Rubiaceae Flora of Northern Sierra Madre Natural Park, Isabela, Luzon, Philippines: Species Richness, Distribution, and Conservation Status

Abstract

The Northern Sierra Madre Natural Park (NSMNP) in Isabela, Philippines holds the record of being the widest remaining tropical rainforest on the island of Luzon. However, this park remains underexplored in terms of its flora. A few botanical studies have been conducted but were particularly focused on the general floristic diversity of the area. No botanical studies have been done yet concentrated on a particular plant family. Species conservation and protection could be more resounding if attention is given to a particular group of plants like the very diverse Rubiaceae family. Hence, an initial checklist of Rubiaceae species in NSMNP is provided with accounts in their richness, distribution, and conservation status. The botanical exploration was conducted in April–August 2019 in NSMNP, specifically in the coastal areas – namely, Palanan, Divilacan, and Maconacon – and a non-coastal zone, Cabagan. In total, 52 Rubiaceae species were recorded belonging to 23 genera and representing 15 tribes of the family. The number of species constitutes about 9.72% of the total number of species in the country. The most specious among the tribes belong to Psychotrieae (9), followed by Spermacoceae (8), Naucleeae and Urophylleae (6), and Coffeeae (5). Of these species, 31 are Philippine endemic, while two are considered narrow endemic to the province. As regards their conservation status based on the International Union of Conservation of Nature (IUCN) list of threatened species, one was assessed as critically endangered (CR) and another one as endangered (EN). Further, two more species were recognized as highly threatened (HT), being CR species, and two other species were assessed as vulnerable (VU) based on the Updated National List of Threatened Philippine Plants and Their Categories found in the Department of Environment and Natural Resources (DENR) Administrative Order No. 2017-11.