## 3 Results
### Relationships between Annual GPP Estimates and Topographical Factors  
As shown in Figure 6a,e,i,m, the ratio between \(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{MOD17}}\) showed similar relationships with topographical factors in forest, shrub, and grass pixels, which can be mainly summarized as (1) decreasing with an increase in elevation or slope, (2) increasing with an increase in SVF, and (3) presenting no apparent relationship with aspect. More specifically, a slight decrease was observed in the ratio between \(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{MOD17}}\) when the slope varied from 0\({}^{\circ}\) to 40\({}^{\circ}\), and then the ratio decreased rapidly in the slope range of 40\({}^{\circ}\)-80\({}^{\circ}\). The ratio between \(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{MOD17}}\) decreased when the elevation varied from 2500 m to 3500 m, whereas it showed no obvious variation in the elevation range of 3500-4500 m.  
As shown in Figure 6b,f,j,n, the ratio between \(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{TL-LUE}}\) in forest pixels presented a higher value than that in shrub and grass pixels, and its relationships with topographical factors were similar in the forest, shrub, and grass pixels. The ratio between  
Figure 6: Relationships between annual GPP estimates and elevation (**a–d**), slope (**e–h**), aspect (**i–l**), and SVF (**m–p**). The first to the forth row indicate the \(\text{GPP}_{\text{MTL-LUE}}/\text{GPP}_{\text{MOD17}}\), \(\text{GPP}_{\text{MTL-LUE}}/\text{GPP}_{\text{TL-LUE}}\), \(\text{GPP}_{\text{MTG}}/\text{GPP}_{\text{TG}}\), and \(\text{GPP}_{\text{TL}}/\text{GPP}_{\text{BEPS}}\), respectively. For each vegetation type in the subfigure, the solid line and ribbon represent the mean and standard deviation values in each elevation (calculated at the interval of 100 m), slope (calculated at the interval of 1\({}^{\circ}\)), aspect (calculated at the interval of 10\({}^{\circ}\)), and SVF (calculated at the interval of 0.01) ranges.  
\(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{TL-LUE}}\) decreased with elevation increasing or SVF decreasing, and no obvious relationship was found with aspect. A slight decrease was also observed in the ratio between \(\text{GPP}_{\text{MTL-LUE}}\) and \(\text{GPP}_{\text{TL-LUE}}\) when the slope varied from 0\({}^{\circ}\) to 40\({}^{\circ}\), and then the ratio decreased rapidly in the slope range of 40\({}^{\circ}\)-80\({}^{\circ}\).  
As shown in Figure 6c,g,k,o, the ratio between \(\text{GPP}_{\text{MTG}}\) and \(\text{GPP}_{\text{TG}}\) in shrub pixels presented a higher value than that in forest and grass pixels, and showed similar relationships with topographical factors in forest, shrub, and grass pixels. In general, the ratio between \(\text{GPP}_{\text{MTG}}\) and \(\text{GPP}_{\text{TG}}\) (1) decreased with increasing slope or decreasing SVF, (2) increased gradually and then decreased with the aspect varying from 0\({}^{\circ}\) to 360\({}^{\circ}\), and (3) decreased gradually when the elevation varied from 2500 m to 3500 m, and then increased when the elevation varied from 3500 m to 4000 m.  
As shown in Figure 6d,h,l,p, the ratio between \(\text{GPP}_{\text{BTL}}\) and \(\text{GPP}_{\text{BEPS}}\) presented similar relationships with topographical factors in forest, shrub, and grass pixels, which can be mainly summarized as (1) decreasing rapidly with increasing elevation, (2) increasing gradually with increasing slope, (3) decreasing and then increasing with the aspect varying from 0\({}^{\circ}\) to 360\({}^{\circ}\), and (4) increasing gradually with increasing SVF.