Chapter 1 - Biogeomorphic evolution and expansion of mangrove forests in New Zealand’s sediment-rich estuarine systems

Abstract

Mangrove forests (Avicennia marina) occupy New Zealand's northern estuaries, which range in size from 1 to 750km2. Contrary to the global pattern of forest loss, areal expansion of New Zealand's mangrove forests has averaged 4%yr−1 since the 1930s. These modern mangrove forests have colonized intertidal flats as they have progressively accreted above mean sea level (MSL), following catchment deforestation (mid–late 1800s) and soil erosion resulting in 10-fold increases in estuary sedimentation rates. The fate of these mangrove ecosystems under rising sea levels depends on maintaining intertidal sedimentary environments. Data from 18 estuaries show that potential mangrove habitat (i.e., intertidal area above MSL) is well predicted by an exponential relationship with the ratio of catchment annual sediment load to estuary tidal-prism volume (r2=0.69, P<.001). Coastal embayments have the smallest areas of intertidal habitat above MSL due to limited fluvial sediment supply. Barrier-type estuaries have the largest intertidal areas above MSL and have also experienced the largest increases in mangrove forest habitat. A case study is presented from the Firth of Thames, where mangroves have colonized some 11km2 of rapidly accreting intertidal flat since the early 1960s. Seedling recruitment is largely governed by tidal and stochastic variations in water level and substrate disturbance by waves. Sediment delivery to the mangrove forest is controlled by spring-tide inundation coupled with onshore winds that resuspend intertidal muds. Surface-elevation gain is constrained by sediment desiccation and compaction during the summer. Simulations of future sea-level rise and sediment-supply scenarios indicate that intertidal habitats in small tidal creeks (0.1-km fetch [F]) and estuaries (1-km F) will maintain their elevation relative to MSL at relatively low sediment-supply rates. By contrast, intertidal flats in the largest estuaries (10-km F) will be vulnerable to inundation by rising seas. Sediment-supply rates twofold higher than historically will be required to sustain these larger systems. The potential for landward retreat of mangrove forests will be limited by storm-defence infrastructure.