Measuring the biodiversity of saxicolous lichens above timberline with reference to environmental factors: the case-study of a Natura 2000 site of western Alps

Following a random sampling approach, the biodiversity of saxicolous lichens above timberline was measured in terms of variability and abundance in a Natura 2000 site of western Alps (Mont Avic Natural Park, Italy), having different habitats, climates (i.e. exposures and altitudes) and lithotypes. Fifty-eight species colonize about 1/3(rd) of the overall lithic surfaces. Few dominant banal species (21%) enter quite all the environments and account for most of the cover (90%), while demanding species restricted to particular microniches (79%, including seven rarities) account for most of the species variability (Zipf-Mandelbrot model). Multivariate analyses relating communities to environmental features highlighted that lithotypes exert a major influence on the biodiversity, hard-weathering rocks showing a richer colonization than soft-weathering ones. Other environmental features, as altitude, habitat and human Influence, operate at a lower scale, being particularly determinant for lichen variability and abundance on soft-weathering rocks. On hard-weathering rocks, lichen variability mainly depends on the geochemistry and, subordinately, on the heterogeneity of microniches due to microtopography. Such a community pattern suggests that the biodiversity of saxicolous lichens above timberline should mostly benefit front the conservation of areas where different lithotypes occur and, secondly, are widespread distributed with respect to other environmental features, i.e. areas warranting conservation of the widest range of microniches favourable for demanding species, including rarities.