Intelligent Design Icon Intelligent Design
Life Sciences Icon Life Sciences

Paper Digest: Are Trees Well Designed?

Photo credit: Dreamy Pixel, CC BY 4.0 , via Wikimedia Commons.

Editor’s noteEvolution News is delighted to continue an occasional series, “Paper Digest,” looking back at past publications in peer-reviewed journals of interest in the debate about intelligent design.

Back in 2004 in the Journal of Engineering Design, Stuart Burgess, a longtime proponent of intelligent design theory, and D. Pasini published on the physics and design principles of trees. Specifically, the study looks at the mass-efficiency of the structural shapes and forms found in trees. Burgess and Pasini explain that their purpose is “to understand how high levels of mass-efficiency are achieved [in trees] and to identify lessons for engineering designers.”

Consistent with Burgess’s general research strategy — using the assumption of good design in nature to guide investigation, further scientific knowledge, and better elucidate how natural systems work — this paper is an excellent example of how ID research can be applied in a scientific discipline.

A Key Step in Reverse Engineering

To classify the function of structural features of a tree, Burgess and Pasini use a methodology called a function-means tree. This is just a graphical way of identifying a hierarchy of functions, beginning with the highest and then depicting how these are fulfilled by lower-level functions. Building this hierarchy of objectives is a very important step in reverse engineering because it helps one to understand the functional reasons for structures observed in nature.

Major Sources of Load

To appreciate the design of a tree, Burgess and Pasini explain that it is important to understand the major sources of loading or forces that a tree must endure. These loads come from the wind and the weight of the tree itself. Using their engineering toolset, Burgess and Pasini offer equations to estimate the aerodynamic force due to the wind, the bending of the trunk, and the maximum stress the tree endures. From these equations, they discover the engineering importance of a tree’s structural design features, such as a tapered trunk and the structural hierarchy of little branches being supported by bigger branches. They also discover fun facts like large trees don’t have greater bending stress than small ones, but taller trees might have greater bending stress due to higher wind speeds further up from the ground. They authors are also able to determine some of the fail points and they note that for storms with winds of greater than 100 mph, trunks and branches are very vulnerable to breaking.

Next, Burgess and Pasini discuss the self-weight of the tree trunk and note that the compressive stress is not significant even for large trees because of their structure. Since trees grow straight up and then have branches emerging from all sides, much of the bending stress is alleviated through this excellent design of counterbalance.

Burgess and Pasini mention that one of the most important things about the overall structure of the tree is its structural hierarchy. There is first the trunk, then the main branches, then the secondary branches, and finally the tertiary branches and leaves. This hierarchy provides several advantages. First, it allows the surface area of the tree’s canopy to be linked to its source efficiently. The hierarchy also allows a relatively direct load path from the canopy to the trunk. Finally, the hierarchy mediates the ability for gradual growth. Through their expertise and with the help of a design lens, the authors can reverse engineer and understand the structural design of trees.

Structural Features of the Trunk and Main Branches

Key structural features in the trunk include tapering and residual stresses. Tapering is the effect of the top of the trunk having a smaller diameter than the bottom of the trunk. Burgess and Pasini explain that this is a good design because the maximum bending varies the least at the bottom and the most at the top. It also reduces the amount of biomass that the tree must produce. The design of the trunk to bend also enables pre-stressing. This helps to improve the tree’s strength by relieving stress a little at a time instead of all at once in a devastating snap. They explain:

When the tree is subjected to aerodynamic loading, bending stresses are superimposed on the residual tensile stresses. Pre-stressing is a beneficial structural feature because when the trunk is subjected to bending moments, the net compressive stresses are less than the net tensile stresses. Since the compressive strength of wood is lower than the tensile strength, the preloading significantly improves the strength of the tree.

Major branches connect the smaller branches to the trunk of the tree, which means they are subject to large loads because of the number of small branches and leaves attached. Burgess and Pasini note that, to compensate, the main branches, just like the trunk, are tapered — with the greatest diameter near the trunk, and tapering to a point. With major branches, the diameter of the branch changes from circular to rectangular, and the depth, especially at the connection point, is increased to support increased load bearing as the branch supports more and more weight.

How Leaves Minimize Aerodynamic Loading

Burgess and Pasini explain that in engineering, designs are often optimized around either strength or stiffness. When optimizing a design for strength, more flexibility is possible because stiffness is not a strict requirement. The authors observe that trees seem to be structurally designed more for strength. This is especially clear when we look at the smaller branches of a tree and its leaves, which readily deform in the wind, thereby minimizing aerodynamic loading. Despite their extreme flexibility, the design of the leaves still keeps them stable enough to provide a flat surface for light collection. In engineering, this is called high bending stiffness but low torsional stiffness.

The Structural Role of Roots

Hold-down bolts in concrete, connecting a building to its foundation, are comparable to sinker roots, a type of root that grows deep into the soil. Unlike hold-down bolts, though, roots are multifunctional, providing not only structural support but also water uptake. Burgess and Pasini explain that the lateral roots extend perpendicular to the sinker roots and provide an anchoring system through the creation of a plate of soil to which the tree structure is bound. For some trees with high growth rates, buttresses provide additional support as the underground root systems develop more slowly than canopy growth.

The Amazing Design of Wood’s Microstructure

Burgess and Pasini briefly discuss the incredible design of wood’s microstructure, which is that of hollow cells with a hexagonal shape. They explain that being hollow reduces the overall density of wood, which reduces the load needed to be borne by the trunk and branches. They also derive an equation for a performance factor, demonstrating how the hexagonal microstructure strengthens the structure significantly.

Inspiration for Engineers

In this publication, Burgess and Pasini describe how trees are incredibly well-designed to withstand the forces of their own weight and the wind. Trees have smart structural design features like a multilayered hierarchy, counterbalance of loads, tapering to preserve resources, flexibility for minimal aerodynamic loading, and an appropriate microstructure. The authors note the structural efficiency of the tree is essential to its survival and ability to fulfill its roles in the ecosystem. Interestingly, engineers use nearly all the structural aspects of trees, but trees still have more multifunctionality than is commonly seen in human engineering. Burgess and Pasini conclude by looking forward, with the expectation that trees still offer additional sources of inspiration for engineers, especially when it comes to “multi-functioning structures with smart, adaptable behavior.”