*3.2. Spring Phenology Detection and Evaluation* 3.2.1. Detection of SOS Dates

Vegetation growth in the alpine grasslands on the Tibetan Plateau has a distinct seasonal cycle, and the SOS can be detected from the time series of five different VIs. We first removed the poor-quality/filled values from the original MODIS datasets and then used a double-logistic fitting method to smooth the noisy time series of VIs in the TIMESAT program [48], since it is more robust in extracting phenological parameters [49–52]. We applied two upper-envelope iterations with an adaptation strength of 2 to reduce the bias of atmospheric effects in the smoothing process [20]. Then, the simple and intuitive dynamic threshold method [53,54] was used to detect the SOS (Figure 2) for its simplicity and robustness [55], and a 20% amplitude was used to determine the SOS, which was consistent with previous studies [20,55,56]. Figure 2 illustrates how the dynamic threshold method detects the SOS under snow and snow-free conditions. The SOS date was detected as the point in time when the VI increased to 20% of the amplitude plus the base value.

**Figure 2.** A schematic diagram of how the dynamic threshold method detects the start of the season (SOS). ΔSOS = SOSsnow − SOSsnow-free.
