3.1. Status Quo of Carbon Storage and Carbon Sink
There is an obvious uneven distribution of carbon storage in the south and north of the YRD region in 2015 (
Figure 2). The spatial distribution of each time period was similar. The entire region was mainly covered by two value-area: the areas with the highest value (20.34 Mg C ha
−1) and the areas with value lower than 13.46 Mg C ha
−1, while the distribution of areas with value between 13.46 Mg C ha
−1 and 20.34 Mg C ha
−1 was very rare. The areas with the highest value (20.34 Mg C ha
−1) were continuously and densely distributed in mountainous area in the south of the region, in Zhejiang Province, southwest of Jiangsu Province, and south of Anhui Province. In addition, small patches of this value area scattered in the middle and north of the region. On the contrast, the areas with value (6.48–13.46 Mg C ha
−1) were densely distributed in plain area in the middle and north of the region (in Shanghai Municipality, Jiangsu Province and east of Anhui Province), and they patchily scattered in the south of the region. The areas with the lowest carbon storage (6.48 Mg C ha
−1) had patchy pattern in the region and relatively clustered in the middle of the region, (in Shanghai Municipality and south of Jiangsu Province near the Yangtze River Estuary and east of Zhejiang Province near Hangzhou Bay).
In 2015, the two main carbon sinks were woodland and cropland in the YRD region, totally accounted by 85.27% of total carbon storage (
Figure 2). Woodland and cropland accounted for 44.37% and 40.90% of total carbon storage, respectively. The woodland has the largest contribution of total carbon storage, while the unused land had very few contributions. The total carbon storage in these two types were much higher than the two types: the total carbon storage of woodland is 14.99-times of grassland, 8.95-times of waterbodies, and 7.26-times of built-up land, while the total carbon storage of cropland is 13.82-times of grassland, 8.25-times of waterbodies, 6.69-times of built-up land.
3.2. Spatiotemporal Dynamics of Carbon Storage and Sequestration
The total amount of carbon storage presented a trend of increasing first and then decreasing (
Figure 3). There was an increase in 1990–1995 in the total carbon storage, and then the total carbon storage kept decreasing in 1995–2015. The total amount of carbon storage increased from 2.91 × 10
9 Mg in 1990 to 2.95 × 10
9 Mg in 1995, and then decreased to 2.90 × 10
9 Mg in 2015. Thus, the total economic value of carbon storage increased approximately from 467.42 million dollars in 1990 to 472.99 million dollars in 1995, and then decreased to 465.01 million dollars in 2015. The amount of carbon sequestration largely decreased from 3.47 × 10
7 Mg in 1990–1995 to −1.20 × 10
7 Mg in 2000–2015. The change rate of carbon sequestration decreased from 1.19% in 1990–1995 to −0.41% in 2000–2015.
There was a decreasing trend of proportion of carbon storage in cropland at average decline rate of 2.25%, while proportion of carbon storage in woodland presented a trend of increased first (5.27%) in 1990–1995 and decreased later (average −0.26%) in 1995–2015 (
Figure 3). Proportion of carbon storage in grassland presented a trend of decreased first (−20.71%) in 1990–1995 and remained relatively stable in 1995–2015. There was a relatively stable trend in proportion of carbon storage in waterbodies, while the proportion in unused land fluctuated in 1990–2015.
In general, the carbon sequestration increased first in the south of the region, while decreased in the middle and north of the region in 1990–1995 (
Figure 4). Then, the carbon sequestration remained stable in the south, while the carbon sequestration in the middle of the region kept decreasing in the next four periods, especially in Shanghai Municipality and south of Jiangsu Province near the Yangtze River Estuary and east of Zhejiang Province near Hangzhou Bay.
In 1990–1995, the increased areas of carbon sequestration were concentrated in the south in Zhejiang Province, while the decreased areas were distributed in the middle and west of the region (
Figure 4). The increased areas were mainly concentrated in the south of the region in Zhejiang Province. The three largest patches of increased areas were distributed in the south of the region in Zhejiang Province. The decreased areas were distributed in Shanghai Municipality, middle and south of Jiangsu Province and Anhui Province. In 1995–2000, the decreased areas were distributed in Shanghai Municipality, south and east of Jiangsu Province and Anhui Province, while the distribution of increased areas were very rare. In 2000–2005, intensive expansion of decreased areas appeared in the middle of the region, especially in Shanghai Municipality and south of Jiangsu Province near the Yangtze River Estuary and east of Zhejiang Province near Hangzhou Bay. The increased areas were relatively rare and scattered in the entire region. In 2005–2010, the decreased area was concentrated in Jiangsu Province and Shanghai Municipality along with the Yangtze River. In 2010–2015, intensive expansion of decreased areas weakened in central of the region, while the increased areas’ distribution was still very rare.
3.3. Land Use Policy Influences on LULC and Spatiotemporal Pattern of Carbon Storage
The spatial disparity of carbon storage mainly caused by uneven distribution of natural resources and intensity of LULC, while spatiotemporal dynamics of carbon storage were mainly influenced by LULC change led by land use policies and environmental policies. On one hand, the natural causes of large spatial disparity of carbon storage is mainly related to landform and landscapes distributions of the YRD region: the north and the middle are covered mainly by low-land plain with cropland and built-up land, while the south is mainly covered by mountainous woodland [
19]. On the other hand, the spatiotemporal dynamics of carbon storage were mainly influenced by LULC change and land conversion led by land use policies and environmental policies.
The area of built-up land continuously expanded during 1990–2015 at an average of 14.32%, with a considerable net increment (11,956.80 km
2), while the area of cropland kept decreasing in 1990–2015 at average 2.25%. Secondly, the area of forest was increased by 4.18% during 1990–2015, showing the largest increase during 1990–1995, and then kept decreasing during 1995–2015 at average 0.26%, while the area of grassland had a large decrease first during 1990–1995 at 20.71%, and then kept decreasing gently during 1995–2015. The above two pairs of dynamics of LULC were strong correlated with land conversion from each other that will be showed in the following
Table 2,
Table 3,
Table 4,
Table 5 and
Table 6.
In 1990–1995, the large areas of increased areas of carbon storage were mainly contributed to the conversion of grassland to woodland (2432.13 km2) in the south of the region thanks to forest restoration policy. The deceased areas of carbon storage were mainly contributed to the conversation of cropland to built-up land (3617.00 km2) in the middle and north region, which was the largest area of land conversion. In 1995–2000, the decreased areas of carbon storage were mainly contributed to the conversion of cropland to built-up land (1178.13 km2) in the middle and north of the region. In 2000–2005, 2005–2010, and 2010–2015, intensive growth of decreased areas in the middle and south of the region were mainly contributed to the conversion of cropland to built-up land. There was a decreasing trend of conversion of cropland to built-up from 2000 to 2015 (2000–2005: 3064.35 km2, 2005–2010: 2472.85 km2, 2010–2015: 2119.65 km2).
The implementation of the Forest Law and the Land Management Law both experiences three stages in the YRD region: (1) pre-conservation of forest and cropland, (2) serves land-centered urban development and economic development, and (3) early Eco-civilization stage.
In 1990–1995, the promulgation of ‘Forestry Law’ in 1981 imposed ‘strict controls of annual forest cutting and vigorously promoted afforestation’. As a result, large areas of grassland were restored into woodland. Farmers in the southern mountains no longer lived mainly on tree cutting and farming, while natural vegetation recovered at a quicker pace, and the forest coverage was increased thanks to the reform and opening-up [
19,
35,
36,
37,
38].
In 1995–2010, the YRD region entered a period of rapid development period informed by ‘land-centered urban development’ and major planned expansion of large cities following the 10th and 11th Five-Year Plan (FYP) focused on economic growth [
35], along with noticeably quicker urbanisation, rapid infrastructural construction. Natural ecological space, including cropland, was rapidly reduced due to urbanization, disorderly urban sprawl, and construction of infrastructural networks in the eastern plain [
19,
35,
36,
37,
38]. The concept of Eco-civilization was introduced in the 17th CPC National Congress of China at the end of this period.
In 2010–2015, after the conceptualization of national strategy of ‘Eco-civilization’ in 2012, environmental policies, e.g., Prime Farmland Policy and Ecological Redline Policy. For ensuring the base line of crop protection and crop security, Anhui Province, Jiangsu Province and Zhejiang Province, and Shanghai Municipality delineated prime cropland protection area of 49,211.26 km
2, 39,220.67 km
2, 16,675.00 km
2, 1334.00 km
2, respectively, in 2015 according to National Bureau of Statistics (
http://www.stats.gov.cn/: accessed on 3 June 2018). For regional ecological redline policy, the provinces and cities in the YRD metropolitan region are embarking on the delineation of ecological red lines and ecological functional zones. For example, Jiangsu Province completed the delineation of provincial ecological red lines, while Nanjing completed the work at the municipal scale. Due to different understandings and definitions of ecological red lines, the delineation results may vary even though the delineated zones are in the same province. Moreover, Zhejiang Province completed functional ecological zoning. For two identical zones, there can be two division versions. Now that the results are not even unified within a province, there may be greater differences when more provinces are involved, and convergence can be more difficult. Therefore, it is imperative to have unified zoning at the regional scale. Therefore, urban expansion and land conversion to built-up land were started to be restrict to a certain extent.