Asymmetric Responses of AGB and fPAR in Northern Forest to Climate Condition

Northern forests exhibit strong sensitivity to recent climate change, which brings risks of carbon reemission from mature forests and threatening biodiversity. Understanding the role of these forests in regional carbon and hydrological cycle is fundamental for implementing effective forest management strategies and projecting future terrestrial dynamics. Currently, most studies investigating vegetation growth or responses to climate change rely on the fraction of photosynthetically active radiation (fPAR) as an indicator of plant productivity. However, while fPAR primarily reflects carbon assimilation within the current growing season, it provides limited insights into long-term carbon storage, such as above-ground biomass (AGB). This discrepancy arises from the complex vertical structure of forests, leading to an incomplete understanding of how forest AGB responds to local climate conditions. In this study we utilize a wall-to-wall AGB dataset derived from microwave remote sensing observations to investigate the asymmetric responses of AGB and fPAR to climatic factors and explore the underlying mechanisms driving these differences. The key contributions of this study are twofold: (1) demonstrating that AGB and fPAR exhibit distinct and asymmetric responses to local climate conditions, and (2) elucidating the factors contributing to the mismatch between AGB and fPAR. Using annual mean precipitation, temperature, and radiation data from 2015 to 2020, we analyzed the climatic responses of AGB and fPAR. Results reveal an asymmetric relationship with precipitation: AGB is negatively correlated with local precipitation, while fPAR exhibits a positive correlation. Temperature and radiation, however, show no significant constraints on either AGB or fPAR. To further investigate this asymmetry, we introduced the difference between normalized AGB and fPAR (AGB-fPAR) as an indicator and divided the study area into 12 sub-regions of 1° × 1° where precipitation and downwelling radiation energy were treated as invariant. Our findings demonstrate that topographic factors account for approximately 60% of the variation in AGB-fPAR, driven by the redistribution of energy caused by local terrain. Additionally, surface runoff reallocates water availability beyond local precipitation, with proximity to open surface water showing a significant positive relationship with higher AGB-fPAR. Forest structural complexity, such as mixed species composition and older tree age, further amplifies the AGB-fPAR gap due to their influence on vertical structure. This study highlights the importance of considering the divergence between fPAR and AGB in assessing forest responses to climate change. Local topographic effects, hydrological dynamics, and forest structural traits jointly drive the disparity between these indicators, which evidences the need for integrated approaches to understand and predict forest-climate interactions.