The key to the aspens’ quaking is the anatomy of the petiole, the stem that attaches the leaf to its twig. Most leaves have rounded petioles — like little cylinders — which gives them a stiff structure and keeps them stable. But the petiole of the trembling aspen is flat along its entire length, which gives it that signature flexibility.
What is the effect of this trembling habit?
Ecologists have a number of ideas, from avoiding bruising, to fending off pests. But to me, the most convincing answer emerged from the work of the tree physiologist John Roden.
He installed hundreds of light sensors at different levels of the canopies of two species of aspens — the trembling aspen with its fluttery leaves and another species with fixed leaves. He documented that sunlight penetrated more evenly and more deeply into the tree crowns of trembling aspens. That resulted in a significantly greater total amount of energy produced and stored by the tremblers.
The next time you walk through an aspen grove, take a moment to lie down beneath them and look up at their quivering crowns. What do you see? I see both a system that has evolved to capture energy efficiently, and thousands of arboreal butterflies about to take flight!
