Bending or swaying: adaptive strategy of submerged flexible vegetation in the shallow streams

The dynamic motion of submerged flexible vegetation under flow conditions is ubiquitous in aquatic ecosystems. This motion alters flow resistance and affects plant growth, reproduction and evolution. While two-dimensional bending surrogates (e.g., seagrass blades) have been widely used to replicate natural plant motion, vegetation in shallow streams such as ceratophyllum often exhibits complex three-dimensional coherent swaying. Typically growing in gentle, shallow unidirectional flows, such plants rely more on buoyancy than rigidity as an adaptive response—they sway with the flow rather than bend against it.

To testify this hypothesis, we propose a novel non-uniform surrogate conceptualizing such stream vegetation. Laboratory experiments reproduced the three-dimensional coherent swaying behaviour observed in the shallow streams. Results reveal a counterintuitive drag non-increase of the collective coherent swaying in the canopy. A theoretical framework integrates hydrodynamic interactions among flexible plants, sheltering effects, and vortex optimization to explain the observed drag non-increase. Optimal tissue buoyancy, canopy spatial configuration and submergence ratio are derived for the benefit of evolution. The adaptive strategy provides new insights into the trade-offs between rigidity and buoyancy in aquatic vegetation and their implications for canopy function and connectivity.