Towards the assessment of biodiversity and management practices in mountain pastures using diagnostic species?

Diversity of plant communities in semi-natural grasslands is strongly dependent on management practices. To characterize community diversity and agricultural practices, we need easy-to-use indicators. The diagnostic species are used to characterize and differentiate vegetation types, but are they useful to assess the various facets of community diversity and the agricultural practices applied to managed grasslands? Based on the fidelity analysis of 311 phytosociological releves, considered as references for mesic grassland classification in the Franche-Comte region (eastern France), we identified eight diagnostic species of mesic pastures (Cynosurion cristati). We then compared the number of diagnostic species observed in 45 mountain pastures with taxonomic, phylogenetic and multi-trait functional diversity indices, as well as with agronomic variables describing stocking rate and fertilization. To account for the role of diagnostic species identity, we compared Spearman rank correlations obtained for these diagnostic species with correlations calculated for 1000 random samples of eight species from the regional species pool of mesic grasslands. The number of diagnostic species in the grassland plot was significantly positively correlated with most taxonomic, phylogenetic and functional diversity metrics. However, only the positive correlations with Simpson taxonomic diversity and Rao functional diversity were significantly different from those obtained from random samples. Moreover, the number of diagnostic species was significantly negatively correlated with stocking rate and fertilization intensity, although only correlations with industrial fertilizer input were significantly different from results obtained from random samples. These results show that the number of diagnostic species observed in a pasture may be a good indicator of a high taxonomic and functional diversity and of a low fertilization. Thereby, a special attention should be given to diagnostic species to assess the status of grassland biodiversity, including functional aspects.