Drag reduction by leaf aquaplaning in Hexastylis (Aristolochiaceae) and other plant species in floods

Abstract

Leaves of a forest-floor plant, Hexastylis arifolia, were subjected to rapid water movement at shallow depths to approximate sheet-flow conditions during flooding. When held <20 mm beneath the water's surface by the basal ends of their petioles, leaves surfaced, apparently through hydrodynamic lift, and aquaplaned with very low drag relative to surface area. When held >20 mm below the water surface, leaves curled into cones that had higher drag. Both measured and extrapolated drag of aquaplaning and submerged leaves were well below petiole breaking strength. Leaves of some other herbaceous plants that might occasionally be exposed to sheet flow behaved similarly; most had blades with upwardly curved basal lobes and fairly long petioles. Tree leaves with similar basal lobes and long petioles did not aquaplane. The biological significance of low drag during aquaplaning remains uncertain because petiole strength and root strength should be enough to hold the plant in place during sheet flow even without aquaplaning. Moreover, flooding with sheet flow is a rare event for most of these plants. © 2006 by The North American Benthological Society.