Blowing in the Wind
If you look in any elementary evolution text or book of evolutionary history for the layman, you will see very little about plants. Even The Book of Life, the well illustrated and well written anthology edited by Stephen Jay Gould, spends almost no space on plants (except as fuel for the animals). Of course, we are particularly interested in how things led to us, and we marvel at how evolution selected for the means by which animals get about. Plants, by and large, don’t move, at least not very fast, and so we tend to dismiss the features they did originate. But there is no question that photosynthesis was a major event in the history of life on Earth. And the systems necessary to live on land are fairly impressive. The flower, when it eventually appeared, quickly spread over the planet, because its ability to use animals for the plant’s own reproductive advantage proved a smashing success.
Despite his early fascination with beetles, the prominence that animal diversity played in his thinking during the voyage on the Beagle, and the overwhelming attention to animals in On the Origin of Species, Darwin in later life spent a great deal of time observing and conducting experiments on plants. Darwin described how climbing plants climbed, he showed the existence of carnivorous plants, he analyzed how certain plants avoided self-fertilization and he wrote a study on how orchids are fertilized by insects. In some respects it is their (relative) immobility that allows for easy observation and experiments. Sometimes plants will even demonstrate experiments all on their own. (Last fall I was reading a book* that referred to the loss of leaves of deciduous trees as a loss of valuable carbon. It struck me, however, that carbon was really not particularly precious. Land plants have been bathed in carbon dioxide for hundreds of millions of years and have the perfect mechanism for extracting it. It seemed to me that nitrogen was the real treasure. And as leaves turned colors, it must be due to the tree re-absorbing nitrogen. I bet that the nitrogen is stored in the tree all winter, I hypothesized. Last winter was particularly brutal with heavy snowstorms following each other until March. The ice and snow built up on a tree in my yard until they snapped it near the base. Despite the fact that it was completely severed from the roots, that tree not only grew leaves in the spring, it also blossomed. How’s that for experimental proof? **)
As for their immobility, far from simplifying their strategies, it presents serious challenges. Lacking a flight response, for example, many plants developed defenses against herbivores—poisons, thorns, corrosive liquids, camouflage, mutualism with the herbivore’s enemy (such as stinging ants), and so forth. (Flowers and fruits are examples of the opposite approach: feeding animals in order to take advantage of their mobility for pollination and seed dispersal.)
A problem common to all plants is the problems associated with shape. Owing to the construction of their vascular systems which was adopted shortly after their invasion of the land to accommodate their need for water (and the ability to transport it), plants tend to be thin and long. As plants began growing vertically, they had to deal with wind. The usual method for doing so was to remain fairly close to the ground, grow horizontally and remain flexible. Cooksonia, the earliest vascular land plant known from mid-Silurian times, seems to have grown this way. But height always beckoned. Up there, there was more sunlight and better spore dispersal. Archaeopteris, among the first woody tree-like plants, appeared in the Late Devonian and could reach about 10 meters (33 feet).
Wind affects every aspect of tree architecture. Leaves, for example, are designed in such a way that they not only maximize surface area for light exposure but also reduce wind load on the tree. Take the black locust (Robinia pseudoacacia). In the wind the leaves fold up and reduce wind drag. The pinnate form of the black locust leaves has a long ancestry and is the basic form of ferns and the fern-like fronds of the fossil frond of Archaeopteris. The similarity is no doubt owing to the limited number of plans for supporting leaves, given that plants arose on land the way they did, because angiosperms (like black locust) are not closely related (phylogenetically) to Archaeopteris.
Wind has profound affect on other parts of trees. It can the affect the shape of stems. It can influence the growth direction of branches and trunk. It can stretch the cells between the roots and trunk. In the tropics some trees develop fluted bases to provide maximum support against being uprooted by wind in the shallow soils.
Wind can even affect the “behavior” of populations of trees. Tree islands of spruce and fir are known to “migrate” in high altitudes in the barren alpine tundra. Trees on the windward side of the island face bitter winds which not only apply physical force but also dry out the needles and branches. On the leeward side branches which touch the ground develop adventitious roots which eventually develop shoots. Thus, the island “moves” with the windward side dying off and the leeward side producing new trees. Radiocarbon dating of the wood debris left by receding islands of spruce (Picea engelmannii) and fir (Abies lasicocarpa) tree islands on Niwot Ridge, Colorado, show the islands to recede at the rate of 1.5 to 2.6 centimeters per year. See James B. Benedict, “Rates of Tree-Island Migration, Colorado Rocky Mountains, USA,” 65 Ecology 820-23 (1984) (abstract; paper behind paywall).
Wind can of course affect the overall structure of trees. (I have a large white pine tree that is completely bare of branches on one side owing to the flagging by wind.) Branches can be permanently wind bent and in some cases grow completely around a tree to leeward. But more interesting is what Swiss botanist Simon Schwendener called “adaptive growth”—a term he applied to the shape of the trunk optimized to withstand vertical and horizontal mechanical loads, but could as easily be applied to any aspect of the tree shape. With respect to shape, it has been found that tree mechanics are more determined by the outer shape of trees than the material properties of the stem. Franka Brüchert and Barry Gardiner found (“The effect of wind exposure on the tree aerial architecture and biomechanics of Sitka spruce (Picea sitchensis, Pinaceae),” 93 American Journal of Botany 1512-21 (2006) (full access)) that trees have different shapes depending on their placement within a stand. Trees exposed to the wind on the edge of a stand are shorter and more tapered toward the top, allowing for flexural stiffness at the stem base due to the larger diameter and a higher flexibility in the crown region of the stem. Trees like this sway with a smaller amplitude and frequency, which might help prevent the root system from becoming weakened. Trees in the middle, protected from the worst of the wind, grow taller with a more slender stem but develop a smaller crown. They become more stiff than outer trees and are more rigid against bending. Although the taller trees in the center of a forest would have oscillations of larger amplitude, the damping ratios are higher than at the edge because of the stand structure and canopy closure.
The study that suggested the foregoing ruminations on wind and trees was conducted not by evolutionary biologists or even botanists, but rather by members of the Mechanics Department of the École Polytechnique-CNRS in Palaiseau, France: Benoit Theckes, Emmanuel de Langre, Xavier Boutillon, “Damping by branching: a bioinspiration from trees,” arXiv:1106.1283v1 [physics.flu-dyn] (June 7, 2011) (abstract and full access). Their interest in the subject is purely practical. Many engineered structures, like antennae, are tall and thin, like trees. They wonder if the design of trees can teach engineers something about the ability to withstand large amplitudes of motion or high levels of vibration with little to no damage. What they were particularly interested in is whether the design of trees provided for a high level of damping—the ability to dissipate mechanical energy.
First they note that previous research suggested three sources of damping in tree construction: 1. the viscoelastic behaviour of wood (in which energy is dissipated during the deformation-recovery cycle); 2. the aeroelastic interaction between tree and wind force (in which amplitude-dependent dissipation occurs by force countering the wind force); and 3. “structural damping”—the transfer of energy to branches where dissipation occurs by the previously described (viscoelastic and aeroelastic) means. Their study (which is entirely theoretical; no trees were harmed or even actually observed during this study) is based on two models. The first is a simple Y-shaped model of a massless trunk and two branches all of which are rigid. The trunk is fastened to the ground by a rotational spring. The branches are fastened to the trunk by rotational springs and thus viscous damping is introduced only in the branches by allowing symmetrical movement in one plane from parallel with the trunk to perpendicular to it (and even more obtuse). I won’t go through the mathematics here (it’s easy enough to access the entire article), and I’ll only highlight what interests me (which is the evolutionary significance of all this). One thing they observe in this simplified model is that depending on the amount of energy applied, the optimal branching angle is between Π/2 and 2Π/3, or, in other words, perpendicular to the trunk to an angle pointing towards the ground (viewing it in two dimensions only). This is strikingly like the basic plan of most pine trees (and other gymnosperms), especially as they mature. The ability to dissipate wind energy in this way probably is one of the factors that ensure the tremendous longevity of these trees in very inhospitable places.
The second model changes certain features but retains the simple Y structure. In this model the three beams are made of a linearly elastic, isotropic and homogeneous material. The trunk is clamped at the base. The two branches are also clamped, but at the tip of the trunk. In this slightly more realistic model (the elasticity of the beams act somewhat like the viscoelasticity of wood), the results were the same; namely, “that branching is the key ingredient needed to obtain the modal energy transfer and the resulting eﬀective damping” that had previously been speculated. The more complex model has certain differences, such as, that damping is maximally effective when the branches are pointed more toward the ground than perfectly perpendicular to the trunk. But the fact is that the architecture of a tree is highly efficient in dissipating wind energy. As the authors note: “Since this nonlinear mechanism originates in geometrical effects, the larger the amplitude of motion, the higher the damping.”
Of course the design of the trunk and branching was not selected solely (or perhaps even largely) for wind tolerance. But until we understand all the complex relationships of organisms with each other and fortuitous weather, climate and geological actions, we will likely not know precisely because it is probably not possible to fully understand what Darwin called the “infinitely complex and close-fitting … mutual relations of all organic beings to each other and to their physical conditions of life.” Nevertheless, understanding the physical laws that govern an organism’s leads to the aesthetic satisfaction that Darwin proposes in the next-to-last (and less frequently quoted) sentences of his great book (even if they have an unmistakable Victorian self-centeredness): “It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us … Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.”
*David Beerling, The Emerald Planet: How Plants Changed Earth’s History (Oxford University Press: c2007). [Return to text.]
**The reason this proves that the tree reabsorbed nitrogen and stored it over the winter for later leaf production is that plants cannot obtain nitrogen from atmospheric source, even though nitrogen is the most abundant of the elements in our atmosphere. Trees of course take up carbon from atmospheric carbon dioxide. With the tree severed from its roots, it could not take up nitrogen (and other necessary elements for leaf production) from the soil. So when the leaves grew, they must have taken the nitrogen from a store within the tree. Thus trees that lose their leaves over winter conserve the elements which they cannot easily replace. Nitrogen, and not carbon, is what the trees conserve, and therefore what the trees, by the necessity of the resources available to them, assign a higher value to. [Return to text.]