Forest Structure and Threatened Tree Species along Elevation and Disturbance Gradients in a Tropical River Watershed

Understanding the relationship between forest structure, elevation, and species conservation status is vital for developing effective biodiversity and carbon management strategies. The current study examined 27 forest plots within the selected River Watershed in the Philippines to assess how disturbance and elevation influence tree diameter, height, basal area, aboveground biomass, and the occurrence of threatened species. Structural attributes varied widely across plots, with mean tree diameter ranging from 13.9 to 29.2 cm, and biomass spanning from approximately 3,100 kg to over 30,600 kg per plot. Undisturbed plots consistently exhibited higher basal area and biomass, reflecting greater structural integrity and ecological resilience. Conservation status data revealed that plots hosted up to six IUCN-listed threatened species and as many as 15 DAO 2017-11 classified species, with elevated and undisturbed plots supporting the greatest numbers. Regression models confirmed that elevation significantly influenced the richness of IUCN-listed species, while disturbance strongly predicted reductions in both biomass and basal area. Plots above 700 meters above sea level emerged as conservation hotspots, emphasizing the value of upland forests as refugia for threatened taxa. Visual analyses reinforced these findings: undisturbed plots had double the biomass and nearly twice the basal area compared to disturbed counterparts. These results align with regional and global literature, highlighting the urgency of protecting intact forest landscapes. By linking forest structure with conservation indicators, the study provides empirical evidence to inform sustainable forest management and biodiversity conservation in tropical ecosystems under increasing human pressure.