Survey for threatened plants in riparian fragmented forests: A case study on three Vatica (Dipterocarpaceae) species in Kapuas Hulu,West Kalimantan

Forest fragmentation potentially increases the extinction risk of plant species. Conservation and effective management of threatened plants in fragmented forests require basic information on their population status. In the present study, we conduct population surveys and provide the most current population status of one Endangered (Vatica rynchocarpa) and two Critically Endangered (Vatica havilandii and Vatica cauliflora) species of Vatica in fragmented riparian forests of Kapuas Hulu, West Kalimantan, Indonesia. Using a focused survey method, we assessed populations of the target species in 13 locations with a total covered distance of c. 26 km. We located 317 individuals of V. rynchocarpa and 568 individuals of V. havilandii of which 31 (10.8 %) and 10 (1.8 %) are mature individuals, respectively. In Kapuas Hulu, the endemic tree V.cauliflora were 191 individuals with only 29 (17.9 %) being in a mature stage. All the individuals were found in an unprotected and relatively flat riparian forest with an elevation range of 30–52 m above sea level. We observed that habitat conversion into rubber (Hevea brasiliensis) and the medicinal plant kratom (Mitragyna speciosa) plantations are the main threat to the species. Further studies are needed in order to understand the effects of land use changes and other factors such as river discharge patters on the survival and population dynamic of the species. In addition, protection of the forest fragments is required to conserve all the target species.