This Avicennia alba seedling must cross 3 definable rootlength thresholds in order to become successfully established.

Intertidal wetlands such as saltmarshes and mangroves provide numerous important ecological functions, though they are in rapid and global decline. To better conserve and restore these wetland ecosystems, we need an understanding of the fundamental natural bottlenecks and thresholds to their establishment and long-term ecological maintenance. Despite inhabiting similar intertidal positions, the biological traits of these systems differ markedly in structure, phenology, life history, phylogeny and dispersal, suggesting large differences in biophysical interactions. By providing the first systematic comparison between saltmarshes and mangroves, we unravel how the interplay between species-specific life-history traits, biophysical interactions and biogeomorphological feedback processes determine where, when and what wetland can establish, the thresholds to long-term ecosystem stability, and constraints to genetic connectivity between intertidal wetland populations at the landscape level. To understand these process interactions, research into the constraints to wetland development, and biological adaptations to overcome these critical bottlenecks and thresholds requires a truly interdisciplinary approach.

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Mangroves have been suggested as an eco-defense strategy to dissipate tsunamis, storm surges, and king tides. As such, efforts have increased to replant forests along coasts that are vulnerable to flooding. The leafy canopies, stems, and aboveground root structures of mangroves limit water exchange across a forest, reducing flood amplitudes. The attenuation of long waves in mangroves was measured using cross-shore transects of pressure sensors in two contrasting environments in New Zealand, both characterized by mono-specific cultures of grey mangroves (Avicennia marina) and approximate cross-shore widths of 1 km. The first site, in the Firth of Thames, was characterized by mangrove trees with heights between 0.5 and 3 m, and pneumatophore roots with an average height of 0.2 m, and no substantial tidal drainage channels. Attenuation was measured during storm surge conditions. In this environment, the tidal and surge currents had no alternative pathway than to be forced into the high-drag mangrove vegetation. Observations showed that much of the dissipation occurred at the seaward fringe of the forest, with an average attenuation rate of 0.24 m/km across the forest width. The second site, in Tauranga harbor, was characterized by shorter mangroves between 0.3 and 1.2 m in height and deeply incised drainage channels. No attenuation of the flood tidal wave across the mangrove forest was measurable. Instead, flow preferentially propagated along the unvegetated low-drag channels, reaching the back of the forest much more efficiently than in the Firth of Thames. Our observations from sites with the same vegetation type suggest that mangrove properties are important to long wave dissipation only if water transport through the vegetation is a dominant mechanism of fluid transport. Therefore, realistic predictions of potential coastal protection should be made prior to extensive replanting efforts.

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