Sometimes it seems like a loss when summer departs. The leaf color changes and you know that the visually quieter winter, season of the voided color palette is coming. This is the time that gives us a chance to see the more subtle aspects of the landscape, such as textures, patterns, and movements that we don’t normally see as clearly when all the trees are in full green.
One of the subtleties I most enjoyed last month, in early November, was the delicate shimmying of the leaves of Populus tremuloides, commonly known as the aspen tree. While other leaves blow in the wind, aspens seem to flicker. Of course, I’m not the first to have noticed this special movement: The “trembling” of an aspen’s leaves is built into one of its most popular common names, quaking aspen.
Years ago I spent a good deal of time looking at aspen leaves and learning about how they move. I was working on a project called Tampa Wind which is comprised of a meandering riverine form mounted with thousands of little silver disks—disks that I wanted to move and shimmy in the wind, just like aspen leaves do. What enables aspen leaves to move in ways that other leaves don’t, I wondered—why are aspens especially able to “quake”?
I discovered that the aspen leaf has a particularly flattened petiole, or leaf stem. This flat connection between leaf and branch allows the leaf to bend and twist in mild breezes, unlike the round-stemmed leaves on other deciduous trees. Why do aspens have this design? It may protect the leaves from being stripped off in forceful gales. The aspen is one of the few deciduous trees that grow alongside the weather-proof evergreens, higher on mountainsides where the wind is fierce. Also, there’s the possibility of another, more complicated purpose that this flattened petiole serves: it may help aspens to drink up more sunlight. Because aspens’ thin bark contains chlorophyll, the trunk portion of the tree can actually photosynthesize. So the changing, “quaking” movement of the leaf may allow more sunlight to penetrate through the leaves and to the bark, giving the tree as much access as possible to the light it needs in order to make food.
Learning about aspen leaves helped me to engineer all those little disks for Tampa Wind. Inspired by the aspen’s flattened petiole, my team and I designed the disks’ connection points (called stand-offs) with a flattened end that serves as a tipping point, so that they echo that delicate movement I first observed in nature. More recently, while designing a project for the Philadelphia Airport, I remembered the lessons I learned from aspen leaves. So if you pass through baggage claim F in Philly this holiday season, look up: Those myriad silver disks that tremble and quake do so in the manner of aspen leaves.
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