草原物候是草原生态系统气候变化的敏感指示器，探索物候变化与气候变化的关系，对草原生态系统保护及全球气候变化研究具有重要意义。本文以呼伦贝尔草原为研究对象，基于2000—2015年MOD09Q1数据和气象数据，利用D-L拟合法对NDVI时间序列进行重构，采用动态阈值法提取草原物候期，利用相关分析法，分析了物候与气象因子之间的关系。研究表明：（1）返青期发生时间介于4月下旬至6月上旬（平均为第138 d），枯黄期发生时间介于9月至10月下旬（平均为第277 d），生长季长度主要在3.5~6个月之间（平均为136 d）。空间上，西部和北部返青期较早，枯黄期较晚，生长季长度最长；中东部返青期较晚，枯黄期较早，生长季长度较短。（2）年际变化趋势上，返青期和枯黄期均以提前趋势为主，提前趋势的像元比例分别为61.2%和63.83%，生长季长度以延长趋势为主，延长趋势的像元占比为54.95%。（3）冬季降水增加是返青期提前的主要原因，而春季降水增多和秋季温度升高是枯黄期提前的主要原因。研究结果有助于加深物候对气候变化响应的认识，以期更好地为草原物候研究和放牧优化管理提供参考。
Accurately monitoring phenological period is of great scientific significance to the optimal management of grassland ecosystem. This paper adopted D-L fitting method and dynamic threshold method to invert the phenological period of HulunBuir grassland, analyzed the characteristics of its spatio-temporal changes, and discussed the relationship between phenological phase and meteorological factors by using MOD09Q1 dataset from 2000 to 2015. The results of the study indicated that: (1) The green-up date occurred in late April or early June (an average of the 138th day), and the withering phase occurred from September to late October (an average of the 277th day). The length of the growing season ranged from 3.5 to 6 months (an average of 136 days). (2) Spatially, in the western and northern HulunBuir grasslands, the green-up date was earlier but the withering date was later, and it had the longest growing season. While in the eastern and central HulunBuir grasslands, the green-up date was relatively later, but the withering was earlier, and the length of the growing season was shorter. Both the green-up date and the withering date showed a trend of moving earlier. The proportion moving earlier on the green-up date and withering date were 61.2% and 63.83%, respectively. The length of the growing season showed a prolonging trend, and the proportion in the prolonged growing season is 54.95%. (3) The main driving factors of the change of phenological trend were clarified based on the spatial correlation coefficients of phenological phase and annual average, seasonal temperature and precipitation at the pixel scale. The main reason for the advancement of the green-up date was the increase of precipitation in winter, while the main reasons for the advancement of the withering date were the increase of precipitation in spring and the increase of temperature in autumn. The research results can provide references for revealing the climate change of the HulunBuir grassland, and guidance for the herdsman to graze and to develop animal husbandry.
