Biophysical Modeling of Mangrove Seedling Establishment and Survival Across an Elevation Gradient With Forest Zones

Mangrove forest development critically depends on the establishment and survival of seedlings. Mechanistic insights into how water levels, waves and bed level dynamics influence the establishment process of individual mangrove seedlings are increasing. However, little is known about how spatial and temporal changes in water levels, waves and bed level dynamics across elevation gradients in mangrove forests facilitate/limit seedling dynamics. For this study, a new seedling establishment and growth model was integrated into a process‐based hydrodynamic and morphodynamic numerical model. This biophysical model was applied to a fringing mangrove forest located in the southern Firth of Thames, Aotearoa, New Zealand. This study quantifies the increasing establishment density and survival probability of mangrove seedlings from the lower‐elevated unvegetated intertidal flat toward the higher‐elevated mature mangrove forest. Three cross‐shore zones with distinctive seedling dynamics were identified: (a) a zone with daily tidal inundation where seedling dynamics are episodic and limited by the dispersal of individual propagules that rapidly anchor to the substrate by root growth, (b) a zone with daily to bi‐weekly tidal inundation where seedling dynamics respond to variations in spring‐neap tidal cycles and, (c) a zone with less than bi‐weekly inundation where seedling dynamics are governed by high propagule supply and seedling survival probability. The seedling establishment density and survival probability are dominated by annual extremes in tidal hydroperiod and bed shear stresses, respectively. The obtained parameterizations can be used to incorporate seedling dynamics in decadal‐timescale mangrove forest development models that are instrumental for mangrove management and restoration. Plain Language Summary The growth of mangrove forests along (sub)tropical coastlines is dependent on the establishment and survival of mangrove seedlings. The seedling establishment process is influenced by tides and waves that occur along these coastlines. Previous studies have considered the conditions under which mangrove seedlings can establish and survive, but little is known about how these conditions vary along coastlines, and how these variations affect seedling establishment and survival near and inside mangrove forests. For this study, a computational model was developed that simulates how tides and waves vary from the sea toward a mangrove forest in Aotearoa New Zealand