Asymmetric response of Northern Hemisphere vegetation to climate change from 2000 to 2018: Phenology leads GPP

Understanding how vegetation phenology affects terrestrial ecosystem productivity (GPP) is key to predicting carbon cycle under climate change. However, the heterogeneity in the trends of the length of growing season (LOS) and GPP, as well as their underlying driving mechanisms, remains inadequately explored. Here, we analyzed the spatial heterogeneity in LOS and GPP trends across regions north of 20°N in the Northern Hemisphere (NH) from 2000 to 2018. A climate‒phenology‒GPP model was constructed to investigate the interactions among these components. Furthermore, the temporal lag effects between GPP and vegetation phenology were examined to reveal the physiological and ecological mechanisms through which vegetation phenology regulates GPP under climate change. The results indicate that LOS and GPP trends exhibit notable spatial heterogeneity in northwestern Eurasia. The most common trend pattern is a simultaneous increase in both LOS and GPP, followed by regions where LOS increases while GPP decreases. Pre-season and growing season climate can explain 23% and 38.9% of the divergence between LOS and GPP trends, respectively. Temporal lag analysis further demonstrates a predominant long-term (over eight years) lag effect between LOS and GPP, accounting for approximately 38.7% of the study area. This study reveals the coupling relationship between vegetation phenological dynamics and GPP, highlighting the complex interactions among climatic factors, phenological processes, and ecosystem functioning. It provides important insights into the regulatory mechanisms of the carbon cycle in the context of climate change.