Beyond the mangroves: a global synthesis of tidal forested wetland types, drivers and future information opportunities

Jeffrey Kelleway , Gregory B Noe, Ken W Krauss, Laura Brophy, William H Connor, Jamie A Duberstein, Daniel A. Friess, Keryn Gedan, Elliott White Jr, Maria Fernanda Adame, Janine Adams, Rafael Carvalho, Alei Freddie, Igu Nwabueze Ikenna, Elsie Rivera Ocasio, Christopher J Owers, Sigit Deni Sasmito, Andrew Swales, Phoebe Stewart-Sinclair, Raymond D. Ward, Inigo Zabarte-Maeztu

There is increasing awareness of the global diversity of tidal forested wetlands (TFWs) and their significance in the provision of ecosystem services. These ecosystems, including mangrove forests, tidal freshwater forested wetlands, supratidal forests and transitional forests together span multiple climatic zones, geomorphic settings, and inundation and salinity regimes. We utilise case studies across five continents to demonstrate the state of knowledge among TFWs. Intertidal mangroves are the best-defined of the TFWs thanks to decades of research on their geomorphology, hydrology and ecology across their broad distribution. Non-mangrove forest settings, however, demonstrate more diverse hydrological, biochemical and vegetation conditions. In many cases, non-mangrove forests are situated at upper intertidal or supratidal elevations, where surface waters and groundwater are subject to interactions between tides freshwater inputs. Salinity datasets show variations ranging from tidal freshwater forested wetlands and ‘low-salinity mangroves’ to mesohaline or marine salinities, often with high temporal variability. While the floristic composition of non-mangrove forests vary among biogeographic regions, locally dominant TFW species are commonly distributed beyond the tidal niche into non-tidal wetland and upland forests. This presents challenges for traditional remote sensing approaches to ecosystem mapping, which are mostly lacking for non-mangrove forests. Geomorphic approaches and developments in machine learning offer opportunities to address this.

The landscape position and forested structure of TFWs supports provision of timber, fuel, foods and other culturally important products, as well as maintenance of aquatic and coastal services and greenhouse gas regulation. Growing evidence of these ecosystem service values can motivate arrest and reversal of historic and contemporary TFW losses. Major knowledge gaps regarding the roles of tidal processes and biophysical controls – and the implications of sea-level rise and climate change – could be addressed to maintain these ecosystem services given contemporary and extensive historic losses of TFW distribution. This gap is particularly significant in major river deltas (including the Amazon and Niger) and lowland peat swamp forests of Southeast Asia. Continued collaboration across diverse settings, and the incorporation of non-mangrove forests into mangrove and blue carbon initiatives presents new opportunities for improved outcomes for all TFWs across local to global scales.