Seasonal and regional divergence in snow-regulated early-season vegetation productivity on the Tibetan Plateau.

Introduction: Seasonal snow significantly regulates vegetation productivity on the Tibetan Plateau (TP), but its pathways and seasonal effects remain unclear. Methods: Using multi-source remote sensing datasets and long-term field observations, this study employed a structural equation model to disentangle the dual-pathway regulatory mechanisms-specifically the soil moisture and growing-season effects-through which snow influences net primary productivity (NPP) across spring and summer on the TP. Results: Snow regulation of vegetation NPP exhibited significant seasonal and spatial differentiation. Spring (March-May) NPP was primarily influenced by the growing-season effect (GS effect), where changes in snow depth were linked to shifts in the start of the growing season (SOS), thereby modulating NPP; this relationship intensified with increasing elevation. In contrast, summer (June-August) NPP was primarily influenced by the soil moisture effect, whereby increased/decreased snow depth was associated with increased/decreased soil moisture, which may subsequently contribute to higher/lower NPP. However, this effect weakened with elevation and even became negative above 4500 meters. Spatially, the total effect of snow depth on summer NPP was positive in the semi-humid central and eastern TP, but negative in the semi-arid southwestern TP. Discussion: These seasonal and spatial variations highlight the importance of accounting for climatic and altitudinal modulation in carbon cycle models, providing vital insights into alpine ecosystem responses to ongoing climate change.