基于近20年遥感数据的藏北草地分类及其动态变化

利用1982—2000年NOAA/AVHRR的旬合成归一化植被指数(NDVI)资料,采用主成分分析和非监督分类方法对藏北那曲地区植被进行分类,分析不同草地类型代表像元的NDVI年内和年际变化特征;定义那曲地区牧草主要生长期平均NDVI≥0.1的地区为植被区,NDVI<0.1的地区为植被稀少区,进一步分析植被区每个像元NDVI的时空变化特征.结果表明:该地区草地类型可分为高寒草甸、高寒草甸草原、高寒草原和高寒荒漠,分类结果与实际情况相符.4种草地类型的NDVI年内变化均呈单峰型,年最大值出现在8月;近20年来那曲地区植被区7—8月平均NDVI由东南向西北逐渐减少,约在0.1~0.6之间变化,变异系数在0.05~0.40之间.NDVI高的地区,变异系数相对较小;NDVI低的地区,变异系数相对较大,年变率的范围在-0.005~0.008之间.近20年来那曲地区植被变化不明显,约20%地区(主要分布在西部的尼玛和东部的嘉黎、比如、索县和巴青等县)的植被活动在减弱.

Classification and dynamic changes of grasslands in northern Tibet based on recent 20 years satellite data

Based on the NOAA/AVHRR 10-day composite NDVI data from 1982 to 2000 and by the methods of principal component analysis and unsupervised classification, the vegetations in Naqu District of northern Tibet were classified, and the annual and inter-annual variations of NDVI on the pixels selected from different grassland types were analyzed. The regions where the mean NDVI values during the main growth season of grass were equal to or great than 0.1 were defined as vegetation regions, while those where the mean NDVI values were less than 0.1 were defined as non-vegetation regions. The spatiotemporal changes of NDVI in each pixel in vegetation regions were analyzed, and the results showed that the grasslands in northern Tibet could be classified into four types, i.e., high-cold meadow, high-cold meadow steppe, high-cold steppe, and high-cold desert. This classification was accorded with practical status. The annual changes of the NDVI in the four grassland types showed one peak value, with the maximum in August. In recent 20 years, the mean NDVI from July to August in vegetation regions decreased gradually from southeast to northwest, ranging from 0.1 to 0.6, and the variation coefficient changed between 0.05 and 0.40, which was smaller in the place of high NDVI than in the place of low NDVI. The annual variability of NDVI changed between -0.005 and 0.008. In recent 20 years, the vegetations had no observable change, but the vegetation development in 20% of vegetation regions, which mainly located in Nima in western Naqu and in Jiali, Biru, Suoxian and Baqing in eastern Naqu, was weakened.