近30年来青藏高原高寒草地NDVI动态变化对自然及人为因子的响应

草地生态系统是陆地生态系统的重要组成部分，在调节气候、水土保持、防风固沙、保护生物多样性等方面发挥着重要作用。青藏高原是全球海拔最高的独特地域单元，平均海拔超过4000 m，素有“世界第三极”之称，亦是我国重要的生态安全屏障，其对气候变化敏感且易受人类活动的影响，属于气候变化敏感区和生态脆弱带。近年来，由于气候变化和人类活动的不断加剧，青藏高原区域气候和环境发生了重大变化，气候变暖、水污染、草地退化和沙化等问题已严重阻碍了当地社会经济的可持续发展。高寒草地是青藏高原主要的植被类型，在气候变化和人类活动加剧的背景下，青藏高原高寒草地植被的动态变化受到人们的广泛关注。归一化植被指数（Normalized difference vegetation index, NDVI）因能有效地反映植被覆盖程度和生长状况而被广泛应用于植被动态的研究中。气温与降水被认为是影响青藏高原植被动态的主要气候因子，放牧强度与人口数量则是主要人为因子。因此，研究高寒草地植被对气候变化和人类活动的响应机制对预测未来草地变化有着重要的意义。基于青藏高原生长季草地的NDVI、气温、降水、放牧强度及人口数量等数据，在县区尺度上，采用趋势分析法探究了1982—2013年青藏高原143个县区生长季草地NDVI动态变化、气候变化及人类活动的变化，同时采用面板数据模型分析了32年来青藏高原143个县区气候、人为因子变化对草地NDVI变化的相对贡献。研究结果显示：（1）青藏高原高寒草地生长季NDVI总体呈增长趋势，草地植被生长状态呈现“整体改善、局部退化”趋势；（2）青藏高原生长季平均气温与降水量整体增加，气候呈现“暖湿化”趋势；（3）在长时间尺度上，气候因子主导了青藏高原高寒草地NDVI的变化，降雨和气温的增加促进草地NDVI的增加，放牧强度的持续增加则导致草地NDVI的减少。

Responses of the NDVI of alpine grasslands on the Qinghai-Tibetan Plateau to climate change and human activities over the last 30 years

Grassland ecosystem, one of the key component of terrestrial ecosystems, performs critical functions in climate regulation, soil and water conservation, windbreak and sand fixation, and the maintenance of biological diversity. The Qinghai-Tibetan Plateau (QTP), also known as the "Third Pole" of the Earth, is the largest and highest plateau in the world, with an average altitude of about 4000 m. As the important ecological security barrier in China, the QTP is regard as a sensitive and fragile area under global climate change. In recent years, the regional climate and environment of the QTP have changed dramatically due to intensified climate change and human activities, such as global warming, water pollution, grassland degradation and desertification, which have seriously hindered the sustainable development of local society and economy. Alpine grassland is the dominant plant functional type on the QTP and its vegetation dynamics in response to climate change and intensified human activities has been widely concerned. Normalized difference vegetation index (NDVI) has been widely applied in the study of vegetation dynamics because it can effectively reflect the vegetation coverage and growth status. Temperature and precipitation are considered to be the primary climatic factors affecting vegetation dynamics, while grazing intensity and human population are considered as the dominant anthropogenic factors. Therefore, understanding the mechanisms of the responses of alpine grassland to climate change and human activities is of great significance to predict the future of grassland under these disturbances. In this study, we analyzed the spatio-temporal changes of the NDVI of alpine grassland, temperature, precipitation, human population and grazing intensity by trend analysis from 1982 to 2013 on the county-scale. Besides, we used the panel data model to investigate the effect of climate changes and human activities on the NDVI in 143 counties of the QTP in the past 32 years. The results showed that: (1) the NDVI increased generally, and the plant growth increased in most of the areas but decreased in some specific areas. (2) there was an increasing tendency for the mean temperature (i.e., warmer) and total precipitation (i.e., more humid) over the growing season. (3) in the long-term, climate factors dominated the change of NDVI in alpine grassland on the QTP. The increase of precipitation and temperature increased the NDVI, whereas the continuously increasing grazing intensity decreased the NDVI in alpine grassland.