## 4 Results and Analysis
### Influence of Vertical Stratification on GPP Estimation
#### 4.2.2 Vertical Stratification of APAR Influenced GPP Estimation by the Multilayer Model  
Forest structure parameters such as LAI influenced the absorption of radiation by the forest canopy, so the total APAR of the forest canopy can be calculated using LAI and incident PAR. In this section, we analyze LAI and incident PAR as influenced by the APAR in different sub-ecosystems and the effect of separate APARs in the sub-ecosystems on GPP modeling. In a natural environment, the variety of vertical LAI distributions leads to different absorbed PAR distributions in the tree, shrub, and grass layers, which have diverse photosynthetic capacities that lead to GPP modeling biases. Figure 7 shows the total GPP in different LAI distributions. We made two assumptions. The first assumption is that the LUE on a daily scale in different layers is the mean value, which is 1.05 gC/m\({}^{2}\)/MJ in the tree layer, 0.83 gC/m\({}^{2}\)/MJ in the shrub layer, and 1.47 gC/m\({}^{2}\)/MJ in the grass layer. These values are the average values of the results of three models considering optimal growing environments. The APAR for each layer is influenced by only the LAI and the total incident PAR in the top layer. The second assumption is that the LUEmax in the shrub layer is lower than that in the tree and grass layers [32]. Similar to the results at the DHS site, when the LAI ratio in the tree, shrub, and grass layers is 2.5:0.6:0.9 (at DOY 249), GPPm is 5% higher than GPPs. At the TMK site during the summer, the LAI ratio in the tree and grass layers is 2:1, and GPPm is 15% higher than GPPs. As in the shrub and grass layers, the understory LAI would thus be the most important factor for GPP, and its ratio to total LAI leads to a range of \(-\)40% to 25% for the GPP estimation uncertainties. Incident PAR in the forest canopy also determined the GPP modeling difference whether or not vertical stratification was considered. Figure 8A,C show that considering the effect of vertical stratification on GPP leads to results that are similar to the GPP values that are measured by the flux towers because understory species such as shrubs and grasses have high photosynthetic capacities despite receiving little incident PAR [82]. When the incident PAR of the top of the canopy is lower than 175 W/m\({}^{2}\) at these two sites, the value of the flux tower-based GPP is 1-2 gC/m\({}^{2}\) lower than GPP, and is closer to GPPm than GPPs. When the incident PAR is higher than 175 W/m\({}^{2}\), GPPm is 0.5-2 gC/m\({}^{2}\) higher than the flux tower-based GPP, which has a lower bias than GPPs. Figure 6: Effects of both considering and not considering the vertical differences in VPD on GPP. The black line is the fit line for the GPP difference scatterplot at different sites. Subfigures (**A,B**) are the GPP difference comparisons based on different canopy mean VPDs at TMK and DHS, respectively. Moreover, at high temperatures at the DHS site, the estimation of GPPm is lower than the estimation of GPPs, and the difference between GPPm and GPPs increases with increasing incident PAR (Figure 8B,D). At the TMK site, the difference between GPPm and GPPs increases with increasing PAR, and reaches a maximum of approximately 70% of GPPs, which is much higher than that at the DHS site. Figure 8: GPP differences considering multilayer and single-layer estimations of incident PAR. Subfigure (**A,C**) show the GPP simulation results with flux tower measurements of GPP. The green points represent multilayer results (GPPmMOD), and the red points represent single-layer results (GPPsMOD). The black points represent the flux tower-based GPP results (GPPactual). Subfigure (**B,D**) show GPP differences under various incident PAR. Figure 7: Ternary contour for the differences between considering and not considering the vertical structure during the GPP estimation with LAI separated into tree, shrub, and grass layers. This graph illustrates that when the total LAI of the canopy is four, there is little temperature and humidity stress on the LUE.