## 4 Discussion  
In this study, we analyzed the trend of GPP, NPP, and NEP in Hangzhou's forest ecosystems and their response to phenology. As can be seen from Figure 6, GPP, NPP, and NEP in different regions of Hangzhou showed significant spatial differences. Such differences may be closely related to the differences in regional climatic conditions. Changes in temperature and precipitation directly affect the photosynthetic efficiency of forests, which in turn affects the GPP, NPP, and NEP [61]. Our results showed that the annual averages of the indicator exhibited a declining trend (Figure 8). This decline can likely be attributed to Hangzhou's urban expansion, which has reduced forest areas, strained land resources, and damaged farmland and natural ecosystems [62, 63]. The negative effects of urbanization highlight the significant disruption of the forest carbon cycle in urban areas caused by human activities [64]. This highlights the urgent need for conservation efforts aimed at safeguarding urban forest ecosystems amid rapid urban expansion. Additionally, we found that the LOS positively influenced the increase in GPP, NPP, and NEP. This result aligns with Piao [23] and Zhao [65], which reported a similar relationship in other regions. The agreement may stem from the similarities in Hangzhou's climatic conditions with other regions and the widespread nature of phenological patterns. With a steady increase in temperature and sunshine length at the beginning of the season, vegetation starts to recover and grow, photosynthesis starts to be active, and the rate of CO\({}_{2}\) uptake by plants increases, and GPP increases in response [52, 66]. When the season comes to an end, vegetation gradually enters dormancy, and GPP may decline [67]. It also reflects the common mechanisms through which phenological changes drive carbon cycle processes in the context of urbanization. Compared with the common Biome-BGC and TRENDY models, the InTEC model has unique advantages. The Biome-BGC model focuses on ecosystem process simulation and is widely used in vegetation productivity and carbon flux simulation, but it has limitations in dealing with complex terrain and urban environmental heterogeneity. The TRENDY model integrates multiple dynamic global vegetation models to analyze carbon cycle at a  
Figure 13: Hangzhou forest ecosystems: direct and indirect effects of phenological variations and climatic change on changes in the carbon cycle, 2001–2020. global scale. However, the simulation accuracy of the TRENDY model at a regional scale, especially in areas affected by urbanization, needs to be improved. The InTEC model, in contrast, explicitly incorporates regional climatic characteristics and land use changes, making it particularly well suited for capturing the carbon dynamics of Hangzhou's urban forest ecosystem. Despite these advantages, certain uncertainties remain. Model validation is a critical step, and future research should prioritize cross-verification of InTEC model outputs with field-based observations. In this study, while we standardized spatial data to a 250 m \(\times\) 250 m resolution, certain datasets, such as nitrogen deposition (originally at 5\({}^{\circ}\)\(\times\) 3.75\({}^{\circ}\)) and meteorological data (originally at 1000 m), required downscaling. The downscaling process may introduce uncertainties due to potential biases in interpolation techniques and the assumption that finer-scale variations follow broader spatial trends. Additionally, CO\({}_{2}\) concentration data were obtained from global monitoring stations, which may not fully capture the fine-scale spatial heterogeneity of CO\({}_{2}\) levels in urban areas. Beyond climatic factors, urban-induced changes such as the urban heat island (UHI) effect and land use transitions also play pivotal roles in shaping forest carbon dynamics. Elevated temperatures in urbanized areas can exacerbate climatic extremes such as heatwaves and droughts, increasing water stress and altering forest physiological processes.