Floristic Composition and Important Value Index of Tree Stratum in Alas Pinggan Mountain Forest Komposisi Floristik dan Indeks Nilai Penting Stratum Pohon di Hutan Pegunungan Alas Pinggan

Abstract

Mountain forest ecosystems harbor complex plant community structures shaped by both biotic interactions and abiotic environmental gradients. However, floristic composition and tree dominance patterns in the Alas Pinggan Mountain Forest area, Ledug Village, Prigen District, Pasuruan Regency, remain undocumented. This study aimed to analyze the floristic composition and Important Value Index (IVI) of the tree stratum in this montane forest ecosystem. Research was conducted on Monday, April 27, 2026, using a systematic plot sampling method across 11 plots of 20×20 m (total observation area of 4,400 m²), targeting trees with a diameter at breast height (DBH) of ≥20 cm. Species identification, DBH measurement derived from stem circumference conversion, and plot-based frequency analysis were performed to model stand structure and species dominance. A total of 90 individual trees belonging to 13 species were recorded. Pinus merkusii emerged as the dominant species with an IVI of 121.12%, contributed by a relative density of 50.00%, relative frequency of 23.53%, and relative dominance of 47.59%. Swietenia mahagoni ranked second as a subdominant species with an IVI of 82.11%. The remaining 11 species, including Syzygium aromaticum, Persea americana, and Ficus septica, were classified as minor or rare species with IVI values below 20%. Mean environmental conditions recorded included air temperature of 26.85°C, relative humidity of 58.45%, and soil pH of 6.13. These findings indicate that the tree stratum of Alas Pinggan Mountain Forest is strongly dominated by Pinus merkusii, reflecting a pine plantation-influenced stand structure within a montane ecosystem. This study was limited to a single sampling day and restricted to mature tree strata (DBH ≥20 cm), suggesting the need for further research covering broader growth stages and temporal variation.