Interactions between above- and belowground vegetation phenology across diverse climate zones in the Northern Hemisphere

Understanding the temporal coupling between aboveground and belowground phenological processes is crucial for a mechanistic prediction of terrestrial ecosystem responses and biochemical cycles under climate change. However, a comprehensive assessment of these interactions between above- and belowground vegetation phenology remains limited, especially across heterogeneous climatic zones. To bridge this gap, this study investigated the linkages between above- and belowground phenology and their response to climatic drivers across diverse Köppen climate zones in the Northern Hemisphere. We integrated multi-source data, including Sentinel and other remote sensing products, minirhizotron imagery, and in situ measurements, to characterise aboveground and belowground vegetation phenology across Arctic tundra, boreal shrubland, temperate grassland, arid savannas, and tropical forest zones. Convolutional Neural Networks were leveraged to identify leaf and root phenophases, while SHapley Additive exPlanations and Structural Equation Modelling analysis were conducted to quantify the temporal synchrony between above- and belowground phenology and determine the primary climatic factors of these coupled processes. Results indicate significant variation in the degree of coupling between above- and belowground phenology across climate zones. In the tundra zone, root activity occurs 17 ± 4.5 days earlier than aboveground phenology and is strongly constrained by soil conditions (β = 0.31, p < 0.01). In contrast, boreal shrubland shows a synchronous onset, with the start of season preceding root growth initiation by up to 9 ± 3.5 days. It is found that the intensity of above- and belowground coupling is primarily regulated by air temperature (SHAP value = 0.206), causing earlier root growth in tropical forests, whereas soil moisture (SHAP value = 0.077) delays leaf growth activity in arid systems. These findings advance understanding of belowground phenology and its coordination with aboveground processes. It also enhances the prediction of how Northern Hemisphere ecosystems will respond to climate change by revealing integrated phenological responses across vegetation compartments.