Habitat diversity of central European fens in relation to environmental gradients and an effort to standardise fen terminology in ecological studies

Mire terminology, subdivision and gradient structure have been subjected to an intense debate intensified in the last years. The conception of Wheeler and Proctor (J. Ecol. 88, 187–203), which divides mires into bogs, having pH<5.5, and fens, having pH>5.5, becomes generally accepted despite a certain critique from the Scandinavian perspective and despite the fact that few contributions to the debate have come from central and southern Europe and from other than botanical disciplines. In this paper, we demonstrated that the bog-fen boundary is clearly determinable not by pH, but by a set of nutrient-requiring species that avoid truly ombrotrophic conditions in central Europe. We therefore defined fens as groundwater-fed wetlands that host low productive nutrient-limited vegetation dominated by Cyperaceae and bryophytes. The fertility gradient within fens, another controversial point in the ongoing debate, is easily distinguishable using both plant and animal data, but it appears primarily within calcium-richer fens. We suggest defining fen grasslands by the high abundance of nutrient-requiring grasses and forbs rather than purely by fen origin and management. Concerning pH/calcium gradient, there is large variation in plant, mollusc, algal, fungal and testacean assemblages within fens with a pH>5.5 in central Europe even though some authors named all mires having such pH ‘(rich) fens’. This clear and extended poor–rich (pH/calcium) gradient in floristical data is independent of the fertility gradient. Conductivity (approximating water mineral richness) contributes significantly to the pH in explaining the vegetation variation. Vegetation composition accounts for a larger amount of the variation in algal, molluscan, testacean and fungal assemblages than even long-term measured environmental data. The chemical limit of the occurrence of any Sphagnum species, even the calcitolerant ones, is the most important and easily recognised natural boundary between major functional fen types, although it varies among regions and hydrological situations. We therefore believe that fen classification based exclusively on floristic data is necessary to avoid circular argumentation and provides the best basis for the characterization of habitats. We propose to subdivide fens into five standard vegetation types with defined boundaries: poor fens (Sphagno recurvi-Caricion canescentis), moderately rich fens (Caricion fuscae), rich fens (Sphagno warnstorfii-Tomenthypnion), extremely rich fens and calcareous fens (both corresponding to Caricion davallianae). Calcareous fens were neglected during the ongoing debate due to its relative scarcity in some traditionally explored regions. Its ecological boundary is the point at which calcium carbonate starts to precipitate, which is connected with marked change in plant and animal species composition. The ecological differentiation of all proposed fen types was tested using a data set from two different regions (Carpathians and Bulgaria). Both conductivity and pH differ significantly between pairs of vegetation types. All proposed fen types also markedly differ in molluscan assemblages.