## 4 Results  
### Regional-Scale Model Comparisons  
The spatial distributions of 12-year (2001-2012) average annual gross primary productivity (AAGPP) of deciduous broadleaf forests in the eastern United States vary across models and datasets (Figure 1). The MOD17A2H product, a widely used dataset in land surface studies, shows an east-west gradient in the spatial distribution of AAGPP, where inland areas have low AAGPP. Differently, AAGPP derived from VPM exhibits a south-north gradient, where southern areas have higher AAGPP than northern areas. TL-LUE generally agrees with VPM in terms of the spatial distribution of AAGPP but TL-LUE has much higher AAGPP estimates than VPM in most areas. Among the six tested methods, GPD matches the data-driven dataset of EC-MOD in terms of the spatial distribution and magnitude of AAGPP. Compared with other methods, the EC-MOD data appear to contain a large spatial heterogeneity of AAGPP. Similar to MOD17A2H, the BESS product has lower AAGPP in inland areas but also shows suspicious mosaicking effects in the modeled AAGPP. The zonally and meridionally averaged AAGPP profiles also vary widely across models (Figure 2). In the south-north direction, zonally averaged AAGPP of MOD17A2H does not change with latitudes (\(p\) value = 0.304) while AAGPP derived from all other models decreases as latitude increases (Table 3). The zonally averaged AAGPP derived from BESS decreases at a much slower rate (\(-21.962\) gC/m\({}^{2}\)/year/degree latitude) along latitude than those derived from the other models. In the west-east direction, meridionally averaged AAGPP derived from MOD17A2H increases as longitude increases (\(6.797\) gC/m\({}^{2}\)/year/degree longitude), meridionally averaged AAGPP derived from BESS does not change with longitude (\(p\) value = 0.944), and all the other modeled results have decreased AAGPP with increasing longitudes. Although the zonally and meridionally averagedAAGPP profiles of TL-LUE, VPM, GPD, and EC-MOD have similar trends, the absolute values of AAGPP could vary largely: for example, AAGPP values derived from TL-LUE are nearly 1.4 times higher than those derived from VPM. Figure 1: The spatial distribution of 12-year (2001–2012) average annual gross primary productivity at 1000 m spatial resolution as derived from (**a**) MOD17A2H; (**b**) two-leaf light use efficiency (TL-LUE); (**c**) Vegetation Photosynthesis Model (VPM); (**d**) GPD; (**e**) Breathing Earth System Simulator (BESS); and (**f**) eddy covariance and MODIS (EC-MOD). MODIS: Moderate-Resolution Imaging Spectroradiometer. Given the mismatched spatial distributions of AAGPP across models, the spatial distribution of average meteorological variables could provide qualitative references to understand the sensitivities of modeled AAGPP to climate conditions. The average LAI, average air temperature, and average VPD have similar spatial distributions, where their values are lower in high-latitude regions than in low latitude regions and are lower in mountainous regions than in low-altitude regions (Figure 3). Note that high radiation absorption (i.e., high LAI and solar radiation) and air temperature mostly enhance vegetation photosynthesis, while high VPD prohibits stomatal openness and negatively impacts vegetation photosynthesis.