## 4 Results
### Spatial and Temporal Patterns of GPP across North America
#### 4.3.2 Spatial Characteristics of Seasonal GPP Simulation in the TL-CLUE Model  
Across North America, GPP showed obvious spatial heterogeneity and seasonal differences (Figure 9a). In the leaf-on season, GPP was approximately 0 north of 60\({}^{\circ}\)N, increasing to 800-1200 g C m\({}^{-2}\) yr\({}^{-1}\) at 40\(\sim\)50\({}^{\circ}\)N, and increasing to the largest value of 1600-2400 g C m\({}^{-2}\) yr\({}^{-1}\) south of 40\({}^{\circ}\)N. This result indicates that GPP increases with decreasing latitude. GPP was less than 800 g C m\({}^{-2}\) yr\({}^{-1}\) west of 100\({}^{\circ}\)W, 800-2400 g C m\({}^{-2}\) yr\({}^{-1}\) east of 100\({}^{\circ}\)W, and below 400 g C m\({}^{-2}\) yr\({}^{-1}\) in the Rocky Mountains and on the Labrador Plateau east of Hudson Bay (Figure 9a). However, values of 800-2400 g C m\({}^{-2}\) yr\({}^{-1}\) appeared in the middle and eastern regions, which are mainly composed of plains and mountains. While the spatial patterns of GPP during the leaf-gathering season were comparable to those during the leaf-on season, the magnitude was significantly lower, especially in the eastern coastal regions south of 50\({}^{\circ}\)N with a range of 800 to 1200 g C m\({}^{-2}\) yr\({}^{-1}\) (Figure 9c). However, during the leaf-scattering season, GPP ranged from 0 to 400 g C m\({}^{-2}\) yr\({}^{-1}\) over most of North America, and only in the eastern coastal regions south of 40\({}^{\circ}\)N did it rise to approximately 700 g C m\({}^{-2}\) yr\({}^{-1}\) (Figure 9d). These findings suggest that the contribution of the leaf-gathering season to GPP is more than that of the leaf-scattering season. In summary, the spatial characteristics of GPP across North America were largely determined by latitude, longitude, and terrain. In the leaf-off season, GPP was approximately 0 in the main areas across North America (Figure 9b) and 400-900 g C m\({}^{-2}\) yr\({}^{-1}\) only in coastal areas south of 35\({}^{\circ}\)N.  
Throughout the year, the spatial pattern of GPP was similar to this pattern in the leaf-on season (Figure 10). GPP was less than 700 g C m\({}^{-2}\) yr\({}^{-1}\) on the Labrador Plateau, the Rocky Mountains, and north of 60\({}^{\circ}\)N, 700-2000 g C m\({}^{-2}\) yr\({}^{-1}\) at 45-60\({}^{\circ}\)N, and 2000-3200 g C m\({}^{-2}\) yr\({}^{-1}\) in the middle and eastern areas and coastal areas at lower latitudes (Figure 10), which reveals that latitude and elevation are the key factors for the spatial pattern of GPP across North America.  
Figure 9: The spatial characteristics of GPP simulated by the TL-CLUE model in the leaf-on season (**a**), leaf-gathering season (**b**), leaf-scattering season (**c**), and leaf-off season (**d**) across North America during 2002–2020. GPP in the leaf-on season shows obvious spatial heterogeneity; the lowest value, ranging from approximately 0 to 400 g C m\({}^{-2}\) yr\({}^{-1}\), occurs in the high elevation areas of the Rocky Mountains and high latitude areas north of 52\({}^{\circ}\)N, and the highest value, ranging from approximately 1800 to 2200 g C m\({}^{-2}\) yr\({}^{-1}\), occurs on the east coast south of 48\({}^{\circ}\)N. GPP is near 0 in the leaf-off season for most areas of North America.