The Tibetan Plateau (TP), also known as the world's Third Pole, is underlain by frozen ground and is highly sensitive to climate change. However, it remains unclear how the variations in soil freeze-thaw could affect vegetation dynamics across the TP. In this study, we adopted the latest datasets for vegetation, climate and soil freeze-thaw in the past two decades to explore the possible impacts of changes in soil freeze-thaw on vegetation greenness and phenology on the TP. According to the satellite-based observations, the TP showed an overall greening trend during 2001-2020, and the growing season length increased significantly at a rate of 3.6 days/10a, mainly contributed by the advances of the start of the growing season (2.7 days/10a). Based on ridge regression and partial correlation analysis, air temperature and precipitation were found to be the major dominant factors of vegetation dynamics on the TP, and precipitation played a dominant role in the relatively warm-dry southwestern TP where vegetation browning and spring phenology delays were observed. In the relatively cold regions, earlier soil thaw onset generally facilitated spring phenology, and longer soil thaw duration tended to increase the growing season soil moisture content, which could in turn enhance vegetation greenness. In the relatively warm regions, however, earlier thaw onset and longer thaw duration could possibly exacerbate the growing season water stress and limit vegetation growth. The negative impacts were more evident in the regions with unstable and completely degraded permafrost according to the results in the source region of Yellow and Yangtze rivers. Our findings highlight the spatially varying role of soil freeze-thaw changes in vegetation dynamics, which have important implications for the carbon budget of the TP in a warming future climate as frozen ground continues to degrade.

青藏高原（TP），也被称为世界第三极，地势冰冻，对气候变化高度敏感。然而，目前尚不清楚土壤冻融的变化如何影响整个青藏高原的植被动态。本研究采用近20年来最新的植被、气候和土壤冻融数据集，探讨土壤冻融变化对青藏高原植被绿度和物候的可能影响。卫星观测显示，2001-2020年青藏高原总体呈绿化趋势，生长季长度以3.6天/10a的速度显着增加，主要是由于生长季开始提前（2.7天/10a)。基于岭回归和偏相关分析，气温和降水是青藏高原植被动态的主要主导因素，而青藏高原西南部相对温暖干燥，植被褐变和春季物候延迟以降水为主。在相对寒冷的地区，较早的土壤解冻一般有利于春季物候，而较长的土壤解冻持续时间往往会增加生长期土壤含水量，从而提高植被绿度。然而，在相对温暖的地区，较早的解冻开始和较长的解冻持续时间可能会加剧生长季节的水分压力并限制植被生长。根据黄河和长江源区的结果，在冻土不稳定和完全退化的地区，负面影响更为明显。