Supplementary file 1_Untangling the drivers of gross primary productivity in Chinese Fagaceae-dominated forests.docx

Forests dominated by Fagaceae species constitute the largest proportion of China’s total forest area and forest volume, playing a crucial role in carbon sequestration and climate mitigation. However, the variation in gross primary productivity (GPP) across different climate zones and taxonomic genera, as well as its driving factors, remains poorly understood. In this study, we employed variance decomposition, partial least squares structural equation modeling, and piecewise linear regression to identify the drivers of GPP derived by SIF in Fagaceae-dominated forests in China. The results indicated that the GPP varied significantly across taxonomic genera and climate zones. Specifically, the forests dominated by Castanopsis and those located in the south subtropical zone exhibited the highest GPP (6553.5 g C m−2 yr.−1), whereas the Quercus-dominated forests and those in the north temperate zone showed the lowest GPP (1300.9 g C m−2 yr.−1). The observed variation in GPP was primarily attributed to climate features (e.g., MAT, MTWM, PAR) (56.83–81.29%) and forest characteristics (8.06–23.58%), with minimal contributions from soil and topographic factors. The structural equation models further demonstrated that forest characteristics and climate factors directly influenced GPP, while topography and soil factors had indirect effects on GPP through cascading pathways. Additionally, the critical threshold of forest age on GPP varied depending on genus and climate zone, peaking at 90.40 years in the middle temperate zone and 91.00 years in Quercus-dominated forests. The findings improve our understanding of spatial patterns and drivers of GPP in Chinese Fagaceae-dominated forests, and the identification of critical forest age thresholds clarifies productivity shifts across development stages, thereby contributing to a better assessment of national forest carbon sequestration potential.