Spatiotemporal evolution of ESV and its response to land use change in the Yellow River Basin, China
Analysis of changes in the value of ecosystem services in the YRB
The results showed that from 1990 to 2020, the total value of ecosystem services in the YRB showed a dynamic trend of decrease-increase-decrease, with an overall upward trend, and a total increase of 31.85 × 10ten USD, with an average annual increase of 1.14 × 10ten USD (Table 2). This changing trend is consistent with the change in land use cover in the region. In 30a, YRB cropland decreased by 8663 km2, due to rapid urbanization. In addition, after the year 2000, China began to implement the policy of returning agricultural land to forest and grassland on a large scale, which accelerated the reduction of cultivated land. The results again showed that the forest area increased by 30,933,093 km2indicating that the implementation of the policy of “conversion of agricultural land to forest and grassland” has achieved excellent results, thus increasing the value of ecosystem services generated by forest land by 167.66 × 10ten usd. Grasslands increased by 738 km2because the corresponding ESV increased by 28.73 × 10ten USD, while unused land decreased by 8131 km2with 9.52×10ten Declining ESV in USD. In general, the ecological protection and management measures in the YRB have achieved remarkable results and the ecosystem service values have been significantly improved due to the increase in forests and grasslands.
In terms of the structure of ecosystem services in the YRB (Table 3), the relative proportions of the various ESVs did not change significantly, resulting in a relatively stable ESV structure. Soil conservation and waste disposal are the most important of these, accounting for approximately 37% of ESV’s total value. The YRB ecosystem, as can be seen, emphasizes the importance of soil conservation and waste disposal within the basin, climate regulation, biodiversity conservation and entertainment representing no than 11.99% of the total. Various services have changed to varying degrees over the study period. Waste disposal and climate regulation, for example, suffered losses of 22.23 × 10ten USD and 20.29 × 10ten USD, respectively. The rest of the services showed an upward trend, among which the value of the food production service increased the most, which was 19.03 × 10ten USD, due to the obvious increase in area of forest land and grassland in the YRB.
Spatial distribution and patterns of variation of ecosystem services in the YRB
The total ESV value of the study area and the changes in the value of each service could not reflect their spatial differences. To describe the temporal and spatial distribution pattern of ESV in the study area, the natural breakpoint method was used. This method was then used to classify the ESV with reference to existing studies and divided the area into four levels: low value, low value, high value and high value areas. Taking the three-level watershed of the YRB as the statistical unit of analysis, the result showed that the higher the level, the higher the ESV. As shown in Figure 2, from 1990 to 2020, the spatial characteristics of the ESV were relatively stable. The upper reaches of the YRB from Shizuishan to the north bank of Hekou City, the Fenhe River Basin from Hekou City to Longmen and the Jinghe River Basin are all rich in high values. Forest and grassland are relatively concentrated in the above-mentioned areas, the ESV coefficient is high, and the watershed area is large, resulting in a high total ESV. The higher value areas are mainly distributed in the areas from the Longyang Gorge to the Lanzhou Main Basin, the Daxia River and Tao River Basin, and the Wei River Basin. The area above Baoji Gorge and the inflow area are in the transition area between the high value area and the lower value area. For example, the transition zone between the Loess and Qinghai-Tibet plateaus is an area of greater value. The lower value area mainly includes the Huangshui River Basin, the Datong River Basin, the basin under Lanzhou, and the Guanzhong Plain area. Thus, the unused land in this area is widely distributed. Due to the large area of buildable land in the Guanzhong Plain, the value of ecosystem services has declined. The low value area is in the lower reaches of the YRB, which contains the largest and largest building land area in the basin, has poor ecosystem service function and is also the most economically developed area of the YRB . In terms of changes in the value of watershed ecosystem services, the number of watersheds at the ESV level has not changed significantly between 1990 and 2020. The average watershed ESV is 40.52 × 10ten usd. There were 7 high value, 5 high value, 12 low value and 5 low value watersheds respectively.
Hotspot analysis revealed the spatial agglomeration characteristics and ESV evolution law in the YRB from 1990 to 2020 (Fig. 3). Throughout most of the YRB, ESV accumulation features were not spatially significant, and significant areas were dominated by high and low ESV accumulation. Maqu-Longyangxia River Basin, Daxia River and Tao River Basin, Datong River Basin and Fen River Basin were the five core areas where ESV had the highest value. The inner river, the northern and eastern margins of YRB, and the lower reaches are mostly low-value settlement areas. High Value Agglomeration Area and Low Value Agglomeration Area did not change significantly in space from 1990 to 2020, but the number of grids in each decreased from 647 to 627 and from 699 to 681, respectively. In general, high-value settlement areas in the YRB are scattered, while low-value settlement areas are scattered.
From 1990 to 2020, the coordinates of the barycenter of the ESV in the YRB remained stable between 106.78°–106.94° E and 36.40°–36.65° N (Fig. 4). During the study period, the coordinates of the barycenter ESV showed a transfer trajectory first to the southwest, then to the northeast, then to the southwest. From the perspective of the overall transfer direction, the ESV barycenter shifted from the northeast of Huanxian County to the southwest from 1990 to 2020. The ESV in the northeast decreased, while that in the southeast has increased. From 1995 to 2000 and from 2000 to 2005, the migration distance from the barycenter of the ESV in the YRB was longer by 16.33 km and 15.75 km, respectively, while the migration distance from the barycenter of the ESV from 2005 to 2020 was shorter.
Response of ecosystem services to land use change
Land use type change area in YRB increased by 64,356 km2 between 1990 and 2020. The area of each land type has changed to varying degrees. Cultivated land and building land are the two types of land that have evolved the most. The area of cultivated land has decreased by 8663 km2while the area of building land increased by 13,109 km2. Compared to water, forest and grassland, unused land has undergone significant transformations. However, compared to 1990, it has decreased by 8131 km2 in 2020. The forest has increased by 3093 km2 while grasslands increased by 738 km2. Ecosystem services are strongly impacted by changes in land use types. Using the spatial analysis method, the researcher introduces a resilience index to reflect the response of ESV to land use change in this paper. Over the period 1990–2000 and 2000–2010, the average elasticity of ESV change in the YRB to land use change was 0.27 and 0.44, respectively, but dropped to 0.04 in 2010–2020. This indicates that the disturbance capacity of land-use change on ecosystem services was low between 1990 and 2000, but increased between 2000 and 2010. Land-use change had less impact on ecosystem services since 2010. land use types was broad during this period, but the average elasticity index was low because there were so many different types of land use change, such as forest land conversion and cropland to building land, and the conversion of forest land. area of land and water to cultivated land. The decrease in ESV caused by land use change per unit area was minor. Also, forest land and grassland in the river basin were actually increased, as the ESV increased. Overall, the value of ecosystem services has remained relatively constant.
Precise spatial statistics on the elasticity index from 1990 to 2020 have been carried out (Fig. 5). The elastic index of the upper YRB and the Loess Plateau is higher, and the impact of land use change on ecosystem services is more apparent in this region, according to the results. This is mainly due to the implementation of large-scale ecological engineering measures in response to vegetation degradation in the upper reaches of the YRB and soil erosion in the middle reaches (Loess Plateau), e.g. the Chinese government. In addition, Lanzhou New District, Guanzhong Plain and the lower Yellow River region also showed higher elasticity index. The regional development and construction mentioned above, together with human activities, have led to a rapid increase in building land, leading to a significant decline in ecosystem services and a higher resilience index due to rapid urbanization.
In general, land use types in the YRB have changed dramatically and conversion of land types is very common. The conversion of ecological land to urban building land, as well as the conversion of unused and cultivated land to ecological land, has led to significant changes in the value of ecosystem services. This demonstrates that the basin’s green construction projects have yielded positive environmental results.
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