Towards the pan-European bioindication system: Assessing and testing updated hydrological indicator values for vascular plants and bryophytes in mires

Abstract

Abstract Bioindication systems based on the occurrence of plant species are widely used in vegetation science, palaeoecology, community ecology, geographical modelling and global change biology. Although the existing systems are mostly regional, the development of large-scale vegetation databases calls for the establishment of a pan-European indication system. Here we present the first step towards this goal, by assessing indicator values for soil moisture and water table depth in European mires and associated grasslands. For each vascular plant and bryophyte species occurring in 24,091 vegetation-plot records of European mires, we developed an updated system of Ellenberg-like Ecological Indicator Values (EIVs) at the scale of 1–12 (species optimum) including ecological valences – minimum value (drought intolerance), maximum value (flooding tolerance) and range of values (tolerance), using both statistical and expert-based approaches for species co-occurrence data. Spearman correlation coefficients, Akaike Information Criterion (AIC) and Akaike weights were used to assess the performance of the updated EIVs against measured soil moisture and water table in two Central-European datasets, and against the proportion of aquatic mollusc species in a European-scale dataset. The updated EIVs performed well in all cases, having stronger correlations with directly measured water table and the combined proportion of aquatic and wetland molluscs than the original EIVs. The original EIVs, however, performed better with soil moisture. In the Central-European datasets, the use of minimum values (drought intolerances) of the updated EIVs weighted by species covers and/or tolerances led to the highest correlations with water table in all cases. This improvement of the indication potential on the regional scale is mainly related to the inclusion of bryophytes into the updated EIVs. Our data show that the updated EIVs are beneficial not only when used in the cross-regional or pan-European datasets, but also when water table rather than soil moisture is to be indicated in the regional-scale data, as it occurs frequently in bryological, zoological and protistological research. Overall, this study demonstrates how EIVs can be updated by including previously omitted taxa of vascular plants and bryophytes and adding ecological valences to reflect accurate conditions of natural habitats at the continental scale.