Habitat characterization of two Pinguicula species (Lentibulariaceae) in the western Alps

Background and aims – The habitat conditions and population characteristics of the Italian members of the genus Pinguicula are presently still scarcely known. In this study we carried out a comparative research on two butterwort species occurring in the same mountain area, but very different in their distribution, the endemic of the western Alps Pinguicula arvetii and the circumboreal P. vulgaris. Methods – We sampled 36 stands of P. arvetii and 29 stands of P. vulgaris with squared plots of 0.25 m2. In each plot we collected data on geology, topography, hydrochemistry and soil, made a complete inventory of plant species, used for calculating the Ellenberg indicator values, and assessed population density and proportion of flowering individuals. Moreover, we calculated indices of size of vegetative and reproductive structures. We compared the environmental characteristics of sites occupied by the two target species and performed ordinations to assess differences in habitat conditions and to analyse the relationships between characteristics of the two butterworts along environmental gradients. Key results – The two butterworts showed significant differences in site elevation and in most of the ecological indicator values. Consistently, the analyses of species composition of stands revealed that the endemic P. arvetii occurred more commonly at higher-elevation habitats, characterized by a greater incidence of species typical of snowbeds, screes and sandy and skeletal poor grasslands. Individual and population characteristics differed in their position along the ordination axes, with a negative relationship between population density and proportion of flowering individual in both the Pinguicula species. Conclusions – The main environmental variable discriminating sites occupied by the two butterworts was the elevation. Growth and reproductive performances of the two Pinguicula species were influenced by site conditions, but environmental gradients differently affected individual and population characteristics of the two target species.