## 4 Discussion
These factors likely contributed to the growth of PFs in and around large cities like Nanjing. The clear upward trend in GPP and NPP is linked to enhanced photosynthesis driven by climatic change in the study region. This result aligns with previous studies reporting an increased canopy leaf area and improved leaf-level photosynthetic efficiency in Chinese forests, particularly in the southeast [83; 84; 85]. Data from forest Fluxnet sites further underscore a strong CO\({}_{2}\) fertilization effect, with nearly all subtropical sites in China showing positive GPP responses to rising atmospheric CO\({}_{2}\)[86; 87; 88]. Meanwhile, rising early growing season temperatures also contribute significantly to increased forest productivity. For example, spring phenology has advanced considerably due to winter and spring warming over the past decades [89]. This is closely associated with the peak in GPP in the early growing season during recent years, as observed in this study (Figure 3). Finally, increased N deposition, serving as a key external nutrient input, may further enhance photosynthetic activity and tree growth. From our analysis, we find that the interannual variability in N \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Indicator**} & **Value** & **Study Region** & **Methods** & **Source** & **Reference** \\ \hline \multirow{8}{*}{160.4–193.3} & 15–60 & Southern China & Model & CEVESA [PERSON] [PERSON] et al. (2007) [74] \\ \cline{2-6}  & 160.4–193.3 & Taihu Lake Basin, & \multirow{2}{*}{Model} & \multirow{2}{*}{BIOME-BGC} & [PERSON] et al. (2017) [75] \\ \cline{2-6}  & 266 & Poyang Lake Basin, & & & & [PERSON] et al. (2013) [76] \\ \cline{2-6}  & 30 & Tropical forests & \multirow{2}{*}{Model} & \multirow{2}{*}{VPSIT} & [PERSON] et al. (2011) [60] \\ \cline{2-6}  & 424 \(\pm\) 95 & Savanna ecosystem in North Australia & \multirow{2}{*}{Site observation} & \multirow{2}{*}{FLUXNET} & [PERSON] et al. (2021) [67] \\ \cline{2-6}  & 403 \(\pm\) 102 & Global tropical forests & \multirow{2}{*}{
\begin{tabular}{c} Site observation \\ and model \\ \end{tabular} } & \multirow{2}{*}{Data synthesis} & [PERSON] et al. (2007) [61] \\ \cline{2-6}  & 254 & Poyang Lake Basin, & & & \\ \cline{2-6}  & China & & & & \\ \hline \multirow{2}{*}{385.36 \(\pm\) 117.81} & \multirow{2}{*}{Sites in Southern China} & \multirow{2}{*}{Site observation} & \multirow{2}{*}{FLUXNET} & [PERSON] et al. (2012) [77] \\ \cline{2-6}  & & & & & \\ \hline \multirow{2}{*}{668} & Tropical forests in the Amazon & \multirow{2}{*}{Site observation} & \multirow{2}{*}{FLUXNET} & [PERSON] et al. (2024) [64] \\ \cline{2-6}  & & & & \\ \hline \multirow{2}{*}{16.32} & \multirow{2}{*}{Jiangsu Province, China} & Model & BEPS & This study \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._deposition aligns well with observed changes in C sequestration indicators. Since many PFs have been established on former farmlands with potential nutrient deficiencies across the study region, the evidence therefore indicates that PFs' productivity can be affected by the ecosystem's nutrient supply [90]. Despite the substantial increasing trend in GPP and NPP, the NEP of PFs is observed to be tempered by corresponding rises in ecosystem respiration. The long-term trend in PFs' NEP suggests that hot weather, especially in late summer, has been boosting ecosystem respiration and partially offsetting the C uptake [91]. Additionally, higher night-time temperatures can increase plant maintenance respiration without any C intake, further reducing daily NEP [92]. In Jiangsu, where late-summer temperature has trended upward [93], trees and soil microbes continue to respire C at high rates, even after seasonal peak photosynthesis has passed, which erodes the NEP during the middle to end of the growing season. The climatic responses of the C sequestration indicators have further implications on the development and management of plantation forests in areas with similar environments.