## 3 Results
### Spatial and Temporal Characteristics of the Terrestrial Carbon Uptake  
Figure 5 shows the regional distribution of the vegetation's multiyear average NEP values in the Dongting Lake basin. Overall, the vegetation ecosystem in the Dongting Lake basin exhibited a carbon sink, with a multiyear average NEP value of 314.95 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). The zones with a strong carbon sink effect (NEP \(>\) 600 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) were mainly located in the western, southeastern edge, and northwestern edge regions of the watershed. The weaker (NEP \(<\) 300 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) zones were mainly located near the Dongting Lake area, the urban construction sites, and their surrounding areas in the central part of the basin. In terms of temporal changes (Figure 6), the overall NEP of the basin showed an increasing trend (\(y\) = 1.8915\(x\) + 295.09, \(R^{2}\) = 0.23, \(p\) \(<\) 0.05), where the maximum NEP value appeared in 2015 (366.49 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) and the minimum value appeared in 2005 (276.23 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)). Figure 4: Spatial patterns of the NDVI trends in the Dongting Lake Basin from 2001 to 2020: **(a)** The Sen trend analysis of NDVI shows the spatial distribution of the greening trend, **(b)** The significance test of NDVI shows different levels of change (*: \(p\) \(<\) 0.05). Figure 5: Spatial distribution of the average NEP in the Dongting Lake basin from 2001 to 2020. Figure 7 shows the spatial characteristics of changes in vegetation NEP from 2001 to 2020. The results of the Sen-MK trend test indicated that the areas with a significant increase in the vegetation carbon uptake accounted for 42.52% of the total area, mainly located in the flat topographic agricultural production areas in the central part of the basin [41]. The areas with decreasing vegetation carbon uptake effect were mainly located in the hilly woodland areas at the northern edge, accounting for 29.91% of the total area. In general, the overall NEP level in the basin showed a fluctuating upward trend (1.40 g C\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\)). However, it is still necessary to strengthen the protection of forests to avoid the risk of weakening the vegetation carbon uptake due to forest degradation. The carbon uptake effect varied significantly over different vegetation types (Figure 8). We found that forestland had the highest carbon uptake effect (347.53 g C\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\)), followed by grassland (327.76 g C\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\)), and cultivated land (261.82 g C\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\)) had the lowest. In addition, the carbon uptake of the different vegetation types all showed a  
Figure 6: Temporal trends of the average NEP in the Dongting Lake basin from 2001 to 2020. Figure 7: Spatial patterns of the NEP trends in the Dongting Lake Basin from 2001 to 2020: (**a**) The Sen trend analysis of NEP reveals the spatial distribution of the trend of carbon uptake, (**b**) The significance test of NEP shows different levels of change (*: \(p<0.05\)). significant upward trend. The growth rate of cultivated land was fastest (1.87 g C\(\cdot\)m\({}^{-2}\).a\({}^{-1}\)), while that of forestland (1.13 g C\(\cdot\)m\({}^{-2}\).a\({}^{-1}\)) and grassland (1.71 g C\(\cdot\)m\({}^{-2}\).a\({}^{-1}\)) was lower. The main reason may be the influence by the development of agricultural production, technological progress, investment increase, and crop yield, which consequently resulted in the increasing carbon sequestration effect of crops during the growth period [38; 42; 43]. The statistical results of the whole region showed that forestland is the main component of carbon uptake in the Dongting Lake basin, with an annual average carbon uptake of 5.17 \(\times\) 10\({}^{7}\) t C, accounting for 65.39% of the total carbon uptake in the basin.