<![CDATA[Pine resin is an important non-timber forest product in Indonesia; however, the productivity of Pinus merkusii tapping remains relatively low and has not yet met industrial demand. This study evaluated the effect of a sulfuric acid–modified ETRAT stimulant on pine resin productivity under operational tapping conditions in Perum Perhutani forests in Central Java, Indonesia. Resin tapping was conducted on 160 trees distributed across four age classes (AC III–VI) using a randomized block design with four stimulant treatments: ETRAT, ETRAT + 5% H2SO4, ETRAT + 10% H2SO4, and a control without stimulant. Resin yield was monitored over 10 consecutive tapping periods, with intervals of 3 days. Resin productivity exhibited temporal variation across tapping periods, with generally lower yields during the initial tapping periods, increasing toward the intermediate periods, and declining slightly thereafter; however, differences among tapping periods were not statistically significant. Stimulant treatment significantly affected resin productivity. The highest productivity was achieved with ETRAT + 5% H2SO4 (11.29 g/quarre/day), representing an approximately 78% increase compared to the control (6.34 g/quarre/day). The addition of 10% H2SO4 did not further improve productivity, suggesting potential physiological stress associated with excessive acidity. Productivity also differed among age-class blocks, with the highest values observed in AC V and AC VI, although these differences likely reflected combined stand-level effects, including stand density and site conditions. These findings demonstrate that low-concentration sulfuric acid modification can enhance the operational performance of organic stimulants by improving ethylene-mediated resin flow while maintaining tree physiological tolerance. These results provide practical implications for improving pine resin productivity in tropical production forests under operational management conditions. Keywords: age class, ETRAT, pine resin productivity, stimulants, sulfuric acid]]>
