Two new species of Melastoma L. (Melastomataceae) from Limestone Karst, Khammouane Province, central Laos

Two new species, Melastoma calcicolum Souvann. & Tagane sp. nov. and M. souladethianum Souvann. & Nob.Tanaka sp. nov. (Melastomataceae), are described from limestone karst habitats in Khammouane Province, central Laos. Descriptions, photographs, distribution, habitat, phenology, and initial conservation statuses of both newly described species are given. A key to the species of Melastoma in Laos including currently reported species is provided. These discoveries result from extensive fieldwork conducted between 2022 and 2025 in protected areas including Phou Hin Poun National protected area and Phou Pha Marn area. M. calcicolum sp. nov. is distinguished from M. saigonenese by its terete twigs, narrow lanceolate leaves, spaced penicillate emergences on the hypanthium, stalk of emergences on hypanthium shorter, glabrous sepals, and longer styles, while M. souladethianum sp. nov. differs from M. pellegrinianum by its terete branches, small inflorescences with 2–4 flowers, and sparsely bristled hypanthium with strigose hairs. Both species are currently known only from their type localities, occurring in exposed limestone areas, and are preliminarily assessed as Endangered and Critically Endangered based on IUCN criteria due to their restricted distributions and small population sizes. These findings highlight the high botanical diversity and endemism of Laos’s limestone ecosystems and underscore the importance of continued botanical exploration in the region.