Drivers of Alpine Mire Vegetation at Their Range Limit

Mires are fragile ecosystems in which plant communities are structured by complex interactions among hydrological regimes and groundwater properties. Although extensively studied in boreal and temperate regions, their environmental drivers in southern European mountains remain poorly understood. We investigated five complex mires in the Pyrenees, sampling 156 plots of vascular plants and bryophytes while measuring water table dynamics and groundwater chemistry over two years. Vegetation was classified into six main groups, including acid and alkaline fens, transition mires and Sphagnum hummocks. Ordination analyses (tb-PCA and RDA) revealed that mean water table depth, groundwater calcium and silicon content, and pH were the most important determinants of floristic composition. Bryophytes responded primarily to pH, whereas vascular plants were more influenced by water table variables, reflecting functional trait differences. Despite these environmental effects, spatial structure explained a comparable or greater proportion of variance, especially for vascular plants, underscoring the roles of local species pools, dispersal limitation, and site history in shaping community patterns. Establishing a reliable baseline is crucial for interpreting the distribution patterns of mire vegetation. Our results demonstrate that both environmental gradients and spatial processes are fundamental to understanding mire vegetation and highlight the importance of analyzing plant taxonomic groups separately.