Vegetation Phenological Differences Between Polar- and Equatorial-Facing Slopes in the Three Rivers Source Region, Tibetan Plateau

Vegetation growth is influenced by the microclimate driven by aspects, as evident in the asymmetric vegetation greenness on polar-facing slopes (PFS) and equatorial-facing slopes (EFS). However, it remains uncertain whether aspects influence vegetation phenology. To address this question, we defined the aspect-induced phenological differences between PFS and EFS from 2019 to 2022 within each 3 x 3 km2 grid, using average phenological metrics extracted from Sentinel-2 data. We found that the start of the growing season (SOS) occurs earlier on EFS in cold and humid regions, but in arid areas, PFS has an earlier SOS. The end of the growing season (EOS) consistently occurred later on EFS due to radiation limitations in autumn phenology. Employing the space-for-time approach, the observed distribution of phenological differences within the climate space could potentially indicate the phenological trends of different slope orientations in the future. Our study provides valuable insights into topographic regulation on vegetation phenology. Equatorial-facing slopes (EFS) receive more solar radiation than polar-facing slopes (PFS), resulting in contrasting microclimate conditions. Specifically, EFS are warmer and drier, while PFS are colder and wetter. These microclimate differences contribute to variations in vegetation greenness between PFS and EFS. Based on this, we hypothesized that the phenology of grassland in the Three Rivers Source Region on the Tibetan Plateau is influenced by aspects. To test this hypothesis, we calculated the phenological differences between PFS and EFS within each 3 x 3 km2 grid using average phenological metrics derived from Sentinel-2. Our findings reveal that, for the start of the growing season (SOS), EFS exhibited an earlier onset in regions with low temperature and high precipitation, whereas for regions with high temperature, the growing season starts earlier on PFS. In contrast, over 70% of the grassland area on EFS experiences a later end of the growing season (EOS) due to radiation being a major limiting factor of autumn phenology. In addition, we utilized the space-for-time approach to project potential future phenological changes on PFS and EFS. Our study enhances comprehension of vegetation ecological management and carbon sequestration in mountainous areas. There are significant phenological differences between polar-facing slopes and equatorial-facing slopes Phenological differences vary under different background climatic conditions Using the space-for-time approach, the phenological differences in climate space suggest future phenological shifts