Ecoregion-specific restructuring of green-season and peak-season phenology across the continental United States

Vegetation phenology regulates ecosystem carbon, water, and energy exchanges; however, continental-scale assessments often emphasize growing-season boundaries while paying less attention to peak-season dynamics. We used MODIS Enhanced Vegetation Index (EVI) time series to quantify phase-specific phenological trends across stable non-crop vegetation in the continental United States (CONUS) from 2001 to 2024. Metrics included start of season (SOS), start of peak season (POS), end of peak season (POE), end of season (EOS), green-season length (GSL), and peak-season length (PSL). EVI-derived dates were benchmarked against USA National Phenology Network observations. Trend robustness was evaluated through threshold and smoothing sensitivity analyses, and environmental associations were assessed using XGBoost and SHAP attribution methods. Validation showed moderate agreement with USA-NPN observations (R 2 = 0.58–0.67; RMSE = 16.28–19.75 days). Ecoregion-level trends revealed phase-specific restructuring: GSL increased most in the Texas-Louisiana Coastal Plain (+0.68 days yr −1 ), Western Sierra Madre Piedmont (+0.63 days yr −1 ), and Everglades (+0.61 days yr −1 ), whereas PSL shortened most in the South Central Semi-Arid Prairies (−0.50 days yr −1 ), West-Central Semi-Arid Prairies (−0.32 days yr −1 ), and Southeastern USA Plains (−0.30 days yr −1 ). Several ecoregions exhibited longer green seasons but shorter peak seasons, indicating expanded shoulder-season greenness without a corresponding increase in sustained peak greenness. Attribution analyses indicated that SOS was associated with a combination of climatic, thermal, and topographic predictors, while POS, POE, and EOS were more closely linked to thermal and hydrologic conditions. These findings suggest that recent greening across CONUS reflects phase-specific restructuring of satellite-observed canopy greenness rather than a uniform extension of the growing season.