Tree invasions in Italian forests

Many forest tree species have been moved outside their native range to provide goods and services elsewhere, but some of them have become invasive, causing negative impacts on biodiversity and human activities. The assessment and knowledge on the degree and scale to which forest ecosystems are invaded by non-native trees is of paramount importance for tailored policies and strategies aiming at forest conservation. By analyzing main databases and literature and applying a four-level scale of invasion (not currently invaded and with low invasibility; potentially invasible; moderately invaded; massively invaded), we assessed the current and potential occurrence of twenty-five invasive non-native trees across forest ecosystems in Italy. Vulnerability to invasion substantially differed across forest categories, with riparian forests being particularly vulnerable to invasion. Acacia dealbata, Juglans nigra, Populus × canadensis and Prunus laurocerasus invaded a relatively small number of forest categories but showed their potential to widen their impact on others. Furthermore, some non-native species showed a recent increase of their invasive status in Italy. This study identified the Italian forest categories and non-native tree species that deserve primary attention, and can support proper management and allocation of funding to protect forests from the spread of invasive tree species.