Sensitivity of Green-Up Date to Meteorological Indicators in Hulun Buir Grasslands of China

Temperature and precipitation are considered to be the most important indicators affecting the green-up date. Sensitivity of the green-up date to temperature and precipitation is considered to be one of the key indicators to characterize the response of terrestrial ecosystems to climate change. We selected the main grassland types for analysis, including temperate steppe, temperate meadow steppe, upland meadow, and lowland meadow. This study investigates the variation in key meteorological indicators (daily maximum temperature (T-max), daily minimum temperature (T-min), and precipitation) between 2001 and 2018. We then examined the partial correlation and sensitivity of green-up date (GUD) to T-max, T-min, and precipitation. Our analysis indicated that the average GUD across the whole area was DOY 113. The mean GUD trend was -3.1 days/decade and the 25% region advanced significantly. T-max and T-min mainly showed a decreasing trend in winter (p > 0.05). In spring, T-max mainly showed an increasing trend (p > 0.05) and T-min a decreasing trend (p > 0.05). Precipitation showed no significant (p > 0.05) change trend and the trend range was +/- 10 mm/decade. For temperate steppe, the increase in T-min in March promotes green-up (27.3%, the proportion of significant pixels), with a sensitivity of -0.17 days/degrees C. In addition, precipitation in April also promotes green-up (21.7%), with a sensitivity of -0.32 days/mm. The GUDs of temperate meadow steppe (73.9%), lowland meadow (65.9%), and upland meadow (22.1%) were mainly affected by T-min in March, with sensitivities of -0.15 days/degrees C, -0.13 days/degrees C, and -0.14 days/degrees C, respectively. The results of this study reveal the response of vegetation to climate warming and contribute to improving the prediction of ecological changes as temperatures increase in the future.