植被作为陆地生态系统的重要组成部分，在调节区域碳循环和水循环过程中发挥着重要作用。青藏高原是全球气候变化的驱动器和放大器，是气候变化背景下最敏感脆弱的区域之一，也是我国关键的生态安全屏障，其生态环境的变化对当地乃至全球气候变化及生态平衡起着极其重要的作用。因其自然环境和生态系统的独特性、原始性和脆弱性，青藏高原已成为研究植被对气候变化响应的重点区域。因此，本文旨在了解气候变化背景下，明确青藏高原植被生长状态时空分布特征，探讨植被对气候变化的响应规律，评估其生态脆弱性的时空变化特征及影响因素，并预测未来气候情景下青藏高原的植被变化趋势及其生态脆弱性分布特征，为我国青藏高原生态安全屏障的优化提供研究基础。论文的主要工作与成果如下：

        （1）基于青藏高原野外植被覆盖度（Fractional Vegetation Cover，FVC）测量数据验证表明，全球陆表特征参量（Global LAnd Surface Satellite，GLASS）FVC产品是目前最符合青藏高原植被生长状态的FVC数据。基于长时间序列的生长季GLASS FVC数据分析表明，青藏高原植被自2000年来呈整体改善，局部退化的趋势，其空间差异性与气候条件、水文条件、地形地貌、土壤性质、人类活动强度等因素紧密相关，高原温带的干旱、半干旱区域以及海拔较低区域的植被生长状态改善更为显著，而植被退化主要由地质灾害、城市扩张等因素导致，多发生于青藏高原中部及南部地区。

        （2）基于植被生长季的GLASS FVC与气象数据，采用统计方法分析表明，植被对气温、降水和太阳辐射均具有较强的响应，但该响应具有较强的时空不一致性，具体表现为：在干旱区域，植被受到降水影响更大；在东部区域，高原温带地区的植被受气温影响更大；太阳辐射在海拔落差大和干旱地区的植被与裸地交错地带对植被生长有更大的影响。此外，气象因子对植被生长的影响还具有一定的时滞效应，其中气温和降水的时滞效应较弱，而太阳辐射的时滞效应更强，尤其是冬季的太阳辐射变化对于大部分区域的植被生长起到了积极的促进作用。

        （3）基于SRP理念，提出了主成分分析与熵权法相结合的客观赋权策略，构建了一套客观、自动的生态脆弱性评估方法，并完成了2000-2018年青藏高原生态脆弱性评价及其时空动态分析。分析结果表明，青藏高原整体生态脆弱性较高，呈现出“东低西高”的空间格局，生态敏感性、生态恢复力与生态压力对青藏高原生态脆弱性的贡献依次减弱。植被的生长状态及其空间格局能够有效缓解生态脆弱性。在研究时段内，青藏高原生态脆弱性显著降低的区域主要分布在青海湖、柴达木盆地、阿里山地以及它们的周边区域，而生态脆弱性较高且显著增加的区域主要集中在雅鲁藏布江大峡谷与青藏高原北部高寒荒漠地带，这些区域是当前青藏高原生态修复需要关注的重点区域。

        （4）基于GLASS叶面积指数（Leaf Area Index，LAI）数据，对陆面过程模型CLM5.0的参数化方案进行了优化，有效提高了CLM5.0模型对青藏高原LAI的模拟能力。基于CLM5.0模型，对两种共享社会经济路径SSP126与SSP585所表征的未来气候情景下的青藏高原LAI进行了模拟，并依此分析了未来气候情景下青藏高原的生态脆弱性。结果表明，在以上两种未来气候情景下，青藏高原LAI均呈现较高的增长趋势，且在高排放的SSP585情景下植被的变绿范围更广、趋势更强，表明未来气候情景下更强的气温、降水、CO₂浓度等对植被生长具有积极作用。此外，青藏高原的植被改善虽然能在一定程度上缓解青藏高原的生态脆弱性，但青藏高原在未来气候情景下仍具有较高的生态脆弱性。与SSP126情景下相比，SSP585情景下青藏高原的生态脆弱性水平更低，且时空变化以生态脆弱性降低为主。在未来气候情景下，生态脆弱性较高且呈显著增强趋势的区域主要分布在青藏高原的高寒荒漠地带以及西部高原山地区域，应重点针对这些区域进行生态监控，制定科学的未来气候适应措施。

As one of the vital components in the terrestrial ecosystem, vegetation plays a significant role in regulating regional carbon and water cycles. The Qinghai-Tibetan Plateau (QTP) is one of the most sensitive and vulnerable regions under global climate change and its ecosystems are suffering from severe degradation. As the ecological security shelter of China, QTP’s environmental change may cast great influence on local or even global climate dynamics. Due to its unique, authentic and vulnerable ecosystems, the QTP has become a key area to study vegetation responses to climate change. Therefore, this research is aimed at providing scientific backgrounds for optimizing QTP’s ecological security shelter by firstly investigated the spatio-temporal dynamics of vegetation and its responses to climate change. Secondly, this research assessed the spatio-temporal characteristics of QTP’s ecological vulnerability and evaluated the important role of vegetation in mitigating ecological vulnerability. Finally, this study projected future vegetation dynamics and its ecological vulnerability under future climate scenarios over the QTP. The main achievements in this paper can be concluded as follows:

(1) Based on field measurements across the QTP, Global LAnd Surface Satellite fractional vegetation cover (GLASS FVC) has been selected as the optimal FVC product in representing QTP’s vegetation status and analyzing its dynamics. A significant greening trend was detected over the QTP during 2000 to 2020 using GLASS FVC. However, considerate spatial heterogeneity was found across the QTP due to variations in climatic and hydrological conditions, topographic features, soil properties, intensity of human activities, etc. Vegetation growth was more significant in arid and semi-arid areas of the temperate zone, as well as regions at lower altitudes, while vegetation degradation, mainly resulted from geological disasters, urban expansion, etc., primarily occurred in the central and southern regions of the QTP.

(2) Based on growing season GLASS FVC and climate factors, vegetation responses greatly to temperature, precipitation, and solar radiation over the QTP. Among which, precipitation has the greatest impact on vegetation growth. However, the responses investigated by statistical method established considerate spatial heterogeneity. In arid regions, vegetation growth responded more to precipitation, while for eastern regions with fine humidity, vegetation responds more positively to temperature. Solar radiation had a greater impact on vegetation over the ecotone between alpine grasslands and bare lands over arid and steep regions. In addition, the impact of climate factors on vegetation growth also has considerate time lag effects. The time lag effects of temperature and precipitation is relatively weak, while the time lag effect of solar radiation is stronger. The change of solar radiation during winter has played a positive role in vegetation growth over most regions.

(3) This study developed an objective and automatic framework for evaluating ecological vulnerability over the QTP under the system ecological sensitivity-ecological resilience-ecological pressure (SRP), and analyzed the spatio-temporal patterns of ecological vulnerability from 2000 to 2018. The spatial distribution of EVI established similar patterns over the study period, with ecological vulnerabilities increasing from eastern to western regions. The contributions of sensitivity, resilience and pressure to ecological vulnerability decreased successively, and vegetation is vital in mitigating QTP’s ecological vulnerability. During the study period, regions with the lowest and significant reducing vulnerability are distributed in Qinghai Lake, Qaidam Basin, Ali Mountain, and their surrounding areas. Moreover, Regions with the highest and most significant increasing vulnerabilities are the kay areas of ecological restoration in the QTP, which are mainly located over the Yarlung Zangbo River Grand Canyon and the alpine desert in the north of the plateau.

(4) Land surface parameters of CLM5.0 was optimized using GLASS Leaf Area Index (LAI). The improved CLM5.0 performed better in estimating LAI across the QTP compared to the original model. Based on the improved CLM5.0, LAI under future scenarios of SSP126 and SSP585 were simulated over the QTP, and hence the ecological vulnerability analysis was performed for these future scenarios. The results show obvious LAI increasing trends under both scenarios, with greater increasing trend under SSP585, which indicates that increasing temperature, precipitation, CO₂ concentration and other factors in the future has a positive effect on QTP’s vegetation growth. However, the QTP still has high levels of ecological vulnerability under both future scenarios. Compared with the results under SSP126, the ecological vulnerabilities under SSP585 were lower and established a more stable trend during the study period due to the higher vegetation increasing trends. Under the future scenarios, areas with high ecological vulnerability and significant increasing trends are mainly distributed in the alpine desert areas and the mountainous areas over the western QTP. Future adaptation and mitigation plans should be focused on these endangered regions to protect the ecosystems of the QTP.