Crop phenology is a significant factor that affects the precision of crop area extraction by using the multi-temporal vegetation indices (VIs) approach. Considering the phenological differences of maize among the different regions, the summer maize cultivated area was estimated by using enhanced vegetation index (EVI) time series images from the Moderate Resolution Imaging Spectroradiometer (MODIS) over the Huanghuaihai Plain in China. By analyzing the temporal shift in summer maize calendars, linear regression equations for simulating the summer maize phenology were obtained. The simulated maize phenology was used to correct the MODIS EVI time series curve of summer maize. Combining the mean absolute distance (MAD) and p-tile algorithm, the cultivated areas of summer maize were distinguished over the Hunaghuaihai Plain. The accuracy of the extraction results in each province was above 85%. Comparing the maize area of two groups from MODIS-estimated and statistical data, the validation results showed that the R² reached 0.81 at the city level and 0.69 at the county level. It demonstrated that the approach in this study has the ability to effectively map the summer maize area over a large scale and provides a novel idea for estimating the planting area of other crops.

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.