Anchoring and Root Architecture Influence Hydro‐Morphodynamic Mechanisms of Dislodgement in Mangrove ( Rhizophora mangle ) Seedlings

Abstract This study isolated hydro‐morphodynamic mechanisms leading to mangrove propagule anchoring and seedling dislodgement under erosive flows. Red mangrove ( Rhizophora mangle ) propagules self‐anchored in estuarine sediments over 112 days under simulated tidal pulses. Seedlings were tested under erosive flows at 1 week, 2 weeks, and 1 month after anchoring. One‐month‐old seedlings exhibited notable resistance, withstanding peak velocities of 17–22 cm/s and severe erosion (mean erosion depth 8.3 ± 0.9 cm, volume 1.9 × 104 ± 2.5 × 103 cm 3 ). Seedlings oriented against the flow were more vulnerable to uprooting. Two‐week‐old seedlings oriented with the flow had a greater mean erosion depth (4.4 ± 1.2 cm, p < 0.05) than seedlings oriented upstream (1.5 ± 0.6 cm). Consistent with dislodgement mechanisms proposed in prior studies, the results conformed to three distinct uprooting pathways: instantaneous uprooting by hydrodynamic forces without bed erosion (Type I), removal facilitated by local scour around the roots of seedlings (Type IIa), and removal after large‐scale bed degradation (Type IIb). Erosion depth and volume at the time of dislodgement varied systematically with the geomorphic mechanism of removal. Seedling resistance was closely linked to rooting structure; seedlings uprooted by Type IIb exhibited the longest single root length (mean: 8.6 ± 0.6 cm), highest total root length (mean: 134.4 ± 19 cm), and greatest root volume (mean: 61.1 ± 9.2 cm 3 ).