## 5 Discussion  
In this research, we examined the addition of vertical stratification parameters such as temperature, VPD, PAR, LAI, and LUEmax to support remote sensing GPP estimation at the site level. Temperature and VPD influenced the LUEmax at various canopy heights because the vertical meteorology environments have greater variations at forest sites than those in other vegetation  
Figure 10: Differences in GPP estimation using different models with carbon flux tower measurements. Subfigure (**a**) showed the condition in DHS, and subfigure (**b**) showed it in TMK. VPMm results from using the multilayer VPM GPP minus the flux tower-based GPP. VPMs results from using the single-layer VPM GPP minus the flux tower-based GPP. The Y-axis on the right is the daily mean shortwave incident radiation. types, such as shrub and grass sites. PAR and LAI were used to determine the APAR of different sub-ecosystems, which would lead to a great deal of uncertainty in energy allocation. Vertical LUE is influenced by the vertical distributions of temperature, VPD, and LUEmax, but there are uncertainties when modeling the LUE in different sub-ecosystems. LUEmax values differ in sub-ecosystems, as explained in Section 4.2.1 of this article. At the TMK site, the GPPdif changes from \(-50\%\) to \(30\%\) when the air temperature at the canopy height increases from approximately \(7~{}^{\circ}\)C to \(20~{}^{\circ}\)C. The variation is more significant at the TMK site than that at the DHS site, and one reason for this difference is that the temperature differences among layers are more obvious at the TMK site than at the DHS site, as shown in Figure 2. The effect of temperature stress on LUE differs for tree, shrub, and grass layers under the same mean canopy weather conditions. As the optimal temperatures of the tree, shrub, and grass layers differ by 2-5 \({}^{\circ}\)C at the same air temperatures in the forest canopy, the temperature stresses of the layers on LUEmax differ, varying by 5-20% around the optimal temperature [78; 83]. However, when the temperature is above \(23~{}^{\circ}\)C, GPPdif changes only slightly because the canopy and the understory layers experience high temperature stress, and the LUE would decrease rapidly at this temperature range. On the other hand, the variation in GPPdif did not significantly change with the increase in VPD, as Figure 6B shows that GPPdif at the DHS site decreases only slightly when the VPD increased. [PERSON] et al. [84] showed that the water contents of plants are more related to soil moisture than to the water content of the air in seasonally moist forests. The plants in the overstory and understory layers share the soil water content. Thus, the VPD does not have a significant effect on the GPP estimation in dense forests. All of these effects result in different LUE values at distinct canopy heights, as shown by previous research [85]. Uncertainties in the vertical LAI distribution lead to uncertainties in APAR in the sub-ecosystems. LAI, which is a key parameter for separating an ecosystem into different photosynthetic components, determines the contribution of different species to total ecosystem photosynthesis, the light transfer process in the canopy, and the micrometeorological environment inside the forest [58]. As many studies have shown that the distribution of PAR through the canopy structure is highly affected by LAD, LAI, and the characteristics of different species in forest canopies with high heterogeneity, the PAR distribution must consider the vertical structure [39; 58; 82]. The overstory tree layer determines the intercept ratio of the incident PAR, so if the LAI in the overstory is high, a large amount of PAR would be absorbed during the first turn of intercepted light.