Inundation and salinity regimes support blue carbon conditions in Australian temperate supratidal forests

Supratidal forests are defined by their position relative to the tidal frame where inundation and salinity patterns are potentially influenced by both tidal and nontidal regimes. Despite their recent inclusion in national blue carbon initiatives, knowledge of the processes that influence their carbon storage in supratidal forests remains limited. In this study, we report on new datasets of vegetation structure, carbon cycling parameters, inundation, and salinity patterns across 18 sites spanning more than 4000 km of Australia's temperate coastlines. We report site-specific ecosystem carbon stocks ranging from 169 to 635 Mg C_org ha^-1, with mean aboveground biomass (134 ± 63 Mg DM ha^-1) and belowground carbon stocks to 1 m soil depth (193 ± 98 Mg C_org ha^-1), which are within the range of national estimates for mangrove and saltmarsh ecosystems. While there are variations in vegetation structure between sites dominated by the genera Melaleuca and Casuarina, this does not lead to discernible differences in above- or belowground carbon stocks. Organic matter decomposition trends within supratidal forest substrates were similar to those of adjacent mangrove and saltmarsh, though there were differences among study sites and between labile and recalcitrant tea litters. Soil-atmospheric flux measurements conducted at one site were also within the range of adjacent blue carbon ecosystems. We hypothesize that the high degree of preservation of belowground carbon and low soil-atmosphere flux of greenhouse gases is driven by a combination of infrequent surface inundation, high water tables, and typically saline groundwater in supratidal forests, as measured across multiple settings. Supratidal forests are carbon-rich ecosystems influenced by coastal processes associated with tidal inundation. While further research is required to understand the full distribution, carbon cycling, and abiotic drivers of supratidal forests, our findings strongly support their inclusion in blue carbon and other management initiatives that support the response and recovery of these endangered ecological communities in a time of change.