Survival of young planted mangroves in a calm bay environment during a tropical cyclone

Abstract

The efforts of mangrove plantations are not always successful, and the reasons for their failure remain unclear. In this study, the growth of planted mangroves (Kandelia sp.) was monitored in a mangrove forest on Amami Island, Japan, to investigate their survival over the first three years. A strong typhoon passed near the island 16 months after planting. Although the mangroves were planted at the frontal edge of the intertidal zone, most of young mangroves survived without visible damage. The external forces acting on the plants, such as wave forces, storm surge forces, wind drag forces, and oscillatory acceleration, were estimated by applying an integrated analysis model for tropical cyclones, storm surges, wind waves, and plant kinematics. These analyses yielded three main results. First, although large waves over 10 m in height were generated offshore, the mangrove forest was located in the innermost part of the bay, resulting in considerable wave attenuation. Second, the wave period in the bay was much shorter than that in the open ocean, which may have promoted resonance between waves and mangrove plants. The oscillatory acceleration of the young mangroves was estimated to have approximately the same magnitude as that of the gravitational force. Third, the young mangroves could withstand a fairly strong typhoon, but this may have been fortuitous because the timing of the adverse wind wave conditions coincided with low tide. It is estimated that if the tides had been higher, the oscillatory acceleration would have been five times that of the gravitational acceleration. Although this study only presents examples of specific mangrove species in a particular location, the findings can be used to improve the attempts to restore mangrove forests and thereby protect coastal settlements from the adverse effects of tropical cyclones.