Floristic Inventory, Diversity, and Community Structure of the Riparian and Coastal Sand Dune Landscapes in the Lower Padsan River Basin, Laoag City, Ilocos Norte, Philippines

Abstract

Padsan River basin, the largest river system in Laoag City, Ilocos Norte, encompasses two vital ecological landscapes: coastal sand dunes and riparian areas. Despite its ecological importance, the floristic diversity of these landscapes still needs to be better understood and documented. A vegetation assessment was conducted to characterize the composition, diversity, and community structure of plants in the lower Padsan River landscapes. Eighteen 10 m x 10 m nested quadrats were established across six sampling stations. The study recorded 82 species of vascular plants from 80 genera and 41 families. Fifty-two percent are introduced species, 48% are native species, and 27% are invasive species. The plant life forms are composed of trees (34 spp.), herbs (17 spp.), vines (13 spp.) and grasses (9 spp.). Fabaceae and poaceae were the most species-rich plant families. Prosopis juliflora (Candaroma) was the most dominant tree species, comprising 29% of the entire population. The lower Padsan River basin landscapes exhibits moderate tree diversity (H’ = 2.7), with Shannon diversity indices ranging from 0.89 to 2.43 across sampling locations. The riparian areas showed higher tree Shannon diversity, richness, and evenness, while the coastal sand dune areas had higher tree dominance but lower Shannon diversity. The plant community structure varies between the two types of ecological landscapes (ANOSIM Global R=0.7, p<0.05). P. juliflora dominates the coastal sand dune, while a mix of the Leucaena-Pithecellobium-Terminalia community characterizes the riparian area. This study provides essential baseline information on the floristic diversity and community structure, serving as a foundation for developing an integrated river management plan and strategies for conserving important ecological landscapes in the downstream section of the Padsan River Basin.