Using woody flora to define phytogeographic units. Analysis of a west–east transect in central Italy

To improve the phytogeographic characterization of peninsular Italy, we analysed and compared the spatial distribution of woody flora at both the species and genus level along an approximate 160 km west–east transect within the central peninsula. Sampling sites were selected using a subjective stratified sampling design. A total of 1710 occurrence records, 138 species, and 74 genera from 153 floristic relevés was used. The congruence in species and genera patterns, supported by Mantel and Procrustes tests, confirmed the validity of the higher‐taxon approach to analyse woody flora patterns. Non‐metric multi‐dimensional scaling and Pearson's chi square analysis pointed out that elevation and related bioclimatic factors were the most important drivers of woody species composition. Three well‐defined woody flora types (coastal low‐elevation, inland middle‐elevation, and inland high‐elevation) were identified using partition around medoids clustering. The species‐rich inland middle‐elevation type supports the recently described Transition unit between the Mediterranean and Euro‐Siberian Regions. Contrary to previous assumptions, floristic and chorological differences between the Tyrrhenian and the Adriatic subtypes were detected only in the inland high‐elevation type and not in the coastal low‐elevation type. The adopted methodological approach, based on higher‐taxon approach and on clustering and ordination of floristic relevés, proved to be valuable to detect woody flora distribution patterns and to delineate the phytogeographic characters of a very heterogeneous and biodiverse area.