Downy oak woods of Italy: phytogeographical remarks on a controversial taxonomic and ecological issue

Abstract

The importance of downy oak as an integral component of the "submediterranean" woods has been underscored in many studies. Nevertheless, terms like "submediterranean" and "downy oak" are some of the most faintly understood concepts in the European phytogeographical and taxonomical research. Downy oak is well known to be a problematic taxon: the name "Quercus pubescens" (= Q. humilis) piles together populations characterized by an increasing phenotypic and genomic polymorphism along north-south gradients, which is commonly explained as the result of a "founder effect" given by a relatively fast post-glacial re-colonization of the northern stands through rare long-distance dispersal events. On the other hand, the southern polymorphism of the downy oak provides evidence for geographic/environmental selection driven by different edaphic conditions along clinal gradients of cold and drought stress, even if the distinction of different species is blurred by systematic hybridization and introgression, enhanced by the recent deforestation. Since downy oak occurs widely all through the Italian Peninsula, we tried to detect some ecological and geographical borders which might be useful to identify climate-vegetation feedback mechanisms as well as to sharpen the syntaxonomical and systematic investigation on such a critical species complex. Our work is based on a well-distributed geo-referenced set of vegetation data, combined with layers of environmental variables (elevation, climate, soil chemistry). The statistical significance of the correlation between vegetation and environmental data has been evaluated through the Mantel's test. Our results suggest that there are some borders in the distribution/prevalence of morphologic traits of "Q. pubescens" (regarded here as a species complex). These borders are not limited by sharp ecological or geographical gaps but instead reflect patterns of selection and phenotypic variability in key traits of the geographical range.