## 3 Results
### Spatial Change Trends of Carbon Cycle in Forest Ecosystems of Hangzhou  
Figure 7 illustrates the spatial distribution of the annual mean values and coefficients of variation (CVs) for GPP, NPP, and NEP in Hangzhou's forest ecosystems, based on a spatiotemporal simulation of the carbon cycle. Figure 7a,c,e depict the spatial patterns of  
Figure 5: NEP Spatial distribution.  
Figure 6: Trends in Hangzhou’s forests’ yearly average GPP, NPP, and NEP values, 2001–2020.  
the annual mean values for GPP, NPP, and NEP, respectively, while Figure 7b,d,f present their corresponding CV distributions. The annual mean GPP (Figure 7a) is higher in the northwest compared to the southeast, with slightly lower values observed in the central region. The CV of GPP (Figure 7b) exhibits clear spatial heterogeneity, showing generally lower values in the northwest. The annual mean NPP (Figure 7c) demonstrates spatial variability, with higher values in the east compared to the west, and in the north compared to the south. The CV of NPP (Figure 7d) follows a similar spatial pattern, with larger variations in the eastern and southern regions. As shown in Figure 7e, NEP values are higher in the southeast than in the northwest, and slightly higher in the east compared to the west. The CV of NEP (Figure 7f) is somewhat elevated in the southwestern region.  
In this investigation, the trend and significance of the changes of GPP, NPP, and NEP were calculated and shown in Figure 8. Figure 8a,c,e display the spatial distribution of the trends of GPP, NPP, and NEP, while Figure 8b,d,f show the spatial distribution of their statistical significance, respectively.  
Figure 7: Annual mean (**a**) and CV (**b**) for GPP, annual mean (**c**) and CV (**d**) for NPP, and annual mean (**e**) and CV (**f**) for NEP in terms of their spatial distribution.  
Analyzing Figure 8a,b, it is evident that the GPP in Hangzhou forests is trending downward in the center and northwest regions while showing an upward trend in the south and northeast regions. Specifically, 8.79% of the area experienced an increasing GPP trend, with only 0.57% showing a statistically significant increase, primarily located in the northwest of Lin'an, the southwest of Jiande, and the southeast of Fuyang. Conversely, 91.21% of the area experienced a decreasing GPP trend, with 44.66% showing a statistically significant decrease, primarily concentrated in Lin'an's western and central regions, as well as along the borders of Tonglu and Chun'an.  
Analyzing Figure 8c,d, the NPP of Hangzhou forests shows an increasing trend in the southern and northeastern regions, while experiencing a decline trend in the central and northern regions. Specifically, the NPP increases by 16.72%, with a significant increase of 0.48%, mainly concentrated in the southern parts of Jiande and Fuyang, and the junction of Lin'an and Yuhang. The GPP shows a decreasing trend, with an overall decrease of 83.28%, including a significant reduction of 13.59%, primarily found at the intersection of Tonglu, Chun'an, and Lin'an.  
From Figure 8e,f, it can be seen that the NEP of Hangzhou forests shows a decreasing trend in general. Only 4.23% of the NEP shows an increase, with just 0.04% showing a significant increase, primarily distributed in Jiande and the junction of Lin'an and Yuhang. In contrast, 95.77% of the NEP shows a decrease, with 31.17% of the decrease being  
Figure 8: Trend (**a**) and significance (**b**) of GPP, trend (**c**) and significance (**d**) of NPP, and trend (**e**) and significance (**f**) distribution of NEP of Hangzhou’s forests in the years 2001–2020.  
significant, mainly in the western part of Tonglu, central and western portion of Lin'an, and the southwestern portion of Chun'an.