Assessing Forest Cover Change in West Sumatra’s Social Forestry Areas Using Vegetation Index Comparative Analysis (2014-2024)

Abstract

Indonesia is facing rapid deforestation, which makes it rank third globally in net forest loss. Social forestry, a community-based forest management approach that grants local communities secure tenure and management rights, has been promoted as a key conservation strategy. However, its environmental effectiveness remains understudied. Most research focuses on socio-economic outcomes rather than quantifiable forest cover impacts, creating a significant knowledge gap in understanding whether social forestry delivers measurable environmental benefits. This study addresses this gap by assessing to what extent social forestry has influenced forest cover change in West Sumatra from 2014 to 2024, comparing vegetation trends within social forestry areas against the whole province. Using Google Earth Engine (GEE), this research analyzed Landsat 8 imagery to calculate three complementary vegetation indices (VIs): Normalized Difference Vegetation Index (NDVI) for vegetation density, Enhanced Vegetation Index (EVI) for high-biomass sensitivity, and Normalized Difference Fraction Index (NDFI) for forest degradation detection. Time-series analysis compared annual mean values of these VIs, calculated from the median composite image. The changes between 2014 and 2024 were then mapped and visualized using QGIS, providing spatial data for effective monitoring and reporting of SDG 15.2.1 progress. This assessment focused on changes between social forestry areas covering 260,278 hectares and the entire West Sumatra region, with social forestry boundaries obtained from Forest Watch Indonesia through 2022. The findings demonstrate that social forestry areas consistently maintained higher baseline VIs compared to West Sumatra overall, with mean NDVI ranging 0.826-0.854 versus 0.794-0.819 regionally. Over the decade, social forestry areas showed positive trends across all indices, with increases of +0.016 NDVI, +0.005 EVI, and +0.008 NDFI, while regional trends were stable or declining. Spatial analysis revealed that areas outside social forestry boundaries experienced more widespread negative vegetation changes, indicating that social forestry areas provided relative protection against regional degradation trends. However, the Sungai Aur sub-district emerged as a consistent degradation hotspot across VIs. Among the VIs, NDFI proved particularly effective in highlighting the subtle difference in forest degradation and structural integrity, offering superior insight for monitoring these specific impacts. These results provide quantitative evidence supporting the environmental effectiveness of social forestry policies in West Sumatra, demonstrating measurable forest conservation benefits. The methodology contributes to SDG 15.2.1 monitoring by offering a reproducible, open-access approach for assessing sustainable forest management outcomes using satellite data, supporting continued investment in social forestry as an effective strategy for forest conservation and sustainable development.