Cassine koordersii (Celastraceae) is an endemic tree known only from the Puger area in Jember Regency, East Java, Indonesia. In 1998, the tree was categorized as Critically Endangered on the IUCN Red List because of its small geographical range (WCMC, 1998, dx.doi.org/10.2305/IUCN.UK.1998.RLTS.T37405A10050197.en). The tree is known from herbarium collections made in 1898 by Koorders from Watangan Mountains in Puger (Kostermans, 1986, Gardens’ Bulletin Singapore, 39, 188–189). Since 1898 there have been no additional records of this tree. It is currently known only from two ex situ living collections in Bogor Botanic Gardens, which were propagated from seeds of a former mature tree that died in 2003.

To gather data for an updated conservation assessment of C. koordersii we conducted a survey in August–September 2020 in the Watangan Mountains. A total of seven localities were surveyed: from the western extent of the mountains at Puger Watangan Nature Reserve, through the central areas of Igir Pletes, Watu Susu, Maelang, Klatakan and Papuma, to the eastern mountains at Tanggul Asri, over an elevation range of 0–391 m. We were, however, unable to locate C. koordersii. We observed many charcoal production sites in the areas surveyed, and we believe this, together with timber extraction, is the most likely cause of our failure to relocate C. koordersii. In addition, the forest lies on periodically dry soil of weathered coral limestone, susceptible to frequent wildfires that could reduce the survival of C. koordersii.

Based on our findings, we have reassessed C. koordersii as Critically Endangered based on criteria A2c, B1ab(iii)+B2ab(iii) (Possibly Extinct in the Wild) using IUCN Categories and Criteria version 3.1. The species remains assessed under criterion B, as at present, with an area of occupancy and extent of occurrence of 8 km² and continuing decline in the area and quality of the habitat, but for the updated assessment criterion A is also used. Given the threats to the species, which have caused a decline in area of occupancy, extent of occurrence and/or habitat quality, the population size is likely to have decreased by at least 80% in the last three generations. This is inclusive of the original year in which the species was collected. The forest of Watangan Mountains continues to be affected by timber extraction and wildfires, and our updated assessment is an urgent call for the conservation of this endemic species.

Ex situ conservation is in progress for C. koordersii. There have been several attempts to propagate the species from the two living collections, including grafting and shoot cutting. Grafting has been successful, with three of four individuals surviving after 6 months. For shoot cuttings, only five of 28 made produced roots and shoots. While working to improve these propagation methods, we will also trial tissue culture propagation methods. We hope these methods will produce viable seedlings that can be used to support additional ex situ conservation and future reintroduction programmes for the species.