A national-scale assessment of the main non-native tree species invading forest categories in Italy

In Europe, many non–native tree species have been introduced and, in some cases, have spread outside the planting areas causing impacts on biological and landscape diversity. Recent studies have focused on the distribution of invasive non–native trees in different natural and semi–natural habitats. However, there is a lack of studies considering their distribution among forest categories, units of classification of forest stands useful for their management. In this work, the forest categories currently or potentially undergoing invasion by invasive non–native tree species were identified for Italy. Based on published works, dedicated databases, and a standard evaluation method, the level of invasion by each non–native tree was assigned to each forest category. The most invaded forest categories were riparian, chestnut, and oak–hornbeam woodlands as well as the Mediterranean maquis. The forest categories most at risk of future invasion by non–native trees were oak, beech and pioneer forests. Tree of heaven and black locust invaded the greatest number of forest categories. Whereas, cherry laurel had the highest number of potentially invasible categories, representing an important future threat to Italian forests. Our results can provide support to forest management aimed at preventing invasions or mitigating their negative impacts on forest ecosystems.