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

Abstract

The importance of downy oak as an integral component of the "submediterranean" woods has been underscored by many studies. Nevertheless, terms like "submediterranean" and "downy oak" are some of the most poorly understood concepts in European phytogeographic and taxonomic research. Downy oak is well known to be a problematic taxon. The name "Quercus pubescens" (= Q. humilis) combines populations characterized by increasing phenotypic and genomic polymorphisms along north-south gradients, which is explained as the result of a "founder effect" produced by a relatively fast post-glacial re-colonization of the northern areas through rare long-distance dispersal events. On the other hand, polymorphisms of downy oak in the south provide 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, which have been enhanced by recent deforestation. Because downy oak occurs widely throughout 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 of 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 test. We assessed that: • the ecological amplitude of downy oak along the Italian peninsula increases southward; • the maximum variance in ecological conditions is found in Sicily, where the morphologic variability of downy oak is also maximized and where potential competitors, like Quercus frainetto, Q. trojana, Carpinus orientalis, and others, are missing; • discontinuities in the distribution/prevalence of morphologic traits of Q. pubescens (regarded here as a species complex) are not determined by sharp ecological or geographical gaps but instead reflect patterns of selection and phenotypic variability in key traits of the geographical range; • the Ellenberg T and U indicator values for the flora of Italy are correlated well with temperature and precipitation.