Habitat characteristics and vegetation context of Osmunda regalis L. at the southern edge of its distribution in Europe

The abundance of Osmunda regalis was investigated in 42 natural populations in central Italy in order to (i) test which habitat characteristics correlate with its abundance at regional scale and at local scale, and (ii) identify the optimal habitats for the species in a Mediterranean area. This knowledge may contribute to the effective conservation of O. regalis, which is under threat in Italy. Ramets (rosettes of fronds) were counted in several plots of 25 m2 within each population and related to habitat characteristics as well as plant species composition using univariate and multivariate statistics. The abundance of ramets at regional scale was positively correlated with mean annual precipitation and altitude. Large populations mainly occurred at sites with northeastern exposure, i.e. a cool and humid microclimate. The density of ramets at local scale was negatively correlated with rockiness, stoniness and shrub cover.Vascular plant species associated with high density of O. regalis were pioneer species typical of open woods and stagnant waters. Spring swamps therefore represent the habitat with the most favourable edaphic, microclimatic and vegetational conditions for the species at the southern edge of its distribution in Europe,where the limiting factor is the annual water availability.