Syntaxonomical and ecological characterization of two main alien forest communities: Robinia pseudoacacia and Ailanthus altissima at their southern limit in Europe

The biological invasions by plant taxa are recognized as one of the main threats to biodiversity, ecosystem services, human health and economy. This research focuses on Robinia pseudoacacia and Ailanthus altissima, two invasive alien tree species, that are widely distributed in Europe and reaches in Italy the southern limit of distribution with a high potential of diffusion. Both species have pioneer characteristics that let them outcompete native species and form dense populations that could be considered as proper forests. Although A. altissima and R. pseudoacacia wide expansion all over Europe and Italy, the threats they impose on native ecosystems and their hypothetical floristic-vegetational autonomy there is a significant lack of knowledge on the ecological behaviour and floristic-vegetational composition in their southern limit of distribution, such as central-southern Italy in sub-Mediterranean and Mediterranean areas. Even if there are some ecological studies in continental Europe, ecological and floristic-vegetational studies of syntaxonomical nature are insufficient. The aims of this research are to broaden the knowledge on these two invasive alien forests presents in central-southern Italy and in the European context, from an ecological and syntaxonomical point of view. The study was carried out in the peri-Adriatic sector of central Italy at altitudes between 10 m a.s.l. and 460 m a.s.l, in a Macrobioclimate ranging from Mediterranean to the Temperate sub- Mediterranean variant. We performed phytosociological relevés and sampling of ecological parameters in R. pseudoacacia and A. altissima forests and in representative and neighbouring native forests that act as elements of comparison and control. Through the comparison to the other central and southern European syntaxa we described 4 new associations (Melisso altissimae-Robinietum pseudoacaciae, Rubio peregrinae-Robinietum pseudoacaciae, Asparago acutifolii-Ailanthetum altissimae, and Aro italici-Ailanthetum altissimae) that belong to the newly describe alliance Lauro nobilis-Robinion pseudoacaciae, included in the Robinietea class. The new alliance Lauro nobilis–Robinion pseudoacaciae brings together the naturalized and invasive alien neoformation forests and pre-forest that are dominated by R. pseudoacacia and have developed in the Mediterranean territories of the Adriatic sector of central Italy and extends into the temperate macroclimate of the sub-Mediterranean variant. In comparison to the other central and southern European alliances is differentiated by the contingent of Mediterranean species, which testify its belonging to the typically Mediterranean biogeographic, bioclimatic and landscape context. We assessed also altered ecological characteristics in paired comparison with native forests. For R. pseudoacacia forests we highlighted that in lower invaded sites (% coverage from 1 to 25%) there is a significant difference in term of the presence of nitrophilous and alien species, Shannon index and changes in Ellenberg indicator values in respect to the Q. pubescens forests of the habitat 91AA*. In A. altissima forests we found that there is a shift in herbaceous layer richness in comparison to the neighboring native forests, with a higher presence of annual ruderal herbs and the absence of herbaceous species linked to the forest environment. Moreover, there is a significant difference in term of total nitrogen, total carbon, and C/N ratio in the soil. The characterization of these alien coenoses at the ecological, bio-geographic, syntaxonomical and landscape levels is an important starting point to understand and manage these novel forest ecosystems