Vegetation spring phenology in drylands is sensitive to climatic and anthropogenic pressures, yet the nonlinear responses of the start of the growing season (SOS) across different vegetation types remain inadequately quantified. Here, we extracted the start of the growing season from 2001 to 2020 Moderate-Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) time series for stable vegetation areas on the Mongolian Plateau (MP) and applied Extreme Gradient Boosting (XGBoost) models with Shapley Additive Explanations (SHAP) analysis to six environmental drivers—precipitation, temperature, windspeed, livestock density, population density, and elevation—across forests, shrublands, and grasslands. The SOS displayed pronounced spatial heterogeneity, with earlier onset in northern forests and shrublands and delayed onset in southern arid grasslands. Forests and shrublands exhibited significant advancing trends of 6.8 and 6.4 days per decade, respectively, while grasslands showed no significant trend. Temperature dominated the SOS variability across all vegetation types, yet the relative importance of other drivers varied; windspeed notably influenced forests, whereas precipitation and elevation were critical for grasslands and shrublands. SHAP analysis revealed strong nonlinearities and threshold effects, including a U-shaped temperature response and a 350 mm precipitation threshold in grasslands, beyond which the SOS responses markedly shifted. These results highlight the vegetation-specific and nonlinear nature of phenological regulation in drylands, suggesting that phenology prediction and ecosystem monitoring should explicitly incorporate vegetation type and threshold-based climatic responses.
Zhang et al. (Thu,) studied this question.