**2. Study Area and Data**

#### *2.1. Study Area*

The study area is on the east of the Tibetan Plateau (29◦35 30N–35◦48 06N, 94◦08 08E–101◦03 36E, Figure 1), covering a large area of alpine grasslands at an average elevation of 4000 m. Our study focuses on alpine grasslands, which are dominated by alpine meadows and alpine steppe [41]. The study area is typical of alpine meadows, which normally grow from May to September. The mean temperature ranges from near 0 ◦C to above 20 ◦C, and precipitation ranges from 100 mm to over 1000 mm along a south–north gradient [4]. Seasonal snow covers the study area from October to May, which may contaminate the satellite signals and introduce bias in SOS detection.

**Figure 1.** Map of the study area. (**a**) MODIS land cover map of the Qinghai–Tibet Plateau. (**b**) MODIS land cover map and (**c**) DEM of the study area.
