青藏高原主要生态系统变化及其碳源/碳汇功能作用*

根据NOAA/AVHRR (National Oceanic and Atmospheric Administration/Advanced Very High Resolution Radiometer)卫星归一化植被指数(NDVI)数据和CASA(Carnegie-Ames-Stanford Approach)模型的计算结果，过去30 年间(1982-2011 年)，青藏高原生长季植被覆盖度和植被净初级生产力(NPP)呈总体上升态势， 植被总体变好。青藏高原生态系统碳汇功能增强，占全国增加碳汇的10%左右。气候条件的变化是青藏高原植被总体变好的最为重要的驱动因子，退牧还草等大型生态工程的生态效应也比较显著。青藏高原植被总体变好的同时，存在着区域不平衡。植被变差的区域主要集中在海拔较高的、生态更为脆弱的藏北高原、西藏“一江两河”和三江源的部分地区，尤其是藏北高原西部的高寒草原和高寒荒漠出现了较为严重的草地退化，其原因是气候变暖变干叠加人类活动(如超载放牧等)的影响。为了应对气候变化和人类活动对青藏高原植被的影响，应该加强青藏高原生态系统变化长期监测系统与平台建设，加大生态补偿和大型生态工程的实施力度。

Change mechanism in main ecosystems and its effect of carbon source/sink function on the Qinghai-Tibetan Plateau

We obtained Normalized Difference Vegetation Index (NDVI) data from National Oceanic and Atmospheric Administration (NOAA) and calculated net primary production (NPP) using the CASA (Carnegie-Ames-Stanford Approach) model from 1982 to 2011. Overall, vegetation coverage and NPP increased during the growing season in the past 30 years, which indicated that vegetation became better on the Plateau. The carbon sink function enhanced on the Qinghai-Tibetan Plateau. The increment of carbon sink on the whole Qinghai-Tibetan Plateau was equivalent to approximately 10% of that in China. Climate change was the primary driving factor for vegetation change on the Plateau. Various ecological projects (e.g. grazing forbidden) also showed obvious ecological effects. However, there were some regions where vegetation became worse. These regions were mainly centered on the Northern Tibetan Plateau, the Brahmaputra River and its two Tributaries and partial district of Three-River Headwaters Region where ecosystems were more fragile and altitude was higher. Especially, there were worse grassland degeneration in alpine steppe and desert on the western of the Northern Tibetan Plateau, which were attributed to the combined effect of climate change (i.e., warming and drying) and human activities (e.g. overgrazing). Therefore, we should strengthen the construction of long-term monitoring systems and platforms for ecosystem changes and implementation of ecological compensation and projects.