## 4 Discussion  
### Uncertainty of Parameter Sensitivity  
The results of EFAST show different vegetation types vary in terms of sensitive parameters. Parameter sensitivity tests were performed for HBSL and HBGL sites located at the same latitude and longitude in the study, resulting in different combinations of sensitive parameters. On the one hand, the input parameters required for different PFTs are not exactly the same. There are more constant parameters for grass in the EPC file supplied by the developer, such as LWT (annual live wood turnover fraction) and SC:LC (new stem C/new leaf C). As grasslands have no woody stems, these parameters only need to be provided to the shrub or forest site; therefore, no sensitivity analysis is required for these parameters at the grassland site. On the other hand, it might be correlated with the differences in the photosynthetic properties of different PFTs, which lead to different sensitivities of the photosynthesis-related parameters for grassland and shrubs [55]. Take for example FLNR, which determines the magnitude of the maximum carboxylation rate in the model photosynthesis module [56], and the Rubisco enzyme directly affects the first step of the vegetation fixation of atmospheric CO\({}_{2}\). This explains the fact that FLNR is a sensitive parameter for HBSL and not for HBGL. However, the climate and environmental conditions varied under different latitudes and longitudes, and the sensitive parameters of the same vegetation type were not the same, as at all nonwoody sites in this paper. This might be attributed to the significant variation in plant traits along the climatic zone and the relative reduction in the degree of influence when the parameters are no longer limited by hydrothermal conditions. Additionally, we found that the parameter sets sensitive to GPP and NEP were not identical for the same PFTs at the same site. It is a more complicated process to calculate NEP compared to GPP. Gross primary productivity (GPP) minus the part consumed by vegetation respiration and the part consumed by heterotrophic respiration (soil respiration) is the net ecosystem productivity (NEP), which involves processes such as C and N cycles, the autotrophic (maintenance and growth) stage and the heterotrophic (decomposition) stage of the Biome-BGC model. As a result, the different geochemical cycling processes involved in GPP and NEP may have contributed to the difference in the sensitivity of different definitions of plant productivity to the same parameter. Through a comprehensive analysis of the existing studies [12; 57], when researchers selected sensitive parameters using different methods (annealing algorithms and the Morris method, etc. ), they obtained not completely identical results. One study conducted a sensitivity analysis of the parameters of the Biome-BGC model with the Morris method and EFAST method, respectively [58]. The parameters' nascent fine-root-to-leaf carbon partition ratio and fine-root carbon-to-nitrogen ratio were screened as sensitive parameters by EFAST, while in the Morris method, they were not. In addition, researchers have set different thresholds for dividing sensitive parameters. For example, some researchers have considered parameters with a total sensitivity index greater than 0.05 as influential parameters in the parameterization of the Biome-BGC model [59]. Other researchers have divided the sensitivity index into three categories in sensitivity analysis when using the Biome-BGC model to simulate forest carbon fluxes, namely, strong-effect parameters (>0.2), medium-effect parameters (>0.1 and \(\leq\)0.2), and low-effect parameters (\(\leq\)0.1) [29]. The diversity of the threshold classification standards leads to different types and numbers of sensitive parameters screened. To summarize, the uncertainty of model sensitivity parameters originates from many aspects, including the type of vegetation at the study site, the spatial location, the output variables of the model (GPP, NPP, or NEP), and different sensitivity analysis methods.