Impacts of non-native nitrogen-fixing trees in Italy: evidence from the invasion of Acacia s.l. and Robinia pseudoacacia (Fabaceae).

Non-native nitrogen-fixing trees are known to be among the most harmful invasive alien plants (IAPs), although their negative impacts on the ecosystem have not always been assessed in the different parts of the invasive range. The assessment of the impacts of these IAPs appears of relevant interest for an informed prioritization of the management efforts. We aimed at providing evidence on the ecological impacts of the main invasive nitrogen-fixing trees in Italy in a number of habitat types. Accordingly, we started, within the Alien Species Working Group of the Italian Botanical Society, a sampling of several case studies of invasion on the whole national territory, through paired vegetation plots (invaded vs non-invaded) for different target IAPs, invaded native habitats and areas/regions. The sampling design foresees the survey of structure and biodiversity of native and invaded plant communities. We included an assessment of the impacts on the topsoil via profile photos and analysis of the main chemical characteristics (N-C-P and pH) of soil samples (A and O horizons). More than 300 plots have been sampled in 8 regions (Calabria, Lazio, Molise, Apulia, Sardinia, Sicily, Tuscany, and Trentino Alto-Adige) for 5 species all belonging to Fabaceae: Acacia dealbata, A. mearnsii, A. saligna, Vachellia karroo and Robinia pseudoacacia, and further data was collected during spring 2022. We individuated more than 15 different natural vegetation types invaded (including habitats worthy of conservation according to Dir. no. 92/43/EEC). The impacts on biodiversity indices such as species richness, plant abundance and cover of canopy and shrub layer, showed high variability among the different IAPs and habitats, but with a general trend towards a degradation of the native biodiversity. The soil profiles showed relevant patterns of changes in the soil horizon stratification, most likely due to the changed condition of litter accumulation and degradation.