3.2. Temporal Variation of Carbon Sink in Terrestrial Ecosystem in China during 2010–2020
During 2010–2020, the annual average NEP value and the interannual variation of carbon source/sink in China were shown in
Figure 4. The average growth rate of NEP fitting was 1.607 gC/m
2 with R
2 = 0.4002. The annual average NEP showed a trend of increasing volatility. The lowest value was 38.47 gC/m
2 in 2010, the highest value was 64.97 gC/m
2 in 2016, and the interannual average was 55.40 gC/m
2. According to the annual average NEP change trend, after 2011, due to the implementation of afforestation, returning farmland to forest and grass projects, and ecological protection and management policies, carbon emissions were less than the carbon sequestration of vegetation, so vegetation played a role of carbon sink. In the past 11 years, the proportion of China’s carbon source area reached 56%, an increase of 8% compared with 2010.
As shown in
Figure 5, the average annual NEP in Northeast China showed a trend of increasing volatility during 2010–2020. The mean NEP value in Northeast China in 11 years was 62.37 gC/m
2, and its annual average NEP was lower than the linear annual average NEP. The forest coverage rate in Northeast China is 47.2%, and the vegetation grows vigorously. Therefore, the NEP in this area is generally high, and most areas show carbon sink properties.
During 2010–2020, the average annual NEP increase in North China was 0.2849 gC/m
2. The range of the average annual NEP in 2013 was 992.37 gC/m
2, far exceeding the range of the average annual NEP in other years in the region. In addition, the range of the average annual NEP range in the region for 11 years was 328.21 gC/m
2, which was much higher than 253.96 gC/m
2 in North China and East China, 240.59 gC/m
2 in Northwest China, and 252 gC/m
2 in Southwest China. This might be related to the heavy rainstorm that hit North China in 2012, resulting in sufficient storage of groundwater on the ground, which contributed to the vigorous growth of vegetation and the significant improvement of carbon sink capacity in the region in 2012 and 2013. From 2011 to 2013, the average annual NEP showed a trend of rapid increase, with the largest increase of 23.65 gC/m
2 from 2011 to 2012, which was related to the implementation of the Grain for Green Project in North China in recent years [
26]. As a result, the vegetation coverage has been significantly improved, the ecological environment has been greatly improved, and the vegetation has played a carbon sink role. During the period from 2014 to 2020, the NEP of terrestrial ecosystems in China basically remained at a stable level.
During 2010–2020, the annual mean and annual change rate of NEP in Northwest China were the smallest among the six sub-regions, which were 16.20 gC/m2 and 0.2714 gC/m2, respectively. There were two time intervals in which the growth of northwest China was more obvious. From 2011 to 2012, it increased by 5.39 gC/m2, while it increased by 6.42 gC/m2 from 2017 to 2018. From 2018 to 2019, the decrease became more obvious with a decrease of 5.29 gC/m2.
The annual mean value of NEP in China’s terrestrial ecosystem in East China showed a trend of increasing fluctuation. The annual average maximum value appeared in 2018, which was 137.03 gC/m2, and the minimum value appeared in 2011, which was 89.07 gC/m2. The annual average maximum value appeared in 2018 in the northwest and central-south regions. Meanwhile, the average annual NEP in East China showed a downward trend from 2018 to 2019, as in North China, Northwest China, Central-South China, and Southwest China. Among them, the average annual NEP in East China, Northwest China, Central-South China, and Southwest China decreased greatly, which might be related to the frequent occurrence of extreme climate, resulting in the change in precipitation pattern and the increase in temperature, thus affecting the growth of vegetation and the function of ecosystems, resulting in the decrease in NEP.
During 2010–2020, the average annual NEP and the rate of change in the central and southern regions ranked first in the six sub-regions, which were 144.22 gC/m2 and 5.2485 gC/m2, respectively. The maximum NEP annual average value in 2018 was 173.76 gC/m2, and the minimum NEP annual average value appeared in 2011, which was 104.85 gC/m2. The range between the maximum NEP annual average year and the minimum NEP annual average year in the past 11 years was 68.91 gC/m2. It could be seen that the annual average value of NEP in the central and southern regions of China’s terrestrial ecosystem fluctuates greatly. In the past 11 years, the annual average value of NEP in China’s terrestrial ecosystem showed a fluctuating increase trend. The obvious valley values between 2011, 2014, and 2019 might be related to the long-term high temperature in the south of the central and southern regions in 2011, the strongest typhoon ‘Rammasun’ hitting the central and southern regions in 2014, and the longest flood season since 1961 in 2019. In 2011–2013, 2014–2018, and 2019–2020, the average annual NEP values had increased significantly. Among them, the average annual NEP growth between 2011–2013, 2014–2015, and 2019–2020 was the most obvious, which may be related to the better effect of the regional government on climate disasters and ecological governance.
During 2010–2020, the annual average value of NEP in Southwest China increased by 2.4258 gC/m2. The maximum annual average value of NEP in 2016 was 76.31 gC/m2, and the minimum value of 32.35 gC/m2 appeared in 2010. In 2011, the annual average NEP value increased by 26.67 gC/m2 and 21.54 gC/m2, respectively, compared with that of 2010 and 2012. The climatic conditions in 2011 were dry, and there was less rain. Although the rainfall was less than that in previous years, the temperature was suitable for vegetation growth. The annual average value of NEP in China’s terrestrial ecosystem increased significantly compared with that before and after, and the carbon sequestration of vegetation was reflected as a carbon sink.
In summary, the average annual NEP in different sub-regions of China showed an increasing trend, but the growth rate is different. In terms of the annual average of NEP, Northeast China, North China, Southwest China, and Northwest China are less than the average level of China (72.20 gC/m2). Except for Northeast China, North China, and Northwest China, the change rate of other sub-regions is greater than the change rate of the NEP mean in China in the past 11 years. Among them, the maximum NEP value in East China in 2018 was 896.96 gC/m2, and the minimum NEP value in Southwest China in 2010 was −586.85 gC/m2. On the whole, except that the northwest region belonged to the significant carbon source, the other regions belonged to significant carbon sinks. NEP values showed a significant increase between 2011 and 2013, 2014 and 2018, and 2019 and 2020. This may be attributed to effective efforts by local governments in managing climate disasters and ecological governance, including a range of positive measures and policies to improve environmental quality. This trend may indicate that the region has made some progress in environmental protection and has taken solid steps toward sustainable development in the future.
3.3. Changes in NEP Gravity Center
This paper analyzed the gravity center migration of China’s sub-regions from 2010 to 2020 to further analyze the spatial variation of the gravity center of NEP in the study area (
Figure 6).
As shown in
Figure 6a, during 2010–2020, the NEP gravity center in Northeast China showed a migration trend of ‘southwest–northeast–southeast–northwest’. The gravity center of 2012–2013 migrated to the southwest compared with that of 2010–2011, indicating that the growth rate and increment of NEP in the southwest were greater than those in the northeast. Compared with that of 2012–2013, there was a trend in northward migration in 2014–2015. In general, the NEP gravity center in Northeast China had shown a trend in moving southward and then northward in the past 11 years, and the distance of moving southward was less than that of moving northward.
As shown in
Figure 6b, the migration of the NEP gravity center in North China had obvious regularity in the past 11 years. The gravity center moved southward during 2014–2017, and the gravity center moved northward in other time periods, indicating that the increase in the NEP gravity center in the north was higher than that in the south. From 2014–2015 to 2016–2017, the gravity center of vegetation NDVI had the largest migration range of 366.8 km, while from 2010–2011 to 2012–2013, the migration range was the smallest, which was 82.43 km. During 2010–2020, the NEP gravity center in North China has shifted northward.
As shown in
Figure 6c, the NEP gravity center in East China moved northward in the first and third periods, which were from 2010–2011 to 2012–2013 and from 2014–2015 to 2016–2017, respectively. In the second and fourth periods, it moved southward in the period from 2012–2013 to 2014–2015, respectively. In these four periods, from 2014–2015 to 2016–2017 had the largest migration distance of 27.02 km. The migration distance from 2016–2017 to 2018–2020 was the smallest, only 15.34 km. The migration trajectory of the NEP gravity center showed a circular migration trend of ‘northwest–southwest–northeast–southeast’.
As shown in
Figure 6d, the NEP gravity center in Northwest China migrated northeastward from 2012 to 2013 compared with that from 2010 to 2011. The gravity center of NEP in 2016–2017 to 2018–2020 and 2012–2013 to 2014–2015 migrated southeastward, while the gravity center of NEP in the period from 2014–2015 to 2016–2017 migrated northwestward with the largest migration range of 242.72 km.
As shown in
Figure 6e, the migration trajectory of the NEP gravity center in Southwest China presents a zigzag shape. The gravity center migrated to the northeast from 2010–2011 to 2012–2013 and from 2014–2015 to 2016–2017. The gravity center migrated to the southeast from 2012–2013 to 2014–2015. The gravity center migrated to the northwest from 2016–2017 to 2018–2020. The migration range of the NEP gravity center from 2012–2013 to 2014–2015 was the largest, which was 72.67 km. It shows that the carbon sink capacity in the southeast of Southwest China has been significantly strengthened from 2014 to 2015, while the gravity center of NEP in Southwest China as a whole shows a trend of moving southeastward.
As shown in
Figure 6f, the change in NEP gravity center in the central and southern regions had a strong roundabout nature, and the change state was more complicated. From 2010–2011 to 2012–2013, the NEP gravity center migrated to the southwest, with the largest migration range of 58.07 km, indicating that the NEP increment and growth rate in the southwest parts were higher than those in the northeast. In the period from 2012–2013 to 2014–2015, from 2014–2015 to 2016–2017, and from 2016–2017 to 2018–2020, the trend of circuitous changes was basically the same.
3.5. Distinction of the Relative Roles of Climate–Human Interactions on NEP Changes
Based on the impact of climate change and human activities on the NEP of terrestrial ecosystems in China, the role of climate change and human activities in the evolution of NEP could be assessed by the changes in the NEP of terrestrial ecosystems in China. The driving factors of NEP changes in China’s terrestrial ecosystems from 2010 to 2020 were divided into six categories: joint action to promote the increase in NEP, climate change to promote the increase in NEP, human activities to promote the increase in NEP, joint action to reduce NEP, climate change to reduce NEP, and human activities to reduce NEP.
It could be seen from
Figure 8 and
Table 1 that zones with joint action to promote the increase in NEP accounted for about 42.58%, mainly distributed in most parts of southern China, Heilongjiang Province, Yinshan Mountains, Shanxi Province, southeastern Tibet Autonomous Region, and other regions. Zones with climate change to promote the increase in NEP accounted for 14.14%, mainly distributed in the eastern part of Inner Mongolia Autonomous Region, northeastern Gansu Province, and Fujian Province. In addition, zones with joint action to reduce NEP accounted for 14.93%, mostly concentrated in Inner Mongolia Autonomous Region, Jilin Province, and southwestern Liaoning Province, and a small amount was also distributed in Tibet Autonomous Region. Zones with climate change to reduce NEP accounted for 8.72%, mainly distributed in the Jiaodong Plain of Shandong Province, Hebei Province, Shanxi Province, Gansu Province, southeastern Inner Mongolia Autonomous Region, and other regions. Moreover, zones with human activities to reduce NEP accounted for 7.78%, mainly concentrated in Xilinhot City, Inner Mongolia Autonomous Region.
The impacts of climate change on NEP changes in China’s terrestrial ecosystems from 2010 to 2020 were quantitatively distinguished.
Figure 9 and
Table 2 showed that the area with a positive contribution rate of climate change to NEP changes in China’s terrestrial ecosystems accounted for about 71.88%. Among them, the area with the contribution rate of climate change ranging from 20% to 40% was the largest, accounting for 40.70%. It was mainly distributed in Xinjiang Uygur Autonomous Region, northwestern Inner Mongolia Autonomous Region, Qaidam Basin, and northwestern Qinghai–Tibet Plateau. The area with a contribution rate of more than 80% was 14.14%, mainly distributed in Heilongjiang Province, Yinshan Mountains, southeastern Gansu Province, southwestern Sichuan Province, Fujian Province, and northern Guangdong Province. The negative contribution rate of climate change to NEP changes in China’s terrestrial ecosystems was about 28.12%, mainly located in eastern Inner Mongolia, Shanxi Province, southern Hebei Province, eastern Shandong Province, and northeastern Qinghai–Tibet Plateau.
The impacts of human activities on the NEP changes of terrestrial ecosystems in China from 2010 to 2020 were shown in
Figure 10 and
Table 3. The area with a positive contribution rate of human activities to the NEP changes of terrestrial ecosystems in China accounted for about 87.01%. Among them, the area with the contribution of human activities ranging from 20 to 40% accounted for 72.60%, which was mainly distributed in Xinjiang Uygur Autonomous Region, northwestern Inner Mongolia Autonomous Region, Qaidam Basin, and northwestern Qinghai–Tibet Plateau. The areas with a contribution rate of more than 80% were mainly concentrated in most southern regions, such as northwestern Shanxi, Shaanxi Province, northeastern Gansu Province, southwestern Henan Province, Sichuan Basin, northwestern Hubei Province, and Guangxi Zhuang Autonomous Region. The areas with a negative contribution rate of human activities to the change of NEP in China’s terrestrial ecosystem accounted for 12.99%, which were more concentrated in the northeast region. In general, the contribution rate of human activities (87.01%) to NEP changes in China’s terrestrial ecosystems was greater than that of climate change (71.88%).
3.6. Partial Correlations with Climate Factors
The second-order partial correlation coefficient between the average annual NEP and the climatic factors, such as precipitation, sunshine, and temperature, at each pixel position in China from 2010 to 2020 was obtained (
Figure 11).
As shown in
Figure 11(a1,a2), the second-order partial correlation coefficients between NEP and precipitation ranged from −0.999 to 0.999. The positive correlation areas accounted for about 47.11%, mainly distributed in the northern Greater and Lesser Khingan Mountains, the North China Plain, the Yinshan Mountains, the Loess Plateau, and the central Qinghai–Tibet Plateau in the southwestern region. The negative correlation area accounted for about 52.89%, mainly distributed in Fujian Province, Guangdong Province, Guizhou Province, northern Guangxi Province in southern China, and the eastern and southern regions of Northeast China. Among them, the significant positive correlation (
p < 0.05) area accounted for 3.82%, and the extremely significant positive correlation (
p < 0.01) area accounted for 3.31%, mostly concentrated in the Greater Khingan Mountains, the northwest of the Loess Plateau, the central region of the Qinghai–Tibet Plateau, and the northeast of Sichuan Province. The areas with significant negative correlation (
p < 0.05) accounted for 3.14%, and the areas with extremely significant negative correlation (
p < 0.01) accounted for 2.32%, mainly distributed in the junction of northwest Liaoning Province and southeast Inner Mongolia in the northern region, the central part of Fujian Province, the southern part of Zhejiang Province, the northern part of Guangdong Province, and the central part of Guizhou Province.
As shown in
Figure 11(b1,b2), the partial correlation coefficient between NEP and sunshine ranged from −0.999 and 0.999. The positive correlation area accounted for about 59.07%, mostly concentrated in the central Tibet Autonomous Region, southeastern Guangxi Zhuang Autonomous Region, southern Zhejiang Province, northeastern Inner Mongolia Plateau, and most of the North China Plain. The negative correlation area accounted for about 40.93%, which was mainly distributed in the east of Daxing’anling, Heilongjiang Province, the central part of the Loess Plateau, Jiangxi Province, the eastern part of Hunan Province, and Yunnan Province. Among them, the significant positive correlation (
p < 0.05) area in the partial correlation coefficient between NEP and sunshine in China’s terrestrial ecosystem accounted for 1.37%, and the extremely significant positive correlation (
p < 0.01) area accounted for 0.99%. The areas with significant negative correlation (
p < 0.05) accounted for 2.12%, and the areas with extremely significant negative correlation (
p < 0.01) accounted for 1.90%, which were mainly distributed in the western part of Heilongjiang Province and the southern part of China.
As shown in
Figure 11(c1,c2), the partial correlation coefficient between NEP and temperature was between −0.999 and 1. The positive correlation area accounted for about 53.09%, mainly distributed in most parts of southern China and the eastern part of northeast China, the Yinshan Mountains, and other regions, which was mainly affected by the temperature on plant growth rate, photosynthetic activity, nutrient cycle, and other processes. The temperature in the southern region was relatively warm, which could prolong the growth season of plants, increase the photosynthetic rate, and promote the growth and productivity of plants, so that the NEP was positively increased. At the same time, the abundant precipitation and rich vegetation types in the south could also promote the increase in NEP. The negative correlation area accounted for about 46.91%, mostly concentrated in the Qinghai–Tibet Plateau, the Loess Plateau, and northern Yunnan Province. From the significance of the partial correlation coefficient between NEP and temperature, the regions with significant positive correlation (
p < 0.05) accounted for 1.85%, and the regions with extremely significant positive correlation (
p < 0.01) accounted for 1.26%, which were mainly distributed in the northeast of Inner Mongolia, the north-central part of Heilongjiang Province, the north of Loess Plateau, the east of North China Plain, the southwest of Sichuan Basin, the south of Guangxi Zhuang Autonomous Region, and the central and western regions of Fujian Province. The significant negative correlation (
p < 0.05) area accounted for 3.01%, and the extremely significant negative correlation (
p < 0.01) area accounted for 1.90%, mainly distributed in the central Tibetan Plateau and Inner Mongolia.
3.7. Dominant Factors of Ecosystem Carbon Sequestration in Different Historical Periods and Sub-Regions
The synergistic effects of different driving factors on the Net Ecosystem Production (NEP) of terrestrial ecosystems in China were analyzed by Geodetector. As shown in
Figure 12, the q values of most interactive driving factor pairs were greater than the single factor q values (diagonal values), indicating enhanced or nonlinear synergistic effects between pairs of factors.
As shown in
Figure 12(a1,a2), the interactions between pairwise driving factors in the Northeast region in 2010 that explain the variation in China’s terrestrial ecosystem Net Ecosystem Production (NEP) were ranked as follows: Precipitation ∩ Temperature (0.628) > Elevation ∩ Temperature (0.628) > Soil Type ∩ Temperature (0.602) > Land Use ∩ Temperature (0.549) > Slope ∩ Temperature (0.540) > Precipitation ∩ Soil Type (0.528). The interactions involving temperature dominated in explaining the variations of NEP. In 2020, the q values of the interactive factors were ranked as follows: Soil Type ∩ Temperature (0.420) > Precipitation ∩ Temperature (0.404) > Elevation ∩ Temperature (0.330) > Soil Type ∩ Precipitation (0.325) > Soil Type ∩ Land Use (0.323) > Land Use ∩ Temperature (0.312). Compared to 2010, the interactions involving temperature still dominated, but less than in 2010. During this period, interactions involving population density had the smallest explanatory power.
As shown in
Figure 12(b1,b2), in 2010 in North China, the interactions between pairwise driving factors that explain NEP variation were ranked as follows: Precipitation ∩ Temperature (0.872) > Precipitation ∩ Soil Type (0.869) > Soil Type ∩ Temperature (0.860) > Elevation ∩ Temperature (0.798) > Land Use ∩ Temperature (0.759) > Elevation ∩ Precipitation (0.756), with temperature interactions dominating. In 2020, the q values of the interactive factors were ranked as follows: Precipitation ∩ Soil Type (0.869) > Precipitation ∩ Temperature (0.860) > Soil Type ∩ Temperature (0.816) > Soil Type ∩ Land Use (0.748) > Soil Type ∩ Elevation (0.744) > Soil Type ∩ Slope (0.699). During 2010–2020, the dominant factor affecting NEP variation in North China shifted from temperature to soil type.
As shown in
Figure 12(c1,c2), in 2010 in East China, the interactions between pairwise driving factors that explain NEP variation were ranked as follows: Temperature ∩ Soil Type (0.530) > Temperature ∩ Precipitation (0.529) > Precipitation ∩ Soil Type (0.517) > Temperature ∩ Gross Domestic Product (GDP) (0.505) > Temperature ∩ Elevation (0.492) > Temperature ∩ Land Use (0.490). During this period, synergistic interactions with temperature dominated, while the q values of the interactions involving GDP and population density were the smallest (0.176), indicating human factors had the least impacts during this period. In 2020, the q values of the interactive factors were ranked as follows: Temperature ∩ Soil Type (0.733) > Temperature ∩ Elevation (0.720) > Temperature ∩ Land Use (0.709) > Temperature ∩ GDP (0.706) > Temperature ∩ Slope (0.703) > Precipitation ∩ Soil Type (0.530). Overall, in East China, the dominant factor influencing NEP variation was the synergistic interaction between temperature and other factors, followed by soil type interactions.
As shown in
Figure 12(d1,d2), the top six pairs of interacting factors explaining the changes in NEP of China’s terrestrial ecosystems in the northwest region in 2010 were as follows: Precipitation ∩ Soil Type (0.739) > Temperature ∩ Soil Type (0.720) > Precipitation ∩ Elevation (0.677) > Precipitation ∩ Temperature (0.673) > Elevation ∩ Soil Type (0.645) > Land Use ∩ Soil Type (0.739). The interaction between soil type and other factors played a dominant role. In 2020, the top six pairs were ranked: Temperature ∩ Soil Type (0.874) > Precipitation ∩ Temperature (0.860) > Precipitation ∩ Soil Type (0.836) > Elevation ∩ Soil Type (0.830) > Precipitation ∩ Elevation (0.828) > Land Use ∩ Soil Type (0.804). Over these two periods, the synergistic effects between soil type and other driving factors were the main factor influencing NEP changes in the northwest terrestrial ecosystem, with this synergy becoming increasingly stronger.
As shown in
Figure 12(e1,e2), in the southwest region in 2010, the top six pairs of interacting factors explaining the changes in NEP of China’s terrestrial ecosystems were ranked as follows: Precipitation ∩ Soil Type (0.569) > Temperature ∩ Soil Type (0.530) > Land Use ∩ Soil Type (0.513) > Elevation ∩ Precipitation (0.506) > Elevation ∩ Soil Type (0.501) > Slope ∩ Soil Type (0.478). During this period, the synergistic effects between soil type and other driving factors were predominant, while the interaction between GDP and population density had the smallest explanatory power (0.083), indicating that anthropogenic factors had the least impacts on NEP changes during this period. In 2020, the top six pairs were ranked as follows: Temperature ∩ Soil Type (0.530) > Precipitation ∩ Soil Type (0.517) > Land Use ∩ Soil Type (0.512) > Elevation ∩ Soil Type (0.473) > Temperature ∩ Elevation (0.442) > Slope ∩ Soil Type (0.435). Compared with that of 2010, the synergistic effect between soil type and other factors remained the dominant influence on NEP changes, but the explanatory power of these interactions showed different degrees of decline.
As shown in
Figure 12(f1,f2), in the central-southern region in 2010, the top six pairs of interacting factors explaining the changes in NEP of terrestrial ecosystems were ranked as follows: Temperature ∩ Precipitation (0.860) > Precipitation ∩ Soil Type (0.857) > Temperature ∩ Soil Type (0.848) > Temperature ∩ Elevation (0.787) > Temperature ∩ Land Use (0.749) > Precipitation ∩ Elevation (0.745). During this period, the synergistic effect between temperature and other driving factors played a dominant role. In 2015, the top six pairs were ranked as follows: Temperature ∩ Soil Type (0.530) > Soil Type ∩ Land Use (0.488) > Soil Type ∩ Precipitation (0.461) > Temperature ∩ Elevation (0.445) > Temperature ∩ Elevation (0.427) > Slope ∩ Soil Type (0.400). Compared to the previous period, the synergistic effect between soil type and other driving factors became dominant, with the synergistic effect among other factors being less than 40%. In 2020, the top six pairs were ranked as follows: Temperature ∩ Soil Type (0.530) > Precipitation ∩ Soil Type (0.517) > Land Use ∩ Soil Type (0.512) > Elevation ∩ Soil Type (0.473) > Temperature ∩ Elevation (0.442) > Slope ∩ Soil Type (0.435). Overall, the explanatory power of various factors showed a weakening trend, with the dominant factors shifting from temperature to the interactions between soil type and other factors. In both periods, the explanatory power of population density and other factors was relatively weak, with Gross Domestic Product (GDP) and population density being the weakest, indicating that anthropogenic factors had the least influence on NEP changes in the central-southern region of China’s terrestrial ecosystems.