## 4 Discussion
### The Contribution of the GGP Implementation to Ecological Change  
Climate change can enhance the ecological condition to some extent, but the GGP only affected new vegetation in the GGP area. To find out the contribution of GGP implementation to the ecological benefit, a paired [PERSON]'s \(t\)-test was conducted to compare the slope of annual \(\text{FVC}_{\text{max}}\), \(\text{LAI}_{\text{max}}\), \(\text{NPP}_{\text{avg}}\), \(\text{WR}_{\text{pua}}\), \(\text{SWAL}_{\text{pua}}\), and \(\text{SWIL}_{\text{pua}}\) changes between two LULC types: (1) the restored vegetation (including forest and grassland) transferred from another LULC type from 2000 to 2010 (LULC A); and (2) the retained forest and grassland that existed before 2000 and remained untransformed from 2000 to 2010 (LULC B). The sample points of LULC A (denoted as Sample A) were selected from the pixels that were transferred to forest and grassland in 2000-2010 at a 1 km spatial resolution. The sample points of LULC B (denoted as Sample B) were randomly selected from the intersecting pixels of forest and grassland in the 2000, 2005, and 2010 LULC dataset in the nearest neighborhood of Sample A, to ensure similar climate conditions and further exclude the influence of climate change. Both samples had about 15000 points, involving 56% forest and 44% grassland. The null hypothesis and alternative hypothesis are listed in Table 7. Results showed that the slope of annual \(\text{FVC}_{\text{max}}\), \(\text{LAI}_{\text{max}}\), \(\text{NPP}_{\text{avg}}\), and \(\text{SWAL}_{\text{pua}}\) changes passed the paired [PERSON]'s \(t\)-test at a very significant level, whereas the slope of annual precipitation and temperature changes did not, indicating that the recovery/improvement rates of EQ and SC in the revegetation area were higher than existing vegetation under similar climate conditions. Therefore, the GGP implementation rather than climatic change was the primary cause for the estimated increase of ecosystem quality and services, which is supported by many studies [15; 19; 26]. The slope of annual \(\text{FVC}_{\text{max}}\), \(\text{LAI}_{\text{max}}\), and \(\text{NPP}_{\text{avg}}\) changes in LULC A increased by 14.23%, 9.94%, and 8.23%, respectively, compared with those in LULC B, while the slope of annual \(\text{SWAL}_{\text{pua}}\) change in LULC A decreased by 30.45% compared with LULC B. This change should be considered as the contribution of the GGP implementation. As the slope of annual \(\text{WR}_{\text{pua}}\) and \(\text{SWIL}_{\text{pua}}\) changes did not pass the t-test, we further conducted paired [PERSON]'s \(t\)-tests on them between LULC A and LULC B in forest and grassland separately, and found that the slope of \(\text{SWIL}_{\text{pua}}\) in restored grassland passed the \(t\)-test with high significance and it decreased by 18.05% more than that in LULC B grassland (\(\mu_{\text{A}}=-0.2260\), \(\mu_{\text{B}}=-0.1914\), P < 0.01). The slope of annual \(\text{SWIL}_{\text{pua}}\) change in restored forest and the slope of annual \(\text{WR}_{\text{pua}}\) change in neither restored forest nor grassland did not pass the t-test. The results further confirmed our findings in the last section. Afforestation with inappropriate species in arid and semi-arid regions can damage local water balances because of the high consumption of plant transpiration and evaporation from the soil