We live in a part of the world where there are many trees. We also experience heavy winds that frequently blow down human-made structures. It is interesting that healthy trees are almost never blown down. When you stop to think about it, you would expect trees to be major victims of high winds. That is not the case, and it is due to leaf designs to preserve trees.
To survive strong winds, trees need two things. The most obvious is structural support–strong, flexible branches, sturdy trunks, broad bases, and good root anchorage. A more subtle requirement is leaf designs to preserve trees. Leaves must have minimal wind drag. A fluid, such as air, flowing around an object generates drag. To minimize drag requires some streamlining to reduce the amount of friction between the fluid and the object. A highly streamlined object will usually be gently rounded upstream and elongated and pointed downstream.
For healthy trees, the leaves offer the most surface area and thus the most drag. Trees most commonly blow over when in full leaf, so leaf design is critical to the survival of the tree. Different trees have different design features, but all of them are designed to avoid destruction in a wind storm. American holly leaves have a method that involves the leaves being able to flatten themselves against each other. When the wind becomes strong, the leaves turn and lie flat significantly reducing the drag.
Tulip tree leaf design allows the leaves to roll up when the wind gets strong. The blade of the leaf points away from the stem. As the wind blows against the leaf, it forms a cone pointing upwind at the stem. The blade forms the broad area of the cone away from the wind direction. The higher the wind, the tighter the cone and the less the wind resistance. Black locust leaves similarly roll together to produce a cylinder.
Each of these designs depends on the properties of the leaf. If the leaves were too stiff, they could not assume the right geometry. The flexibility of their stems has to be high, and the surface of the leaf must be carefully designed and restricted. You can argue that natural selection does all designing and that given enough time it will select the proper shape. But remember that changes in climate mean you don’t have infinite time to apply the process.
God’s engineering wisdom gave us leaf designs to preserve trees. The leaf design allows the longest season for each tree. Sit in your backyard on a breezy day and watch what the leaves do to preserve that tree you prize so highly.
–John N. Clayton © 2018