Vascular plant richness along elevation gradient at Monte Velino (Central Apennines, Italy)

Species richness was studied during two years in 100-m elevation bands along an elevation transect from 1100 m up to 2487 m asl on the southwestern slope of Monte Velino. The 1300- to the 2200-m elevation bands have a similar area in the range of 0.36 to 0.40 km2. A total of 636 taxa (species and subspecies) was recorded. Each species was attributed a life form, an elevation category and a chorotype. The richness per elevation band decreases monotonously along the transect with a plateau between 1300 and 1600 m asl. Because of the similarity of the area of the elevation bands between the 1300-m and the 2200-m elevation band, the richness pattern along the transect is not merely a function of the area. The rejection of the area hypothesis, at least between 1300 and 2300 m asl, is also demonstrated by the species traits. Most of them show a monotonous pattern with elevation, either decreasing or increasing. The cumulative increase of species richness along the transect follows the Arrhenius’ power function S = C x Az with a z value of 0.291. Endemics peak at an intermediate elevation in the 2000-m elevation band and follow the isolation-area hypothesis. In that the species-density matches the adiabatic decrease of air temperature, we consider the latter as the driving factor for the pattern of the species richness in relation to elevation. Colder temperatures (a) reduce the species pool in relation to elevation due to an increasingly colder and shorter vegetation period; (b) drive geomorphological processes that both reduce habitats number and spatial heterogeneity, and enhance their convergence and homogenization (temperature-physiography hypothesis). Therefore, area, temperature and isolation are the three main parameters to explain plant richness with elevation for Monte Velino.