them as wild saplings from elsewhere on our land, and now I am surprised to realize that the elder tree is 17 inches in diameter and the younger 14 inches. My first impression of the swelling butts of these trees is that they look remarkably like elephant feet, or at least what I conceive elephants’ feet look like. The stocky legs swell seamlessly into the foot, with rough gray bark similar to an elephant’s rough gray skin.
Once I get over my elephant musings, my thoughts wander down into the soil. Having dug these trees up when they were youngsters, I know they send one or more sturdy tap roots straight toward the center of the earth. How deep are they now? Did they run into ledge a few feet down and turn sideways? Or was it just a big rock that they were able to scout their way around and resume their relentless pursuit downward? I cut the central taproot when I transplanted them, so I’m quite sure there are now at least two principle roots. Perhaps one hit the rock and another didn’t and therefore became the dominant root. I’m comfortable with the fact that I’ll never know.
Not far away (about 35 miles as the raven soars) some scientists have been studying tree roots for over 50 years. Their studies involve the Hubbard Brook Experimental Forest, a 7,800 acre watershed in the White Mountain National Forest, located a little southwest of Woodstock, N.H. If you go to their website (hubbardbrook.org) you’ll find more information about the intricate details of northern hardwood forests than you could ever read. Most of it you wouldn’t want to read anyway; the arcane writing of hardcore science is not exactly riveting. But there is also a wealth of information for the layperson, contained in an online book summarizing the wide range of ecosystem research that has been accomplished since the watershed was designated experimental in 1955.
Two hours south of us is another renowned experimental forest, the Harvard Forest in Petersham, Mass. Here, too, researchers have long explored the mysteries of tree roots. I discovered a research paper from the 1970s by Walter Lyford, all about the roots of red oak. Lyford detailed the development of red oak roots from seedling to mature trees. Just as I suspected in my front porch musings, the primary tap root forks if injured and goes around obstacles before heading straight down again. Off the primary root, about 5-10 major secondary laterals develop near the soil level which grows thick and woody, extending out from the trunk on all sides and providing considerable structural strength. It is the growth of these big lateral roots just below the soil surface that causes swelling at the base of the trunk. Lyford also pointed out in his paper that the taproot, so important in the early life of the tree as it pushes deep into the soil, is not dominant in the mature tree. The lateral roots often end up going deeper and growing bigger than the original taproot. Red oak roots develop a tight, interwoven mass near the trunk, extending down through the B horizon (the layer of weathered soil below the topsoil or A horizon). As a result, a wind-thrown oak pulls up a large ball of soil, leaving a deep pit that will be evident for decades after all trace of the wood is gone. By contrast, a wind-thrown red maple or balsam fir whose roots are wide and shallow pulls up a thin layer of soil that will be obscured by leaves and detritus by the time the tree and its roots are rotted away.
When Lyford excavated a main lateral on a 60-year-old red oak he found it to be 50 feet long and averaged 12 inches below the soil surface. Excavation of a red maple in the same soil type found its lateral roots concentrated in the top four inches of soil. Red oak grows best in well-drained soil and has therefore evolved a root system that forms a tight ball at the base and penetrates as deep as it can. Red maple is adapted to wet soil and has evolved to keep its roots shallow, above the saturation level.
Chapter 9 of the online book from Hubbard Brook is about roots. “Studying the structure and function of tree root systems requires brute strength or clever techniques because they are difficult to access. At Hubbard Brook considerable attention has been applied to understanding root systems of the trees because they are crucial to forest ecosystem production and biogeochemistry.” It turns out that tree roots, concentrated (80 percent) in the upper eight inches of soil, account for about 25 percent of the forest’s plant biomass. I was a wee bit surprised to learn that the finer roots of trees are constantly dying and being replaced. Hubbard Brook has found that a typical fine root lives about a year and a half, though some only live a few weeks and some live for three or more years. They don’t understand what causes a root’s death but speculate that the root may have exhausted the nutrients in its narrow domain. Or little invertebrates in the soil may be chewing, or soil fungi may parasitize the root. They think that the depression of soil pH by acid precipitation during the 20th century and the current slow rise in pH as the soil adjusts to lower acid inputs have impacts but they don’t yet understand them.
I’ve written in past columns about the important relationship between soil mycorrhizae (symbiotic fungi) and tree roots. The importance of mycorrhizae is apparent but the mechanisms are not well understood; nor are the effects of our disturbance of the mycorrhizae as we cleared away the forest and used the soil for agriculture. In the case of my two oak trees growing out of our lawn, are there mycorrhizae left in the soil from the days 200 years ago when this land was the forest? Or were there enough mycorrhizae clinging to the roots of my trees when I transplanted them to colonize our lawn? I suspect the latter and that a prime reason that it takes transplanted trees several years to start growing again is the time it takes for mycorrhizae to colonize their new soil environment.
And finally, there is the fascinating prospect of communication between trees, presumably transmitted through the roots. I can’t wait to read the newest book on the subject, “Finding the Mother Tree” by researcher Susan Simard, which delves into the theory that the biggest, most successful trees in a group somehow help their smaller neighbors. Perhaps my smaller oak tree didn’t hit its stride for several years because it hadn’t yet established that communication link with the bigger, older tree 40 feet away.
Obviously, my musings from the porch will continue, as will the research into the mysteries of the subterranean world of tree roots.