## 4 Discussion
The average annual sunshine hours in Zhejiang Province reached 1710-2100 h, and the average annual temperature between 15 and 18 \({}^{\circ}\)C. Especially in recent years, high temperatures and dry weather may have affected the carbon sink function of forest ecosystems. Research has also shown that high temperatures and drought can cause a reduction in the carbon sink capacity of subtropical forest ecosystems. For example, high temperatures and drought occurred in the summer of 2010, which reduced the gross primary productivity (GPP) and net primary productivity (NPP) of southwestern China by 0.65 billion and 0.45 billion tons [70], respectively. The extremely high temperatures and drought in July-August 2013 reduced the carbon fixation of forests in southern China by about 100 million tons, accounting for 46% of the net carbon sinks of the national terrestrial ecosystem [71]. From 1997 to 2007, the annual carbon absorption in the north subtropical evergreen coniferous forests in the United States decreased by 37.4% due to prolonged drought [35]. The above cases show that weakening of the carbon sink function of subtropical forest ecosystems, especially in hot weather, is a common phenomenon. However, Zhejiang Province has a subtropical monsoon climate, with an average annual precipitation of 980-2000 mm. It has abundant precipitation, which is an important condition for the growth of young forests; therefore, NEP is positively correlated with precipitation. The forest NEP in Zhejiang Province is weakly correlated with relative humidity. An increase in radiation will lead to an increase in the proportion of direct radiation, and the proportion of scattered radiation will decrease. Most canopy sun leaves are in a state of light saturation, and the canopy shade leaves are in a state of light loss. Therefore, an increase of radiation has no obvious effect on the sun leaves, but the photosynthesis of the shade leaf will decrease because the photosynthesis of the shade leaves accounts for more than 70% of the photosynthesis of the entire canopy. Therefore, the photosynthesis of the whole vegetation will be reduced, and the increase of radiation will lead to an increase in soil carbon emissions, so there is a negative correlation between radiation and forest NEP. The relative contribution rate of temperature, precipitation, relative humidity, and radiation data to the forest NEP in Zhejiang Province, on the time scale from 2000 to 2015, is shown in Figure 12. As with the correlation analysis, precipitation is the main influencing factor and its relative contribution rate is 61%, which is also in line with the actual situation of vegetation growth in some arid regions. The relative contribution rates of temperature and radiation are similar, 18% and 17%, respectively, and the relative contribution rate of relative humidity is the lowest, at only 4%. Combined with the results of the correlation analysis, that is, within the appropriate range, an increase of precipitation will lead to an increase in forest NEP, while an increase in temperature and solar radiation will lead to a decrease in forest NEP, and a change in relative humidity will have little effect on forest NEP. However, Figure 13 shows the spatial distribution of the correlation between NEP and temperature, precipitation, relative humidity, and radiation in the forest ecosystem of Zhejiang Province from 2000 to 2015. From the analysis of Figure 13a, 84% of forests' NEP values in Zhejiang Province are negatively correlated with temperature. Among them, 10% of forests' NEP values had an extreme, significantly negative correlation with temperature, and 44% were significantly negatively correlated with temperature. However, abundant precipitation conditions made 85% of forests in Zhejiang Province show positive correlations between NEP and precipitation (Figure 13b), in which 19% of forests' NEP values were extremely significantly positively correlated with precipitation, and 40% were significantly positively correlated with precipitation.