Geomorphological control on mangrove height and factors arresting their development

Mangroves reach maximum height when substrate conditions are optimal for extended periods. A sudden change in canopy height along a shore normal profile can create a 'wall-like' feature, where tall forest is bounded by what is known as a 'height discontinuity.' Considering mangrove forests at a national scale, this study uses LiDAR-derived canopy height models to identify height discontinuities across Australia, the species responsible and potential causes. Notable height discontinuities were identified at the West Alligator, Leichhardt and Starcke Rivers and Port Douglas, where tall mangrove zones dominated by Rhizophora were located at the highest elevations of the intertidal slope and tree heights decreased sharply by similar to 15 m at the landward margin. Shorter landward forests, dominated by Avicennia and Ceriops, were at the highest intertidal positions. This discontinuity marks a boundary in forest structure and a shift in substrate elevation and tidal inundation. Progradation occurred at some sites seaward of the tall forest suggesting ample lateral accommodation space. Analyses of normalized difference vegetation index (NDVI) across 36 years revealed highest NDVI in tall Rhizophora zones. Dead Rhizophora or sparse mangroves were observed at the landward margin of the height discontinuity. Our findings highlight that height discontinuities mark zones of maximum biomass storage, with transition to shorter trees, sparse forests and dieback zones likely due to sub-optimal conditions driven by sedimentation and tidal inundation patterns. These results underscore the complex interplay between species, sedimentation rates, long-phase tidal cycles and climatic events in influencing mangrove growth, dieback, and carbon sequestration.