Abstract: Optical greenness indices, such as the fraction of absorbed photosynthetically active radiation (fAPAR), are critical in constraining and guiding the modelling of forest carbon storage. However, as optical sensors have limited penetration capacity, it remains uncertain whether greenness indices accurately reflect the true response of aboveground biomass (AGB) to local climatic conditions. In this study, we integrate a wall-to-wall AGB dataset derived from microwave remote sensing to examine the consistency between AGB and fAPAR in their climatic responses. Meanwhile, we use an AGB-fAPAR Difference Index (AFDI) to quantify the driving mechanisms underlying their divergent responses, which is defined as the difference between AGB and fAPAR after standardization and normalization. We find that AGB is negatively associated with local precipitation, whereas fAPAR exhibits a positive correlation, leading to pronounced response differences in AFDI across precipitation gradients. Micro-topography contributes 75% to the spatial patterns in AGB and fAPAR (with a correlation coefficient of 0.75), further shaping AFDI through changes in elevation and slope. Using the AFDI, we demonstrate that topography-driven surface water redistributes water availability beyond local precipitation — proximity to surface water shows a significant positive relationship with higher AFDI, yet radiation after topographic correction shows no significant contribution. Moreover, more complex tree species composition and near-mature stand age amplify the AFDI due to their impact on vertical structure. Our results suggest that AGB and fAPAR exhibit inconsistent responses to precipitation. Local topographic effects, hydrological dynamics, and forest composition and age structure collectively drive the disparity between AGB and fAPAR, highlighting the constraints of greenness indices in capturing forest biomass dynamics and providing a new perspective to enhance the accuracy of forest carbon dynamics predictions.

Key words: Aboveground biomass, FAPAR, Climate condition, Topography, Northern forest