气候变化与放牧对西藏典型高寒荒漠草地植被指数变化的相对影响

高寒荒漠草地是青藏高原天然牧场的重要组成部分,对气候变化的响应敏感,且受放牧活动干扰较大,然而,目前仍缺乏数据解释气候变化与放牧对高寒荒漠草地生长的相对影响。西藏阿里地区的日土县,处于季风带和西风带的过渡区,形成以高寒荒漠草原为主的植被类型。基于西藏日土县2000—2016年间的MODIS遥感植被指数(NDVI),以牲畜存栏数(LN)作为放牧的关键指标,以气温、降水和标准化降水蒸散指数(SPEI)数据作为气候变化的指标,分析了研究区草地NDVI的时空变化格局以及气候变化与放牧对NDVI变化的相对影响,据此评估已实施的退牧还草生态建设工程效应。主要研究结果如下:1)2000—2016年间NDVI先减后增,总体呈现增加趋势,这与温度、降水、SPEI以及LN的变化密切相关,其中,LN对NDVI变化的影响相对更大;2)偏相关分析结果表明,退牧还草工程实施前(2000—2007),LN对NDVI变化的影响更大,而工程实施后(2008—2016),表现为SPEI对NDVI变化的影响更大。研究结果表明,高寒荒漠草地生态系统十分脆弱,在不同空间以及时间尺度受到气候变化和放牧活动的影响也有所不同。退牧还草工程通过控制牲畜数量,减轻放牧压力,在很大程度上遏制了该地区植被尤其是草原和湿地NDVI的降低趋势。

Relative impact of climate change and grazing on NDVI variations in typical alpine desert grasslands in Tibet

As an important part of the alpine pasture on the Qinghai-Tibet Plateau, the alpine desert grasslands, which are assumed to be sensitive to climate change, are suffering from rising grazing intensity. However, due to the lack of data, the relative impact of climate change and grazing on the growth of alpine desert grasslands is still in debate. Ritu County in Ali area of Tibet is located in the transition zone between the monsoon and the westerly zone. The most typical vegetation type here is alpine desert grasslands. In this study, based on data of the satellite-derived normalized difference vegetation index (NDVI) during 2000-2016, we analyzed the relative impacts of climate change and grazing on NDVI changes for Ritu County. Data of air temperature, precipitation and standardized precipitation evapotranspiration index (SPEI) were used as indicators of climate change, and those of livestock (LN) were enrolled as a key indicator of grazing. The results showed that:1) NDVI tended to increase during 2000-2016, which is associated with the variation of temperature, precipitation, SPEI and LN. And LN seems to exert relative larger impacts on the variation of NDVI. 2) Partial correlations indicted that LN was a main driving force for the variation of NDVI before the launch of the ecological restoration project (Grazing Withdrawal Program, GWP) in 2000-2007. However, the main driving force shifted to SPEI since the GWP started (during 2008-2016). The results indicate that alpine desert grasslands are vulnerable to grazing activities and climate change. The impacts of climate change and grazing on vegetation differs between different time scales or space scales. By controlling the number of livestock and therefore reducing the pressure of grazing, the GWP effectively suppressed the significant decrease of NDVI in grasslands and wetlands in Ritu County.