Never Give Up!: The Philosophy of Multi-trunk Trees
Since starting to teach what I affectionately call "my tree course," I have been reading every book I can find on the subject of trees. A recent one, The Wild Trees, chronicles the career of botanist Steve Sillett, pioneer in the study of redwood and coastal sequoia canopies. While swinging through these treetops, he discovered that a single redwood can develop more than one hundred trunks, almost none of them visible from ground level. In fact, one redwood giant reverently named Ilúvatar --after the supreme being in J.R.R. Tolkein’s Middle Earth novels—contains 220 trunks. Competition for light and space is fierce in a redwood forest, so trees that make it into the upper realm grab as much space as possible by thrusting up smaller “trees” from their limbs. Explaining how a single horizontal branch can support a vertical trunk and branches which then reproduce themselves repeatedly in fractal fashion is a challenge for even the most accomplished physicists or engineers. The resulting canopy of Ilúvatar is “so dense with foliage that you could put on a pair of snowshoes and walk around on top and play Frisbee there” (202).
Sillett’s map of the trunks of Ilúvatar illustrates a phenomenon called “reiteration.” The first scientist to notice this “reiterative” process in trees was French botanist Francis Hallé who in 1986 flew across tropical forest canopies in an aerostatic balloon he named Radeau des Cimes (Raft of the Peaks). Peering downward from his balloon, Hallé could see that the complex architecture of a tree is “made up of nooks and crannies and shaded, moist spots and fertile pockets where all kinds of living things can become established and interact with each other” (Preston, 203). Sillett has seen this fertility up close. While exploring the redwood canopy three hundred feet above the ground, he frequently snacks on blackberries growing in the crannies of reiterated trunks.
Of my many reactions to the wonders revealed in The Wild Trees, two were primary: the tree is a marvel of creation, and research in botany may be exciting. Inspired by Sillett’s and Hallé’s work, I set out to discover whether North Carolina forests contained any tree-trunk wonders. Slightly acrophobic, I was happy to do my research from ground level rather than in the treetops. Had I been acrophilic, I would have been frustrated. No forests in North Carolina have trees that approximate the age and height of the redwoods, although when settlers arrived in Orange County in the early 1700’s, there were some mighty tree specimens. A sycamore located on what is now Mason Farm had a trunk “ten or twelve feet across.” Its hollow middle was roomy enough for the Morgan family to live in until they built a more traditional house (Terres 6). Many of the forests we have now in the piedmont are actually the results of economic downturns which caused farmlands to be abandoned and fill up with trees. Most mountain forests nowadays are second growth after extensive logging campaigns in the late nineteenth and early twentieth centuries. Despite the age and height differences between Sillett’s forest and North Carolina’s, once I started looking—in both piedmont and mountain forests-- I began to see trunk reiteration all around me. Sometimes the trunk split just above the ground (see figure 1); other times the original trunk split a few to many feet above the ground. (See figure 2). I saw examples of trunk-splitting in poplars, maples, pines, hornbeams, black cherries, oaks, and beeches.
Research into the phenomenon of reiteration introduced me to a wonderful new word, THIGMOMORPHOGENEISIS, coined by botanist MJ Jaffe in 1973. (Telewski 238). English professors rarely have opportunities to coin words, but we do love to trace etymologies. Broken down into its Greek roots, thigmomorphogenesis equals “thigmo” (touch), plus “morpho” (change in shape), plus “genesis” (creation). In other words, because something touched the tree, it changed shape and generated new life. In an ideal world, no gnawing insect, browsing animal, windstorm, falling tree, ice storm, flood, or chainsaw would ever interfere with a tree’s progress from earth to sky; but since a forest is, in reality, full of competing forces, rarely does a tree live this ideal life. Trunk-reiteration is a strategy to recover from stress on the tree’s development. Such a focus on recovery is quite admirable, even inspiring, attributable to what Rachel Carson heralds as the tree’s philosophical “obligation to endure.” The obligation to endure can be seen in figure 3. This hickory tree suffered a “plastic” or irreversible strain--as opposed to an “elastic” or temporary strain (Tewlewski 238)—when a large pine tree fell on it, forcing its trunk earthward. But the bottom half of the tree did not surrender its sap. Instead, it sent a new trunk skyward.
Biology professor Berndt Heinrich explains trunk-twinning more scientifically. Upon emerging from seed and thrusting down roots, a young tree’s first priority is to develop a vertical “leader” which can quickly reach the sunlight that nourishes the tree. When, for whatever reason, the leader is compromised or destroyed, buds which would have become branches, re-prioritize and start reaching vertically rather than horizontally. Ideally one of these buds will be the clear winner, but sometimes the contest results in a tie and the tree ends up with two leaders which become the two trunks. Even though some trees manage well with double trunks—the poplar in figure 1, for example—such twinning frequently makes the tree more vulnerable to wind and weight stresses than a single-trunked tree, and also makes its energy needs higher. It will need more leaves to photosynthesize the sunlight into food (Heinrich 89-104; Frank).
Foresters and arborists have additional theories about how trees adapt after stresses. The theory of “adaptive growth” acknowledges that trees are engineering marvels, achieving “optimization of strength that is invariably more elegant than that achieved by the human design process” (Wood 133). A tree’s roots, may, for a host of reasons, be poorly anchored in the soil, or they may be insufficiently strong to withstand the force of winds which sway the tree. To stay upright, the tree needs to find a way to keep the trunk from swaying at its base and to eliminate the strain of constantly bending back and forth. A tree can adaptively lay down wood fiber where the stresses are the greatest. The tree thereby “compensate[s] for damage or for a changing environment” (134). This adaptive strategy can be seen in the red oak shown in figure 4.
Because of the tree’s location just to the side of a slope where rainwater rushes into a stream, the tree roots have most likely experienced shifting soil—from rainwater movement, as well as from expansion and contractions due to rain and drought cycles. To adapt to this perhaps-frequent destabilization of soil, the tree has added wood mass in a form that architects might call “buttresses.” The added mass at the base of the tree holds the two trunks vertical and helps to support the additional weight of the branches. (See figure 5). The strategy has obviously worked since the tree’s diameter of 35.4 inches suggests an age of 140 years ( Missouri).
Another example of adaptive growth can be seen in figure 6. This large maple was partially uprooted during Hurricane Fran in 1996. For several years it rested against another large tree until most of the tree’s top died and fell to the ground. Nonetheless, the tree, having adapted to its diagonal situation, remains “aloft.” As a sign of hope for the future, it has even sent up a new trunk, halfway along the old trunk.
A tree may look like a simple construct to those of us strolling through the woods, but it is in a constant struggle to reach “functional balance” between its roots and crown, a structural ideal also known as the root-shoot balance (Coder). The leaves photosynthesize sunlight into carbohydrates, which are sent downward to nourish the roots. The roots, meanwhile, are absorbing nitrogen, which gets sent upward to the leaves. Trees “adjust the mass of roots or shoots to correct any deficiency in photosynthesis or nitrogen uptake.” Therefore, “carbohydrate shortages may initiate more shoots and nitrogen shortages will initiate more roots” (Coder).
This root-shoot relationship may help explain the very unusual four-trunk tree shown in figure 7. The straight trunk soars upward nearly twenty feet, at which point it suddenly forks into a trident shape. All “tines” of the fork look equal in diameter, suggesting that they all began growing upward at the same time. The tree’s crown must have been seriously injured, perhaps by a strong wind or a falling tree. Without the leaves to gather carbohydrates for the roots, the roots pumped nitrogen upwards to encourage new growth. The triple trunks produced plenty of leaves to grab the maximum amount of sunlight and thereby nourish the large root system. In its gravity-defying feat of balancing three trunks on top of one, the tree has successfully maintained an equipoise between roots and shoots. In the words of forestry professor Kim Coder, trees are, from moment to moment, “attempting to solve a series of biologically simultaneous equations. The answer for the tree . . . is a never-ending optimalization process played out among a wild and varied mixture of site, tree, and other organisms.” In other words, a tree never gives up its attempt to reach the light above it.
I have always thought trees were lovely, stately, noble, and essentially serene. Having embarked on my little investigation of tree trunks, I now realize how determined, alive, competitive, and ingenious they are as well. They are intrepid problem solvers. Sometimes the solution is elegant, as with the pine and the winged elm in figure 8, both competing for space between two larger trees; sometimes it is defiant, as in figure 3's hickory, refusing to share the fate of the dead pine. One need look no further than the nearest forest to see the life force at work.
Carson Rachel. Silent Spring. New York: Houghton Mifflin, 1962.
Coder, Kim D. “Control of Shoot/Root Balance in Trees.” University of Georgia Cooperative
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Missouri Department of Conservation. Missouri Department of Conservation, Web. 20 Nov.
Preston, Richard. The Wild Trees. New York: Random House, 2007. Print.
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Hill: University of North Carolina Press, 1993.
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