## 4 Discussion  
Accurate phenological models are crucial tools for predicting vegetation phenological responses to future climate scenarios and their subsequent impacts on ecosystem functions [31, 32]. Given the significant influence of developmental factors on autumn phenology, this study enhanced the traditional autumn phenology model by incorporating growing season GPP and SOS. The results showed that whether considering GPP or SOS alone or incorporating both factors, the modified autumn phenology models outperformed the original temperature-based model, significantly enhancing the simulation capability of autumn phenology (Figure 3). Traditional temperature-based models predict that under future warming conditions, vegetation productivity will increase due to extended growing seasons [8, 33]. However, in temperate forests of the Northern Hemisphere, increased growing season productivity and earlier spring phenology triggered by rising CO\({}_{2}\) concentrations, higher temperatures, or changes in light levels may accelerate plant senescence, potentially offsetting the promotional effects of warming on growing season extension [9, 15]. Therefore, current phenological models that do not consider developmental factors may seriously overestimate future growing season length, leading to significant biases in global carbon cycle estimates [8, 9]. Future research should fully integrate developmental factors into predictions of plant phenology, which may revise our expectations of plant carbon sequestration capacity and avoid overly optimistic estimates of ecosystem carbon sink potential under future climate change. In recent years, numerous studies have employed various methods to estimate the carbon sink capacity of China's terrestrial ecosystems [4, 5, 25]. While estimates vary across methodologies, there is consensus that China's terrestrial ecosystems act as a significant carbon sink [2, 3]. [PERSON] et al. (2022) [3] systematically compared estimates from different periods and methods, revealing that China's terrestrial ecosystems exhibit a carbon sink capacity ranging from 0.12 to 1.11 Pg C/yr. Our study estimated an average NEP of 0.49 Pg C/yr for the period 2003-2021, which falls within the credible range but is slightly higher than estimates from ecosystem process models. This discrepancy likely stems from variations in temporal coverage, as China's NEP has shown significant increases since the 21 st century [34], while other studies include NEP data from before 2000, lowering the overall average in those estimates. On the other hand, the spatial distribution of NEP estimated in our study aligns with previous studies [2, 4, 25], with strong carbon sink areas primarily located in the southeastern and southwestern parts of China. Forests serve as the primary carbon sinks, while shrubs, grasslands, and croplands act as weak carbon sources (Figure 4). Forest ecosystems have high productivity and carbon storage capacity, and studies have shown that existing global forests alone can account for the land carbon sink [35]. In contrast, shrubs and grasslands have low biomass and limited carbon storage capacity, with carbon absorption and respiration being generally balanced, resulting in a weak carbon sink or even a carbon source [36, 37]. Consistent with other studies, China's terrestrial carbon sink showed a significant upward trend from 2003 to 2021 (Figure 5), likely driven by multiple factors. First, the fertilization effect of increased atmospheric CO\({}_{2}\) concentrations significantly enhanced plant photosynthesis efficiency, thereby promoting carbon uptake [17, 38]. Second, in both the northern and southern forest regions, climate warming extended the growing season, which may have further strengthened carbon sequestration capacity [8]. Additionally, in southeastern China, increased anthropogenic nitrogen deposition has been shown to have a significant positive impact on plant growth [39], potentially providing crucial support for the enhancement of carbon sinks [40].