Changes in vegetation phenology and its responses to climatic factors in the Mu Us Desert

  Objective  This study aimed to quantify the long-term trends and interannual variability (IAV) of vegetation phenology in the Mu Us Desert during 2001−2020, and to examine the phenological responses to climatic factors.

  Method  We extracted vegetation phenology from the normalized difference vegetation index (NDVI) timeseries, which was calculated from the MODIS MOD13Q1 product, using the dynamic threshold method in TIMESAT3.3. Extracted phenological indices included the start of growing season (SOS), the end of growing season (EOS), and the length of growing season (LOS). Partial correlation analysis was used to examine the relationships between phenological indices and climatic factors.

  Result  Multi-year mean phenological indices showed distinct spatial variations: average SOS was the (140 ± 15)th day of year (Julian day), displaying an advancing pattern from west to east and from north to south; average EOS was the (291 ± 6)th day of year, displaying a delaying pattern from north to south; and LOS averaged (151 ± 18) d, displaying an extending pattern from west to east and north to south. All phenological indices showed long-term trends during the study period: mean regional SOS across the Mu Us Desert showed an advancing trend (0.58 d/year); mean regional EOS showed a delaying trend (0.25 d/year); and LOS showed an extending trend (0.83 d/year). Temporal trends in phenological indices also showed significant spatial variations, weakening from west to east. The decadal trends and interannual variability of vegetation phenology (SOS and EOS) were influenced by the same set of climate factors: SOS was negatively correlated with the previous-month temperature and precipitation; while EOS was positively correlated with the temperature in the previous month and cumulative precipitation over the previous months.

  Conclusion  Significant longitudinal and latitudinal patterns are observed for SOS and LOS in the Mu Us Desert, and the temporal phenological trends are strengthened from east to west. The temporal trends of phenological indices are mainly reflected by advancing SOS and extending LOS. Advances in SOS and delays in EOS are mainly attributable to rising temperature in central and eastern Mu Us Desert, but are mainly explained by precipitation in the western regions with less precipitation.