Spatiotemporal variation and influencing factors of vegetation water use efficiency in Yunnan Province

The study of the spatiotemporal characteristics and influencing factors of vegetation water use efficiency（WUE） in Yunnan Province could provide a more comprehensively understanding of the regional response to global climate change. In this study, MODIS data were used to quantitatively estimate the water use efficiency in Yunnan Province from 2000 to 2014, and trend analysis and correlation analysis were used to investigate the spatiotemporal patterns and influencing factors. The results showed that:（1） the overall WUE in Yunnan Province indicated a significant upward trend, with a growth rate of 0.0078 gC mm-1 m-2 a-1, and exhibited an "M" shape trend during the year. The drought events that occurred during 2009-2013 had a lagged positive effect on the vegetation WUE in this area. The vegetation WUEs of different land use types, in descending order, were forest, shrubland, grassland and the cultivated land.（2） The spatial distribution of WUE was higher in the west than in the east with a tendency of increasing in the north and decreasing in the south. The highest WUE was found in Lijiang, the region connecting the Yunnan-Guizhou Plateau and the Qinghai-Tibet Plateau, where the WUE was greater than 2.5 gC mm-1 m-2 overall. The vegetation WUE in the Three Parallel Rivers of upper Lancang River showed a striped distribution with the trend of the mountains, and was a concentration of low vegetation values. But it was also a focus where the WUE increased by more than 10%. In addition, northeastern and southeastern Yunnan were also areas with low WUE. Overall, except for Lijiang, there was a decreasing trend of high WUE and an increasing trend of low WUE from 2000 to 2014.（3） With the rise of altitude, WUE showed a tendency to increase first and then decrease, and the increase of WUE was more obvious in the higher altitude, with a change rate of 0.01%. Both evapotranspiration（ET） and Gross Primary Productivity（GPP） were prominent at low altitude, and with the rise of altitude ET plummets and GPP fluctuates and decreases until the peak of WUE was reached 3000 m. Above about 3000 m, the GPP dropped linearly and the ET fluctuated until the WUE was almost zero at about 5000 m.（4） Compared with precipitation, WUE was more obviously influenced by temperature, especially in the Three Parallel Rivers, while other regions were more significantly influenced by non-climatic factors. In the context of global climate change, the findings demonstrated the regional response t