青藏高原2000-2015年草地质量的时空变化研究

基于植被净初级生产力（net primary productivity,NPP）和归一化植被指数（normalized difference vegetation index,NDVI）合成了综合草地等级指数（grassland degree index,GDI），计算了青藏高原2000-2015年草地质量变化的程度和速率，综合分析了草地退化的影响机制.研究表明：2000-2015年，青藏高原Ⅰ级（优质）草地区和Ⅳ级（劣质）草地区的面积比例基本保持不变；Ⅰ级草地区中约28.11%变为Ⅱ级；Ⅱ级草地区转化为Ⅰ级和Ⅲ级的比例分别为18.19%和21.53%；Ⅲ级草地区约有28.54%转变为Ⅳ级.由此可见，2000-2015年间，青藏高原草地经历了退化-恢复-退化的过程：高原北部大部分地区草地质量变化不明显，且呈现好转的趋势；南部和东部地区普遍出现草地退化现象，其中局部地区退化情况较为严重.

Spatio-temporal changes in grassland degree in the Qinghai-Tibet Plateau from 2000 to 2015

The study of spatiotemporal variation of grasslands shed light on understanding the causes and effects of transition of grassland quality．This article aims to provide basic data on the cause and effects as well as the mechanisms of grassland degradation．The grassland degree index (GDI) calculated from weighted index of NPP and NDVI was applied to represent the quality and coverage of grassland．Transition matrix and the slope index of GDI demonstrated the spatiotemporal pattern．From 2000 to 2015, total area of Class Ⅰ (high-quality grass) and Class Ⅳ (poor quality grass) remained unchanged．Some 28.11% of class Ⅰ grassland changed into class Ⅱ．The proportion of class Ⅱ that transformed into class Ⅰ and class Ⅲ were 18.19% and 21.53%, respectively．About one third (28.54%) of class Ⅲ transformed into class Ⅳ．From 2000 to 2015, grassland has undergone a trend of degradation-restoration-degradation pattern．The map of slope demonstrated that grassland conditions in the north has improved, while in the south has deteriorated, several local areas had undergone severe degradation in particular．