## 4 Discussion
This is mainly because the southwestern part of Zhejiang Province is dominated by mountains and hills, with less man-made disturbance and better forest quality, while the northeastern part of Zhejiang Province is dominated by plains. Rapid urbanization, which has caused changes in land use type, forest destruction, and area reduction to a certain extent, has affected the carbon sink capacity of forests. The slope map of the NEP changes and the histogram of the frequency distribution of the cities in Zhejiang Province from 1985 to 2015 are shown in Figure 11. It can be seen from Figure 11 that most of the areas with slope values less than 0 are located along the coast and around the city; urban expansion has led to forest destruction in these places. Areas where NEP has increased are widely distributed throughout Zhejiang Province. It can be seen from Figure 11 that the NEP in most forests in Zhejiang Province has shown an increasing trend in the past 30 years, and the NEP slope is mainly concentrated between 0 and 36. Only 21% of forest NEP values have shown a slight decrease. Among them, 90.9% of these areas have a NEP slope between \(-\)8 and 0. Statistics show that during the period of 2000-2015, more than 97% of forests in Zhejiang Province were carbon sinks, and the total annual NEP of the forest ecosystems was 32.02 Tg\(\cdot\)C\(\cdot\)yr\({}^{-1}\) and the annual average NEP was 441.91 gC\(\cdot\)m\({}^{-2}\cdot\)yr\({}^{-1}\). The carbon sink capacity was higher than that of the Poyang Lake Basin forest ecosystem in the same subtropical region with an annual average  
Figure 11: (**a**) NEP changes in various cities in Zhejiang Province and (**b**) histogram of the frequency distribution from 1985 to 2015. NEP of 204 gC-m\({}^{-2}\)-yr\({}^{-1}\)[28], as well as the forest ecosystem of Jiangxi Province with an annual average NEP of 135.48 gC-m\({}^{-2}\)-yr\({}^{-1}\)[62]. Of course, the Zhejiang confierous forests' average NEP (416.36 gC-m\({}^{-2}\)-yr\({}^{-1}\)) was higher than found in any other study [63], and the broadleaf forests' average NEP (632.47 gC-m\({}^{-2}\)-yr\({}^{-1}\)) simulated in this study, to some extent, was lower than other related studies [63; 64]. This may be due to the fact that forests in Zhejiang Province are still dominated by young and middle-aged forests [65], but the productivity of coniferous forests is larger than that of broad-leaved forests. NEP with higher productivity also increases with age, showing rapid growth at a young age and a rapid decline in old age [21; 66]. Many studies have found that meteorological factors such as temperature, precipitation, solar radiation, and water pressure have a great influence on NEP [67; 68; 69]. Therefore, this study analyzed the relationship between these meteorological factors and NEP in detail. This study analyzed the relationship between NEP and climatic factors across two stages: 1985-1999 and 2000-2015. Before 2000, the forests in Zhejiang Province were mainly dominated by young forests. However, the age of young forests has a great impact on NEP, and after 2000, 97% of forests in Zhejiang Province were carbon sinks; this is more convenient for our later research. Because of the uncertainty in the relationship between the NEP and meteorological factors over a short period of time, we need to analyze the relationship over a relatively long period of time. Studies have shown that NEP had a weak correlation coefficient with temperature, precipitation, relative humidity, and radiation before 2000. This also proves that the age of the forests and other factors in this period had a great impact on NEP. After 2000, the NEP began to show an extremely significant positive correlation with precipitation and significant negative correlation with temperature and radiation and was weakly correlated with relative humidity.