Author(s): Julia Mullarney; Karen Fisher

Linked Author(s): Julia Mullarney

Keywords: Mangrove forests; Nature-based solutions; mātauranga Māori; science-policy interface; New Zealand

Abstract: Mangrove forests are highly productive ecosystems, which provide many physical, societal, and ecological services in tropical and subtropical regions. In particular, these environments can function as nature-based defences against damage from waves, storm surge, and tsunamis. Moreover, some forests have demonstrated the capability to trap sediment at a sufficient rate to keep pace with sea-level rise. However, the ability to retain these functionalities, in the face of global sea level rise and changes in sediment supply, depends on the complex interactions between vegetation elements, water flows, and sediment transport. Through a review of observations from different mangrove systems, I outline some of the key challenges of using, and quantifying the efficacy of, mangrove systems as nature-based solutions: 1.Reconciling predictions across spatial and temporal scales. As mangrove forests become inundated, tidal and wave energy is converted into dissipative wake-scale turbulence, through the processes of vortex shedding and eddy generation around the trees’ aerial root systems (pneumatophores). Hotspots of intense turbulence are created, particularly in the fringing regions between mudflat and forest. This spatial variability in vegetation density at sub-metre scales acts as a control on sediment transport across both small and large-scale. In some cases, erosion and accretion have been observed to be correlated with near-bed turbulent kinetic energy, but not mean current speeds. 2.Accounting for site-specific physical characteristics. The ability of mangroves to dissipate energy, and trap sediment has been shown to be highly species- and environment-dependent. For example, wave energy is differentially attenuated across frequencies, depending on the local hydrodynamic forcing and vegetation characteristics. Additionally, features such as channels within forests can provide a conduit for water and sediment movement, leading to different drag relationships from those applicable in more homogenous environments. 3.Reconciling location-specific cultural and societal values, including incorporation of fundamentally different worldviews (such as Te Ao Māori in New Zealand). At present, there is a surge in restoration efforts across the globe aimed at arresting the overarching trend of mangrove decline. However, New Zealand is a notable exception to this trend with rapidly expanding mangrove coverage. This expansion is often negatively perceived by local residents, who associate mangroves with ‘muddification’ of estuaries, lowered value of recreational amenities, restricted navigation, and reduced ecosystem function. Consequently, communities have advocated for mangrove removal, thereby also removing the capability to attenuate waves and tidal currents. Resolution of these knowledge gaps is crucial to improve the design and implementation of nature-based solutions, to assess the sustainability of the services provided, and to enhance the overall resilience of coastal systems.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022SS2227

Year: 2022