Settling aerodynamics is a driver of symmetry in deciduous tree leaves

Leaves shed by deciduous trees contain 40% of the annually sequestered carbon and include nutrients vital to the expansion and health of forest ecosystems. To achieve this, leaves must fall quickly to land near the parent tree - otherwise, they are lost to the wind, like pollen or gliding seeds. However, the link between leaf shape and sedimentation speed remains unclear. To gauge the relative performance of extant leaves, we developed an automated sedimentation apparatus capable of performing approximately 100 free-fall experiments per day on biomimetic paper leaves. The majority of 25 representative leaves settle at rates similar to our control (a circular disc). Strikingly, the Arabidopsis mutant asymmetric leaves1 (as1) fell 15% slower than the wild-type. Applying the as1-digital mutation to deciduous tree leaves revealed a similar speed reduction. Data correlating shape and settling across a broad range of natural, mutated and artificial leaves support the fast-leaf hypothesis: deciduous leaves are symmetric and relatively unlobed partly because this maximizes their settling speed and concomitant nutrient retention.