From terrestrialisation to disturbance: long-term trajectories of a fen ecosystem and its conservation implications in the Xistral Mountains, NW Iberia

Abstract Peatlands in north-west Iberia are globally important carbon reservoirs, yet their fen communities remain less well understood than those of bogs. Here, we present a multiproxy study of a ~11,100 cal. yr BP peat sequence cored at the Toxiza fen (TOX, Xistral Mountains, Galicia). Our analyses of palynology, rarefaction, charcoal, elemental composition and CIELab colour identify three main ecological phases. First, early Holocene mire initiation through terrestrialisation: wet meadows progressively replaced by a transitional fen at ~10,350 cal. yr BP, and subsequently by a Sphagnum -rich fen. Second, the mire experienced a prolonged phase of hydrological stability during the Holocene Climatic Optimum, characterised by Sphagnum -rich vegetation, high organic-matter content and sustained peat accumulation. Third, late Holocene increasing disturbance, including catchment erosion (~4900-4200 cal. yr BP) and fire-intense phases leading to an Ericaceae-rich fen; with Sphagnum partially recovered in reduced-burning intervals. Phases of ecological transition (i.e., early Holocene terrestrialisation and late Holocene disturbance) showed higher pollen richness, whereas comparatively stable values were found during the mid-Holocene, reflecting long-term ecosystem stability. Comparisons with nearby ombrotrophic mires highlight the importance of hydrological setting (minerotrophy vs. ombrotrophy) in shaping peatland vegetation trajectories under similar climatic forcing. Today the fen shows clear signs of degradation, including fire activity, sediment inputs and afforestation pressure, with no clear Sphagnum recovery. The TOX record, therefore, provides a process-based reference baseline for conservation where the mid-Holocene Sphagnum -rich phase represents a benchmark of hydrological stability and minimal disturbance, offering a framework for understanding biodiversity change and for guiding restoration of minerotrophic peatlands under ongoing environmental change.