## 4 Discussion
### Contribution of Major Restoration Activities to GPP Increase  
Afforestation significantly contributed to GPP increase (14.9% and 24.6% for BESS and MODIS GPPs, respectively) in afforestation and grass planting areas due to its large area coverage. Over 54% of LUCC areas and 98% of major restoration areas were attributed to afforestation. In addition, planting trees can considerably improve vegetation cover and production due to the high biomass of trees. Although grass planting contributed to a small proportion of the total actual GPP (0.03% and 0.41% for BESS and MODIS GPPs, respectively) in the entire restoration area, it contributed over a quarter of the actual GPP increase in the grass planting area. In addition, grass planting was mostly distributed around the west of Sichuan and cannot be replaced by afforestation due to the environment of the fragile ecosystem, such as the occurrence of water deficit. Therefore, grass planting also plays an important role in vegetation recovery in the study area. Overall, the annual GPP increase in afforestation and grass planting areas exerted a positive effect on GPP increase in the entire study area. Afforestation and grass planting areas contributed more than 1% of the annual GPP increase because they accounted for less than 1% of the total study area. In addition, ERP also includes closed hillside afforestation to conserve forests and nature reserve areas [4], which are difficult to recognise in remote sensing images and disregarded in the current study. Therefore, the contribution of ERPs to GPP increase may be larger than that reported in this work.  
Generally, precipitation and temperature have a large impact on the dynamics of vegetation growth such as the normalised difference vegetation index [54; 55; 56], leaf area index and GPP [57]. In this study, annual precipitation and temperature exerted a small influence on GPP dynamics in restoration areas. Only a minor part (i.e., 8% for BESS GPP and 17% for MODIS GPP) of GPP exhibited significant correlations (\(p<0.05\)) with the annual precipitation and temperature in the entire restoration area (Figure 12). Furthermore, this influence due to precipitation and temperature might lead to a decrease in the GPP (Table 3). The GPP decrease was largely related to the reductions in annual precipitation and temperature, which were also found in a previous study conducted during the same period [58].  
In terms of climate change, vegetation recovery through ERPs should be the focus of future investigations to alleviate the GPP decrease caused by precipitation and temperature. Even if an extreme drought occurred during the period of 2009-2010 [52; 53], annual GPP in the afforestation area exhibited minimal decrease and even increased as shown by MODIS GPP (Figure 9). As an effective method for improving vegetation recovery, ERPs can help enhance an ecosystem by improving vegetation structure and weakening the negative effects of climate factors (e.g., precipitation and temperature) on GPP dynamics. In addition, trees and grasses resulting from afforestation and grass planting will grow as  
Figure 11: (**A**) Evaluation of BESS GPP and MODIS GPP products against flux observations from Xishuangbanna and (**B**) the annual variability of flux GPP, BESS GPP, and MODIS GPP during the period of 2003–2010.  
time progresses and continuously contribute to the improvement of the regional ecosystem environment. Therefore, ERPs, including afforestation and grass planting, will increase GPP and reduce an ecosystem's sensitivity to climate perturbations.