Hydrological heterogeneity rather than water chemistry explains the high plant diversity and uniqueness of a Pyrenean mixed mire

Este artículo contiene 18 páginas, 4 figuras, 4 tablas, 3 apéndices.In the Alpine mountains, mire systems are found in the form of small, scattered landscape units. Nevertheless, they maintain noticeably high diversity in terms of plant specialists and community types. A unique, species-rich example in the central Pyrenees, Bassa Nera, is analysed here by describing the major gradients that drive species composition and their relationship with abiotic drivers. We studied the specific vegetation composition, water chemistry and seasonal dynamics of the water table at 35 sampling points. The floristic data confirmed the uniqueness of Bassa Nera in relation to similar Pyrenean mire systems, both in terms of total species richness and regionally rare mire specialists. We distinguished seven plant community types and identified the depth to the water table as the principal driver of the variation in species composition. The water chemistry determined smaller composition shifts, since variations in pH and cationic contents were moderate. Flooded areas were species poor whereas soligenous sub-alkaline fens supported the highest numbers of species. However, the harshest mire habitats (floating Sphagnum carpets and Sphagnum hummocks) stood out as they included the highest number of rare plants within relatively species-poor assemblages. Therefore, the high species richness and uniqueness of the Bassa Nera system mirrors a combination of a wide range of hydrological conditions and a moderate variation in water chemistry. This case study reveals how a few mire systems stand out as exceptional sites. Thus, in Alpine ranges a thorough survey on mire vegetation is needed to select and effectively protect these threatened relict ecosystems.This research was funded by projects REN2002-04268-C02-01 of the Spanish Education and Science Ministry and 634S/2012 of the Spanish Organismo Autónomo Parques Nacionales. A. P.-H. is grateful for the support provided by a grant given by AGAUR of the Catalan Government.Peer reviewe