Functional composition outweighs taxonomic and functional diversity in maintaining ecosystem properties and processes of mangrove forests.

Biodiversity loss can have significant consequences for human well-being, as it can affect multiple ecosystem properties and processes (MEPP) that drive ecosystem services. However, a comprehensive understanding of the link between environmental factors, biodiversity, and MEPP remains elusive, especially in mangrove ecosystems that millions of people along tropical coastlines worldwide depend upon. Here, we collated a comprehensive dataset on forest inventory, plant traits, and environmental factors across 93 plots in the Sundarbans Reserved Forests, Bangladesh. The functional composition (FC) of leaf area showed a stronger positive association with MEPP, being determined by total biomass and productivity of the mangroves, sediment organic carbon, and ammonium, phosphorus, and potassium contents of the sediment, than species richness (SR) or functional diversity (FD). Further, FC mediated a strong negative association of sediment salinity, and a positive association of SR, with MEPP. The similar but opposite total associations of SR and sediment salinity with MEPP suggest that species-rich mangroves could offset the negative impacts of rising salinity on MEPP. When focusing on a single aspect of MEPP, both FD and FC mattered, with the FD of leaf area showing a strong association with mangrove productivity and sediment potassium content, while the FC of leaf litter nitrogen showed the strongest associations with sediment ammonium and phosphorus contents. Therefore, to sustain mangrove ecosystems as a reliable nature-based solution for climate change mitigation, conservation and (re-)establishment projects should prioritize regionally dominant species with high leaf area and nitrogen content, plus functionally different species to support the ecosystem processes and services provided by mangroves.