Spatiotemporal evolution of vegetation net primary productivity in the karst region of southwest China from 2001 to 2018

植被净初级生产力（Net primary productivity，NPP）是陆地生态系统碳循环的重要指标，定量评价NPP的变化规律是碳循环机理的主要研究内容之一。以我国西南地区为研究区域，基于土地覆盖和NPP产品分析了2001-2018年西南地区NPP的时空变化特征，利用土地转移矩阵和统计分析方法，定量描述了土地覆盖类型变化导致的NPP变化量，对NPP变化的主要原因进行分析。结果表明：（1）2001-2018年，西南地区NPP均值呈波动上升趋势。（2） NPP变化趋势在空间分布上呈南高北低，NPP呈增长趋势主要集中在四川的中东部、重庆、贵州西部和云南东部；NPP呈减少趋势出现在云南、四川中西部和贵州东部。（3）森林、草地和耕地转灌丛以及灌丛转森林是对NPP变化量影响较大的土地覆盖类型转变，土地类型转变导致的NPP净增量为20.643 TgC。（4）降水与NPP相关性低，气温与NPP有一定相关性，植被叶面积指数与NPP有显著相关关系，表明气候因子对NPP的影响较小，植被生长密度对NPP有显著影响。退耕还林还草等生态工程实施，导致耕地面积减少和森林面积大量增加，是西南地区NPP上升的主导因素，因此生态工程的实施是西南地区NPP增加的重要影响因素。研究可为生态工程实施背景下NPP的变化机理研究提供参考。;Net primary productivity (NPP) of vegetation is an important indicator of the carbon cycle of terrestrial ecosystems. Quantitative evaluation of the changing law of NPP is one of the main research contents of carbon cycle mechanism. This article analyzes the spatiotemporal changes of NPP in southwest China from 2001 to 2018 based on land cover and NPP products. We use land transfer matrix and statistical analysis methods to quantitatively describe the amount of NPP change caused by land cover changes and analyze the main reasons for the NPP changes. The results showed that:(1) from 2001 to 2018, the average value of the NPP in the southwest China presented a fluctuating upward trend. (2) The spatial distribution of NPP showed a high in the south and a low in the north. NPP showed a growing trend, mainly in the central and eastern parts of Sichuan, Chongqing, western Guizhou, and eastern Yunnan; NPP indicated a decreasing trend in Yunnan, central and western Sichuan and eastern Guizhou. (3) The conversion of forests, grasslands, and the cultivated land to shrubs and shrubs to forests were land cover types that had a greater impact on the amount of NPP changes. The net increase in NPP caused by land type conversions is 20.643 TgC. (4) The correlation between precipitation and NPP is low. There is some correlation between air temperature and NPP and the vegetation leaf area index has a significant correlation with NPP, indicating that climatic factors have a small impact on NPP, and vegetation growth density has a significant impact on NPP. The implementation of ecological projects such as returning farmland to forest and grassland has led to a decrease in cultivated land and a substantial increase in forest area, which is the leading factor in the NPP increase. This study can provide a reference for the change mechanism of NPP under the background of ecological engineering implementation.