Alien Flora of the Himalayan Highlands: Naturalised and Invasive Plants in the Trans-Himalayan region of Ladakh, India

Abstract Biological invasions pose a significant threat to sustenance of biodiversity and ecosystem services, with their incidence expected to rise due to globalization and climate change. Regional inventories of alien species are important for monitoring and managing the biological invasions, particularly in world’s mountains which till recently were immune to invasions but are increasingly becoming vulnerable to invasive species. This study presents the first comprehensive inventory and status assessment of the alien flora in Ladakh – a remote and unique biogeographic region in Indian Trans-Himalaya. Based on field data collected from 2018 to 2023, we provide taxonomic composition of alien flora in Ladakh, including information on invasion status (cultivated, naturalized and invasive), biogeographic affiliation, habitat and life-form characteristics, and pathways of introduction. In total, we recorded 104 alien plant species, accounting ∼ 5.7% to the region's flora. Of these, 24 alien species are under cultivation, while the remaining 80 are aliens growing in wild, with 52 naturalized and 28 invasive. The most species-rich families are Asteraceae (20 species), Amaranthaceae (12 species), Poaceae (7 species), Salicaceae (6 species), and Fabaceae (5 species). Most alien species in the region are native to temperate Asia (34 species), followed by Europe (30 species). The alien species primarily colonize human-dominated habitats, such as roadsides and agricultural fields. Additionally, we found that the elevational distribution of the alien species richness in the region exhibits a unimodal hump-shaped pattern, with the highest number of alien species (73 species) occurring between 2600–3000 meters. The elevational width and maximum elevation of alien species was positively correlated with the maximum elevation in their native region. In an era of rising risks of climate change and rapid land-use transformation in the Himalayan highlands, this first assessment of the alien flora of the Ladakh will foster further research, inform early eradication, and guide pro-active plant invasion management in the region, with lessons for such environments elsewhere.