Alien Plants in the Hortus Botanicus Karalitanus (HBK): Current and Future Threats to the Biodiversity of Sardinia, Italy

Botanical gardens have historically introduced alien species for agronomic, medicinal, and ornamental purposes, but they also contribute to plant invasions. The Hortus Botanicus Karalitanus (HBK) is a historic botanical garden stablished in 1866, as an acclimatisation arboretum for tropical plants, by the University of Cagliari. This study inventoried alien vascular plants in the HBK that showed some degree of spontaneity, analysing their status, origin, life form, introduction pathways, reproductive strategies, and presence in Sardinian habitats. A focus on invasive species and their historical introduction based on their first records on the island and in the HBK was made. Field surveys from 2015 to 2024 allowed us to identify 146 alien taxa, primarily neophytes (83%), 45% of which were naturalised and 12% were invasive. Ornamental plants contributed to 70% of introductions, while accidental taxa (weeds and hitchhikers, 61%) were the most invasive. Seed reproduction was the most common way of propagation (41%). About the diffusion in Sardinia, 25% of taxa had no records in nature outside the HBK, 39% were found only in anthropogenic habitats, and 36% in natural habitats. Among them, 8% were invasive in both the HBK and Sardinia, with wetlands being the most affected (5%). However, only 3% of common invasive species were first recorded in the HBK. The results highlight the role of the HBK in alien species spread and early detection, aiding in invasion management and biodiversity conservation in Sardinia.