## 4 Discussion
### Response of GPP to Drought Conditions  
The main physiological process affected by drought stress is photosynthesis [36]. GLASS and LUE-CI products are light use efficiency products, and vegetation indices are important input data for light use efficiency models [37], which are only sensitive to changes in canopy structure and pigment concentration and are not directly related to photosynthesis, so the lag effect of GPPGLASS and GPPLUE is significant and the precision is low. In contrast, SIF data are directly related to photosynthesis, and GPPLIF has a clear response advantage for drought monitoring. The spatial pattern distribution of the monthly average value of GPPd and the drought index sc_PDSI in Yunnan Province from 2009 to 2011 is relatively consistent, and there is a good correspondence in terms of values, indicating that GPPLIF responds well to drought. The response of GPPLIF to drought varies from region to region, with severe drought in Eastern Yunnan. Most of the regions respond to drought rapidly and strongly, and GPPd also decreased significantly. In particular, in Region 8, Southeastern Yunnan, GPPd reached 21.58% of the monthly mean value of GPP. The region is home to severe, stony desertification, mostly grows scrub, and the vegetation demonstrates poor self-regulation, Long-term severe drought causes the vegetation in the region to fail to grow normally and GPP decreases significantly. This is also consistent with the findings of carbon flux observation [38; 39], large-scale remote sensing inversion [40; 41], and model simulation [3; 41], showing that \"drought significantly weakens the carbon sink function of ecosystems\". Meanwhile, the smaller decline value of GPPd in Region 5, Eastern Yunnan may be related to the smaller GPP of grassland vegetation itself. Southwest Yunnan responds slowly to drought, and drought has less impact on it. Evergreen broad-leaved forests and tropical rainforests are widely distributed in Southwest Yunnan, with extremely high forest cover and abundant precipitation, so this region is less sensitive to water stress. Although long-term severe drought still causes GPP to decline, GPPd in some areas showed positive values, indicating that drought promoted vegetation growth in these regions, which is consistent with the findings of [PERSON] et al. [33] based on the MODIS enhanced vegetation index (EVI) for the 2005 drought event in Southwestern Amazonia. [PERSON] et al. [32] also found that observations from some flux sites indicated that low precipitation and low cloudiness led to more CO\({}_{2}\) uptake in tropical forests limited by radiation intensity during drought events. It is worth noting that the GPPd values in Kunming in Central Yunnan and Qujing in Eastern Yunnan are also positive, which is due to the fact that Kunming is a national garden city, and Qujing is mostly vegetated with farmland, so the vegetation growth in these areas is mainly influenced by human factors, leading to an increase in GPP. The drought is less severe in the alpine areas of Northwest Yunnan, GPPd increases significantly in areas that are 4000 m above sea level, which echoes the negative correlation between SIF and sc_PDSI in some areas of Northwest Yunnan in Section 3.1. Snow and ice cover in high altitude areas and melting snow replenish water during drought, which relieves the hydraulic pressure on vegetation growth [42], coupled with increased solar radiation to promote vegetation photosynthesis. Thus, GPPd increased, indicating that mild drought can promote vegetation growth at high altitudes. After the drought is relieved, the recovery time of GPP has strong spatial heterogeneity. The fastest recovery is in Southwest Yunnan, which is firstly due to the fact that Southwest Yunnan has lush vegetation and mostly evergreen broad-leaved forests and tropical monsoon rainforests, which are more resilient and recovering to drought stress than other regions [43]. Secondly, the recovery capacity of vegetation will gradually decrease as the drought time continues to intensify [44].