Distinct Thresholds and Interactive Effects of Environmental Drivers on Forest Gross Primary Productivity Across Temperate and Cold Climate Zones

Forest gross primary productivity (GPP) is sensitive to environmental change, and assessing its complex responses is essential for evaluating ecosystem carbon sinks, yet the thresholds and interactions among driving factors remain unclear. By integrating long-term eddy covariance measurements with an optimal ensemble learning model and the SHapley Additive exPlanations algorithm, we identified the key environmental factors, threshold characteristics, and interactions driving daily forest GPP across the temperate climate zone (TCZ) and the cold climate zone (CCZ). Forest GPP exhibits clear nonlinear responses that differ across climate zones: solar radiation and air temperature are the dominant drivers of daily GPP variation in the TCZ and CCZ, respectively. In the TCZ, the threshold at which radiation switches from limiting to promoting GPP is estimated at 151.0 W m−2 (95% confidence interval: 149.6–152.5 W m−2), with thresholds of 12.3 °C (12.2–12.4 °C) for air temperature and 7.5 °C (7.3–7.6 °C) for soil temperature. In the CCZ, the corresponding thresholds are 156.0 W m−2 (153.7–158.1 W m−2), 9.1 °C (9.0–9.2 °C), and 8.6 °C (8.5–8.8 °C). Dominant factors operate primarily through main effects, whereas non-dominant factors exert stronger interaction effects. The direction of the radiation effect on GPP is modulated by temperature, and the radiation–temperature interaction is stronger in the cold than in the temperate climate zone. These findings clarify the nonlinear response characteristics and interaction regulation of forest GPP across climate zones, providing applicable insights for climate-adaptive forest management and carbon policy design.