Response of spring vegetation phenology to soil freeze–thaw state in the Northern Hemisphere from 2016 to 2022

The research on spring vegetation phenology is crucial to the investigation of terrestrial ecosystems and climate change. Changes in the soil freeze–thaw (F/T) lead to variations in soil moisture, directly impacting vegetation activity. The start of the season (SOS) is the initial and important phenophase for vegetation activity, and thus, this highlights the need to understand the response of spring vegetation phenology to soil F/T state.This study first comprehensively investigates the consistency of the SOS and three soil F/T state indexes, i.e., the start day of the F/T state (SFT), the end day of the F/T state (EFT), and the length of days of the F/T state (LFT) via satellite data source.Results reveal that: (1) All 3 F/T state indexes impact SOS values, and the EFT outperforms others. The correlation coefficients between EFT and SOS gain around 3.07%. (2) A temporal overlap between SOS and EFT occurs in May, suggesting that parts of the plants begin active growth before average temperatures reach above 0°. (3) Small differences of SOS and EFT exist between savannas, and croplands, with an average difference of less than 10 days; in contrast, the largest differences occur in broadleaf evergreen forests. The results can fill the knowledge gap on the response of spring vegetation phenology to soil F/T state, and help to investigate the reasons for the nonlinear dynamics of SOS under global warming.