A hydrological switch drives the transition from saltmarsh to peat-forming reedland ecosystems

Understanding how carbon-rich coastal peatlands form is crucial for future carbon storage under environmental change. Phragmites australis (common reed), a key precursor species, facilitates peatland development in saltmarshes by triggering a hydrological switch: creek networks clog through root and organic material accumulation, increasing freshwater retention. Here, we use Holocene peat paleo-records from the Netherlands to reveal past coastal-to-freshwater system transitions consistent with this mechanism. With a numerical biogeomorphic model, we demonstrate how reed expansion reduces creek network complexity, promoting a homogeneous freshwater landscape that further facilitates reed growth. Elevation and vegetation time-series, together with salinity measurements linked to species composition from Saeftinghe (southwestern Netherlands) confirm that reed expansion is associated with creek infilling and a shift in hydrological conditions. Our results identify a strong positive feedback that triggers the hydrological switch accompanied by Phragmites expansion, offering new insight into past peatland establishment and future carbon-rich wetland formation.