## 4 Discussion
### Improvements of MTL-LUE, MTG, and BTL in Simulating GPP over Mountainous Areas
#### 4.1.1 Improvement of MTL-LUE over MOD17 and TL-LUE  
MOD17 is a big-leaf LUE model with the assumption that the whole canopy can absorb direct and diffuse solar radiation simultaneously. Various studies suggested that sunlit leaves, which absorb both direct and diffuse radiation, are easily light-saturated, and the photosynthetic capacity of shaded leaves, which only absorb diffuse radiation, is normally constrained by low APAR [10; 29; 30; 31]. TL-LUE and MTL-LUE are both two-leaf LUE models with different treatments of sunlit and shaded leaves. In the TL-LUE model, an empirical relationship established from the diffuse and total incoming radiation measurements at four sites [29], is adopted to separate the direct and diffuse radiation from the total radiation [33; 35]. This empirical relationship may cause biased separation of direct and diffuse radiation over mountainous areas because (1) the direct radiation is affected by local solar zenith angle (which can be expressed by slope and aspect) [38; 84] and (2) the diffuse radiation would reduce when the hemispherical sky dome is partially blocked (can be expressed by SVF) [38; 40; 85]. Moreover, TL-LUE also neglects the effect of topography on the portion of sunlit and shaded canopy areas, which may lead to bias in the separation of sunlit and shaded LAI, because the sunlit canopy area can be altered by the relative positioning of geotropic trees [43; 44; 45]. The MTL-LUE model incorporates topographic attributes (i.e., slope, aspect, and SVF) into the TL-LUE model to improve the characterization of direct and diffuse radiation (as shown in Figure 7a) and the separation of sunlit and shaded LAI (as shown in Figure 7b).  
At the watershed scale, the GPP estimates from MTL-LUE were found to have a lower annual value than those from MOD17 in all the forest, shrub, and grass pixels, with the mean value decreasing by 127 gC m\({}^{-2}\)yr\({}^{-1}\). The lower annual GPP estimates from MTL-LUE may stem from the fact that the MTL-LUE model considers the decrease of incoming solar radiation caused by the shielding of topography: (1) direct radiation varies with the geometric relationship between the tilt of ground surface and the solar position and (2) diffuse radiation decreases with the tilt of the ground surface itself and surrounding terrain. Results also indicated that the GPP estimates from MTL-LUE were observed to have a similar value with those from TL-LUE. Over mountainous areas, the difference between GPP estimates from MTL-LUE and TL-LUE at the watershed scale depends on the gap between the GPP variations of sunlit and shaded leaves: (1) the sunlit GPP estimates from MTL-LUE would be higher in the sunlit terrain (due to the higher sunlit LAI and direct radiation) and lower in the shaded terrain (due to the lower sunlit LAI and direct radiation) than those from TL-LUE and (2) the shaded GPP estimates from MTL-LUE would be lower in the sunlit terrain (due to the lower shaded LAI and lower diffuse radiation) and higher in the shaded terrain (due to the higher shaded LAI) than those from TL-LUE.  
Our finding that the ratio between annual GPP estimates from MTL-LUE and MOD17 (or TL-LUE) decreased with increasing slope or decreasing SVF, is expected because more heterogeneous terrain (e.g., a higher slope and a lower SVF) would cause a greater decrease in received solar radiation. When the elevation varied from 2500 m to 3500 m, although the diffuse radiation increased due to the increase of SVF (as shown in Figure 8), direct radiation would decrease because of the increased slope, which may be the reason for  
Figure 7: Effect of topography on incoming solar radiation (**a**), canopy shadow (**b**), and water (**c**) over mountainous areas.  
the decrease in the ratio between annual GPP estimates from MTL-LUE and MOD17 (or TL-LUE).