Settling aerodynamics is a driver of symmetry in deciduous tree leaves

TL;DR

This study investigates how leaf shape influences sedimentation speed, revealing that symmetry and unlobed forms promote faster fall, which is crucial for nutrient retention and ecological success in deciduous trees.

Contribution

The paper introduces an automated sedimentation apparatus and demonstrates that leaf symmetry enhances settling speed, providing new insights into leaf shape evolution.

Findings

Symmetric leaves settle faster than asymmetric ones.

Mutant asymmetric leaves fall 15% slower than wild type.

Symmetry in leaves likely evolved to maximize sedimentation speed.

Abstract

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 (ASAP) capable of performing $\sim100$ 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 Arabidopsid 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 thefast-leaf-hypothesis: Deciduous leaves are symmetric and relatively unlobed in part because this maximizes their settling speed and concomitant nutrient retention.