Scientists have long recognized a global pattern in leaf shape between tropical and temperate climates: tropical leaves have smooth margins while plants living in cold regions have leaves with teeth or lobes, like a birch or a maple. The relationship is so strong that paleobotanists use leaf shapes in fossil floras as a proxy for temperature, allowing for estimates of climate change through time. But there is still no mechanistic understanding of why this relationship exists. The PIs hypothesize that the evolution of complex leaf margins in temperate plants results in part from the way that young, immature leaves are packed into overwintering buds. This is inspired by recent mathematical models of efficient leaf folding within buds, and by observations of within-plant differences in leaf form. Viburnum is a lineage of ~170 species of woody shrubs and trees that have shifted between tropical and temperate forests many times throughout the past 50 million years, and exhibit the expected changes in leaf shape. The PIs will document timing of growth, leaf lifespan, and patterns of leaf-packing within buds, and gather additional data on a suite of relevant morphological and physiological traits on a set of Viburnum species spanning the tropical-temperate gradient. They will also continue long-term phenological monitoring of ~30 Viburnum species in a common garden. This is a highly collaborative and interdisciplinary project, involving biologists from four other nations as well as an applied mathematician. The PIs will also develop educational modules designed for elementary school children aimed to bridge the gap between math and biology, using origami and basic geometric principles to understand common leaf shapes.
