## 3 Results
### Drought Evolution and GPP Response to Drought in Yunnan
#### 3.3.3 Response Characteristics of GPPSIF to Drought
The highest values of GPPd are mainly located in Region 1 in Northwest Yunnan, Region 6 in East Yunnan, and a small part of Southwest Yunnan (Figure 14a). The distribution area of high values is small, and the average down is only 4.25-24.09 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). Most of the areas are negative values of GPPd. The lowest values of GPPd are mainly located in Chuxiong in Central Yunnan, Zhaotong in Northeast Yunnan, and most of Southeast Yunnan, and the down values in these areas are about \(-34.79\)\(\sim\)\(-78.27\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). The response characteristics of GPP can be found more intuitively from the small regions analysis. Southwest Yunnan was less affected by drought. For example, Region 4 of Southwest Yunnan began to have a drought in June 2009, when the GPPd was \(-0.29\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in July-September (Figure 13), which was still basically the same as the normal period. The monthly average value of GPPd in this region was \(-20.28\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) throughout the drought period, which was less affected. Region 3 of Western Yunnan started to have a slight drought in January 2009 and the GPPd was only \(-6.82\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in July 2009; the sc_PDSI was \(-1.42\) at this time. The monthly mean value of GPPd in Region 3 during the drought was \(-18.07\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), combined with the severe drought in this region (sc_PDSI in 2009 mean value of \(-2.14\)) and GPPd indicates that the drought had little effect on the region. On the contrary, Region 6 in East Yunnan and Region 8 in Southeast Yunnan experienced severe drought. The monthly average values of GPPd in drought years are  
Figure 14: The total value of GPPd and the mean value of sc_PDSI in all months, 2009-06–2011-06. (**a**) total value of GPPd; (**b**) mean value of sc_PDSI. \(-34.74\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) and \(-63.22\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), respectively, which are significantly higher than the monthly average value of GPPd in the province \(-23.31\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). This was especially evident in Region 8, where GPPd reached 21.58% of the monthly GPP mean value, indicating that this area responds strongly to drought. Meanwhile, Region 5, located in Eastern Yunnan, responded to drought in June 2009 when drought conditions worsened (the sc_PDSI monthly mean value of \(-3\)) although the GPPd decline value was relatively small, with a GPPd monthly mean value of \(-26.08\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). As for Region 1 in Northwest Yunnan, the mean value of GPPd from 2009 to 2011 was 1488.63 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), which was higher than the mean value of 1392.48 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in previous years, indicating that drought promoted the photosynthesis of vegetation in this region. Figure 15 shows the scatter plot of the raster values of the two plots in Figure 14, with the horizontal axis representing the mean value of sc_PDSI and the vertical axis representing the total value of GPPd. We found that the coefficient of determination R\({}^{2}\) between GPPd and sc_PDSI was 0.46, indicating that GPP\({}_{\rm{SIF}}\) responded in some degree to the drought.