The Mediterranean tree Acer monspessulanum invades urban greenspaces in Berlin

Because biological invasions by some introduced tree species pose a major threat to biodiversity, early detection of invasion risks is important for managing existing and future plantings and mitigating negative impacts of invasions. Acer monspessulanum is a European tree species with a large Mediterranean and sub-Mediterranean range. Due to its high drought resistance, it is considered well adapted to climate change and a promising future tree for urban plantings. This study aimed to determine whether invasion risks are associated with plantings in cities outside the species’ natural range. Rare old plantings of A. monspessulanum in Berlin, Germany, were used as a model to investigate whether urban plantings can be invasion foci in cities with a temperate climate. For this purpose, the surroundings of cultivated trees were examined with regard to natural regeneration and the number and height of naturally regenerated individuals and their distance from the parent tree were determined. Acer monspessulanum started to spread 273 years after the first cultivation in Berlin. Each of the sampled four plantings had local spontaneous populations, mostly colonizing loose, semi-shaded anthropogenic hedges and forest patches. A total of 814 spontaneous individuals were detected, with a maximum height of 4.5 m. The maximum distance to the next parent tree was 106 m. However, most individuals grew below or close to the canopy of parent trees. The results indicate that increased planting of A. monspessulanum can induce invasion processes in cities beyond its native range. However, negative invasion impacts from urban plantings are not to be expected in cities with similar environmental conditions as Berlin. Therefore, the species is not considered invasive. Despite a decades-long spread period, the spontaneous populations were confined to the adjacency of propagule sources and the invaded urban greenspaces had a low conservation value. As a positive effect, natural regeneration of A. monspessulanum in such settings could increase the resilience of urban forest patches to climate change. However, further spread should be monitored and plantings near rocky sites with dry grasslands of conservation concern should be avoided.