1. Introduction
An ecosystem is the life support system that human beings depend on for survival and development, and it is also the material basis of social and economic development [
1]. Although the world has experienced the invasion of COVID-19 in 2020, which has caused a great negative impact on the global economy, after the epidemic, the global economy will continue to grow in the next 30 years [
2]. This means that human demand and consumption of biological and material resources will continue to increase dramatically [
3]. At the same time, the impact of unreasonable human activities on the ecosystem and its services has gradually increased, for instance, overfishing, resource exploitation, overgrazing, etc., causing the ecosystem to continue to degenerate, resulting in a continuous decrease in the ability of the ecosystem to provide Ecosystem services (ES) [
4]. As the fastest growing country in the world since 1978, China’s economic development has accelerated. In the past 42 years, the average annual economic growth rate has reached 9.2%, and the economic scale has expanded 225-fold [
3], and this trend of rapid economic development will continue. However, the rapid growth of China’s economy is accompanied by environmental pollution and the destruction of ecological balance in many regions of China. The pressure on the ecology will continue to exist in the future. To achieve the goal of sustainable economic and social development, it is necessary to manage the ecosystem more reasonably, which is a severe challenge for China and provides a rare opportunity.
Although there are many studies on ES [
5,
6,
7,
8,
9,
10,
11], these studies lack an in-depth ecological understanding of most of the ES. The ecological information they provide as a basis for decision-making is still limited [
12], especially in the cross-regional ecological compensation (EC) which is similar to Payment for Ecosystem Services. In China, ecological compensation is a policy that restores ecosystem services, and coordinates the contradictions between environmental protection and economic development in different regions. Due to the lack of scientific and reasonable ecological theoretical basis, stakeholders have ambiguity on the service beneficiary areas (SBA) of ES, which limits the implementation of this measure and hinders the development of the EC cycle [
13]. It is now widely recognized that there should be a more intuitive method to enable policymakers to clearly, and accurately understand the SBA, and the benefit level and benefit amount of SBA [
13]. The evaluation of the radiation benefits of regional ES can help decision makers determine the SPA-SBA relationship between regions, which is the basis for implementing interregional ecological compensation schemes (ECS) [
14].
However, there is not much research on the ES radiation assessment, and it is still under exploration, and a mature research framework has not yet been established. For example, Xie et al. tried to link the ecological land with the construction land by using the theory of ES radius and the technology of geographic information system (GIS) to construct the ES radiation model. Based on the experience of experts [
15], Wang et al. established an ES radiation model, which used the total value of ecological radiation intensity each year and the ecological impact of ecological land on construction land to express the effect of ecological barriers [
16]. Han et al. conducted an early study on the radiation benefits of ES. They selected the lower reaches of the Heihe River, which is an essential ecological function area in China as the study area [
13]. They used the sand dust spatial transport model to evaluate the radiation range of sand-stabilization service in this area. However, the establishment of the above models is based on expert experience, and these models lack the analysis of ES radiation effects based on ecological processes, nor do they quantitatively explain the spatial response mechanism of ES.
Here, we try to solve this problem based on the theory of ES flow. The process of ES flow is the process of transferring ES to the outside through water, air, organisms, and other media and ES radiation. The transfer process of ES provided by some specific ecosystems, such as wood, fish, and agricultural products, provides a new perspective for studying ES [
17]. However, the determination of the beneficiary area of such services is based on logistics information [
18], and it is difficult to determine the beneficiary area for those services that cannot be tracked and located. Xu et al. (2018) carried out active exploratory research. They used the atmospheric transmission model to simulate the wind speed transmission process to study the flow characteristics of sand-stabilization service to determine the scope of SBA [
19]. This is a critical study to study the radiation effects of sand-stabilization service from the perspective of physical processes. However, their research lacked reliability verification. There are still many things that need to be discussed, such as the instability of the atmospheric structure and the simplification of flow paths.
In this paper, three specific regions in China’s national ecological sheltering zone (NESZ) are selected as examples to illustrate the application of the ecosystem service radiation assessment framework (ESRAF). Our work uses models to evaluate ES in typical areas and then uses ESRAF to determine the SBA, benefit level, and benefit amount of SBA. Our work on ESRAF contributes in two main ways. First, ESRAF is a new method of studying ES, which can quantify the contribution of natural ecosystems (key areas) to the surrounding areas. Because the ultimate goal of ES radiation assessment is to provide intuitive and simple ecological information for the coordinated management of regional ecosystems. Second, through the application in specific regions of NESZ, we show how to use the existing data for radiation effect assessment, which will help other scientists understand and improve this framework.
In recent years, China has moved forward with ecological conservation and improvement. The second group of trial projects to conserve the ecosystems of mountains, forests, farmland, rivers, and lakes have been launched [
20,
21]. Constructing ESRAF to assess the radiation effect of ES in crucial regions will provide useful information for ecological construction and decision-making, especially evaluate the performance of policies through radiation effects, and according to the cross-regional ES flow to explore the ECS based on radiation benefits between areas.
3. Results
3.1. Ecosystem Services in Typical Areas
The typical areas of NESZ provide essential ES. Spatial distribution showing where ES are produced within typical ecosystems (Figure 9A–C).
The results of the amount of sand-stabilization (ASS) estimated by the RWEQ model show that the ecosystem of Ordos provided an important sand-stabilization service and reduced a considerable amount of wind erosion. In 2015, the actual amount of wind erosion in this area was , which is a massive amount in terms of numbers alone. However, this only accounts for 33.37% of the potential wind erosion, and about 66.63% of the wind erosion was fixed locally due to the ecosystem. In other words, the sand-stabilization service provided by Ordos’s ecosystem had reached about . This not only reduces the sandstorm disaster in the downwind area but also reduces the loss of fertility of the local soil due to wind erosion. In every spring, although wind erosion still occurs in the Ordos, forming sand storms and affecting the downwind area, the affected scope and degree show a downward trend. Therefore, the role of the ecosystem or the effectiveness of Ecological construction cannot be denied, the sand-stabilization service provided by the ecosystem of Ordos plays an important role. If there is no vegetation and ecosystem in the area, air pollution caused by wind erosion would be more serious.
Soil retention in the LPES ecosystem was calculated using the Universal Soil Loss Equation (USLE). The results show that in 2015, the soil conservation service generated by the LPES ecosystem reached
, and the soil per unit area was 539.49 t·ha
−2·a
−1. Although the spatial difference in soil conservation service is significant, the ecosystem in the barrier area still plays a vital role in soil conservation. Moreover, compared with existing studies, the amount of soil conservation in 2015 increased by about 0.98
compared with that in 2000 [
16]. Although the region is still a serious area of soil erosion in China, the annual loss of water and soil is enormous. However, it is undeniable that the soil loss caused by water erosion will be more serious if there is no vegetation cover and EC in this area.
The area of forest resources in Northeast China is 6 × 1011 hm2, accounting for 28.1% of the total forest area of the country. The NFB is the most significant area of China’s water conservation changes. The amount of water conservation in the NFB was m3 in 2015. It plays a vital role in restraining evaporation, replenishing groundwater, regulating the runoff of the Songhua River, and ensuring domestic and agricultural water use. The spatial distribution of water conservation of NFB is different. The areas with higher water conservation capacity are mainly distributed in Jilin and Liaoning, and the lower areas are concentrated in the northern periphery of the study area.
Several studies have shown that 137Cs can be used as a tracer for soil erosion processes [
40]. We verify the accuracy of the wind erosion modulus simulation in this paper by comparing it with the wind erosion modulus monitored by 137Cs collected in related studies. As shown in
Figure 7, the coefficient of determination (R
2) between the two is 0.8641, which shows that the simulation results of the wind erosion modulus in this paper are reliable.
To verify the simulation results of the USLE model, we selected the water erosion census results of eight provinces including Shanxi, Shaanxi, Hebei, and Henan from the “First National Water and Soil Conservation Survey” published by the Ministry of Water Resources of China to compare with the simulation results of the USLE model [
41] (
Figure 8). The results show that the water erosion areas in each province simulated by USLE are basically consistent with the census results with a high fitting degree (R
2 = 0.967,
p < 0.01), indicating that the simulation results using the USLE model are relatively accurate.
Since there is no census result or recognized research result of water conservation service in the NFB, in order to verify the simulation results of the water balance equation used in this study, we adopted a horizontal comparison with the results of similar studies (
Table 4). The results of this study on the estimation of water conservation service in the NFB are consistent with other studies in magnitude.
3.2. Beneficiary Areas of Ecosystem Services
The ES generated in typical areas have apparent ESR effects, which provide benefits for other areas and the location of beneficiaries in recipient provinces (
Figure 9D–I). The beneficiary areas of sand-stabilization service are mainly located in the eastern and northeastern parts of Ordos, with about
km
2, accounting for 17.21% of the country. Additionally, the area of SBA is approximately 19.12 times the area of the Ordos. Among all beneficiary areas, the Ordos has the highest level of benefit. The benefit levels of central and Southern IM and Northern SX (≥30%), which are closer to the Ordos, are also higher, indicating that they can obtain more benefits from sand-stabilization service than other beneficiary areas. Among the benefited areas, the benefit level decreases with distance from the Ordos. However, this is relative, because a high level of benefit also indicates that it suffers from greater wind erosion. Although the SBA far from Ordos has a low benefit level, its wind erosion impact is weak. The quantitative economic benefit results show that the sand-stabilization service provided by the Ordos’s ecosystem reduces a large number of dust-fall and avoids a considerable amount of dust removal investment in SBA.
In 2015, the sand-stabilization service provided by the Ordos ecosystem would reduce about tons of dust-fall for SBA, and save about RMB in dust removal investment. Among all the SBA, the Ordos also benefited from its own sand-stabilization service, and it has benefited the most, with a decrease of about tons. The avoided investment is about RMB, accounting for 36.28% of the total. IM comes second, it avoided about RMB.
The beneficiary area of LPES includes two parts: LPES and the Yellow River. As far as the barrier zone is concerned, soil conservation service reduces soil nutrient loss, traps more soil in the barrier zone, and reduces the adverse consequences of soil erosion. Compared with that in the barrier zone, soil conservation service in the LPES is of greater significance to the Yellow River, which reduces the sediment concentration of the Yellow River and the deposition of sediment on the lower reaches. The results show that in 2015, since the Yellow River passed through LPES, the scour-siltation volume of the Yellow River in each reach was negative, which indicates that there is no siltation. The scour-siltation volume peaked in the Xixiayuan-Huayuankou, which was , while the values of other reaches show a relatively gentle increase. Affected by the narrowing of the river course, the flow rate has accelerated, causing the scour-siltation volume of the Aishan-Luokou and the Lukou-Lijin river sections to rebound slightly. In terms of the radiation effect, the radiation intensity of soil conservation decreased with the increase in distance.
According to the principle of confluence, the NFB mainly radiates the Songhua River Basin in China, with an area of about km2. The water conservation service beneficiary area is primarily located near the NFB, with 266 hydrological response units, covering an area of km2. The spatial autocorrelation analysis shows that the high-high cluster areas are mainly near the Changbai Mountains and the Lesser Khingan mountains, which are red. The water conservation service of the Lesser Khingan mountains mainly radiates from the upper reaches to the middle reaches of the Nenjiang River, the water conservation service of the Changbai Mountains mainly radiates from the upper reaches of the Songhua River and the Mudan River to their middle reaches, and the radiation path is mainly along the main River. The low-low cluster areas are mainly close to the Greater Khingan Mountains, showing dark green. The water conservation service of the Greater Khingan Mountains mainly radiates from the middle reaches to the lower reaches of the Nenjiang River, and the radiation path is also mainly along the main River. About m3 of water is related to NFB. The closer to NFB, the more benefits. With the increase in distance, the degree of benefit is decreasing.
The spatial and temporal distribution of PM10 can accurately reflect the extent and degree of dust storms [
13]. We screened the meteorological stations with PM10 monitoring value greater than 200 during the dust storm, and then analyzed the spatial-temporal distribution characteristics of PM10 at the dust storm before, during, and after, As shown in
Figure 10. The R
2 between PM10 and grid frequency is 0.8083 (
Figure 11), which indicates that it is feasible to determine the SBA based on the HYSPLIT model.
Since the Longmenkou hydrological station, the Yellow River channel is the SBA of the soil conservation service of the LPES. However, due to the lack of sediment monitoring data from the Longmenkou hydrological station and Tongguan hydrological station, the scour-siltation volume of the Longmenkou-Tongguan and Tongguan-Xixiayuan sections cannot be counted. Therefore, we used the available sediment monitoring data of each river section to compare with the simulation results [
44]. The results show a coefficient of determination (R
2) of 0.6845 between them, which indicates that the simulation results of the radiation intensity of soil conservation service are relatively reliable.
3.3. Assessment of Ecological Compensation
The above research shows that the typical areas of NESZ provide essential ES. The cross-regional transmission of these ES brings great benefits to SBA. In terms of sharing the benefit ratio, SPA is the area that benefits more, but other SBAs have also benefited from the ES provided by SPA. In fact, SPA is mainly located in the underdeveloped areas in the west of China, and SBA is mainly located in the developed areas in the east, especially Beijing, Tianjin, and other places. From the perspective of coordinated regional development, SBA should pay reasonable ECF to SPA to relieve the financial pressure of SPA located in the west, which will promote the sustainable development of ecological construction, so that SPA can continue to provide ES with high efficiency. For this, there should be a more credible scheme for determining the ECF paid by SBA to SPA. So based on the principle of “more benefits, more compensation”, this paper proposed a different ECS (
Figure 9J–L).
Similar to the benefits of sharing sand-stabilization service, the Ordos shares the most sand-stabilization service among all SBA, so it should bear the most ECF. That is, the Ordos should pay about RMB, which accounts for about 36.28% of the total amount of ECF. The remaining ECF will be shared by 16 regions, except for Ordos according to their beneficial benefits. Among these 16 regions, IM paid the most, about RMB. SX comes second, requiring RMB. The ECF was payable by other SBAs decreased with distance. The Hb province (Suizhou) that paid the least ECF is only RMB.
The lower reaches of LPES benefit from about RMB, which should be paid as EC to LPES. The Longmen-Yuyuan section paid the most, about RMB, accounting for 24.32% of the total amount. Followed by the Yuyuan-Xixiayuan section, which is about RMB, accounting for 12.91%. The cost of ecological compensation decreases with the increase in distance, and the minimum payment for the Sunkou-Aishan section is about RMB.
The total amount of payment to SPA in the beneficiary areas of the Songhua River Basin is RMB. According to the SPA-SBA location, for the Lesser Khingan mountains, RMB should be paid by 57 hydrological response units benefiting from its ES. The 166 hydrological response units should pay RMB to the Greater Khingan Mountains. The Changbai Mountains should receive about 33,954.13 RMB of EC from 49 hydrological response units.
4. Discussion
4.1. Policy Implications
The ESRAF can provide policymakers with clear and compelling evidence of the monetary value of the ES radiation effect. The results of the typical area show that it is feasible to use available data and methods to estimate ESRAF. ESRAF can give prominence to the values of nature and their contributions to human well-being, similar to the radiation effect of the city on the surrounding countryside. If the results of ES radiation assessment are included in government decision-making, it will contribute to the realization of important social goals (such as Sustainable Development Goals) (
Figure 12). ESRAF can not only be used for an ecosystem service assessment but also provide a basis for determining the EC of SBA. At present, the evaluation of ES is often focused on locality but ignores the cross-regional flow characteristics of ES. ESRAF is a useful method for cross-regional assessment of ES. In particular, it is also a powerful tool for assessing the effectiveness of government ecological protection or ecological restoration and ecological construction. Moreover, it is very imperative to enable policymakers to understand the previously inaccurate range of SBA and the ambiguous ES radiation effects intuitively and clearly. This is of great significance for strengthening ecological construction and ecological restoration in critical areas in the future, that is, determining the best area through multi-scenario simulation to obtain the most ES radiation effects. At the same time, by providing scientific and reasonable ecological information, the government can save public investment and use the saved funds where it is more needed. In addition, ESRAF is an essential basis for determining the EC of SBA. Because in the past and current ECS, the schemes are either unable to reach a consensus because of the widely different expectations, determined based on game theory, or uniformly stipulated by the central government. Ecological information is often missing in these programs. Although some programs have become templates for promotion in China, there are still many problems. For example, in the Xin’anjiang model, the EC between Anhui Province and Zhejiang Province is based on the water quality monitoring results at the boundary of the two provinces [
45]. If the water quality of the river section between the two provinces does not meet the evaluation standard (
p ≤ 1), Anhui Province will pay RMB 100 million to Zhejiang Province due to the unqualified water quality management of the upper reaches of Xin’an River (RMB; in 2020, the average exchange rate between us dollar and RMB were 6.8974). Otherwise, Zhejiang Province will pay 100 million yuan to Anhui Province. This scheme provides an advanced model for river cross-regional ecological compensation in other regions of China, but there are still some issues that have not been resolved. For example, the fairness of this ECS needs to be considered. Although 100 million yuan is a drop in the bucket for the two provinces, the economic development of Anhui Province is different from that of Zhejiang Province. As we all know, compared with Zhejiang, Anhui is much behind. In a study in 2020, the per capita GDP of Anhui in 2019 was 58,496 yuan, which was only 54.4% of that of Zhejiang [
46]. Even more remarkable though, how are the 100 million yuan ECF determined? Our research can solve this problem by simulating the radiation benefits generated by the ecosystem services flow. Perhaps the “simulation result” is more than 100 million, but stakeholders can negotiate according to our research results to achieve an ECF acceptable to both sides.
The ESRAF provides a basis for cross-regional economic compensation by measuring the value and location of the production and use of ecosystem services, and can also play an important role in reducing poverty. The areas involved in the NESZ are mostly located in central and western China. Many areas have rich ecosystem assets, but they are relatively poor in conventional economic indicators (per capita GDP). Provinces that benefit from the ES of NESZ tend to be far wealthier. Ecological compensation can maintain the ecosystem in SPA, which is also conducive to alleviating poverty and promoting sustainable economic development [
47,
48]. The results of studies in typical districts show that it is important to incorporate ESRAF into decision-making. For example, in the Ordos, the grassland grazing prohibitions bans have been implemented for many years, and the deterioration of the local ecological environment has been curbed and gradually evolving in the direction of improvement [
49]. In addition, the local government has also invested heavily in implementing a series of ecological protection and ecological restoration projects, such as protecting virgin forests, turning vulnerable farmland into forests and grasslands, and stopping grazing to let grasslands recover, etc. However, no matter the grassland grazing prohibition order, or the ecological protection projects such as returning farmland to forest and natural forest protection, these measures have some conflicts with the current individual economic interests of local farmers and herdsmen. According to a study, according to one study, about 70% of farmers and herdsmen indicated that they may be grazing illegally during grazing prohibition or rest periods [
50]. Therefore, it is necessary to pay ECF to SPA through cross-regional EC to compensate farmers and herdsmen for the opportunity cost lost due to grazing prohibition or rest grazing. Moreover, to some extent, it has restricted the development of the local economy, due to the policy leaning towards “ecological protection” [
51,
52,
53]. ESRAF will provide decision-makers with clear information about the scope and benefits of ES, reduce the differences among stakeholders in the negotiation of compensation costs, form a positive interaction between SBA and SPA, and promote the harmonious development of economic development and ecological protection.
In addition, the ESRAF provides the possibility to evaluate the performance of policies, especially policies on ecological construction. The NESZ has a total area of 315,200 km
2, accounting for about one-third of the total land area of China, involving 732 districts and counties, and is aimed at maintaining and improving the ecosystem services of the entire country [
16]. Moreover, the government has invested a huge amount of money in ecological construction to improve its stability [
54,
55,
56,
57]. For these regions, ESRAF provides an effective tool for evaluating policy performance.
4.2. Limitations and Next Steps
The research of ESRAF is at an early stage of development, there is still a lot of work to be conducted, and it needs continuous exploration and improvement to be mature.
First, we need to recognize that ESRAF is at an early stage, which may lead to inaccurate estimation of ES due to the influence of data. Moreover, regarding the three ecosystem service radiation models constructed in this article, there are still many details that need to be perfected. However, at present, an important function of ESRAF is to provide a technical roadmap for ES radiation assessment and use cases to explore the areas affected by ecosystem services in cross-regional transmission and the benefits it brings to SBA. Moreover, with the advancement of remote sensing technology, there will be more and more remote sensing data with higher resolution (time and space) and more spectral information, and the limitations of the data will gradually decrease. Nevertheless, there are still some data used to assess other types of ecosystem services that cannot be obtained through remote sensing technology and need to be measured in the field.
Second, the radiation effect evaluation model based on ES flow is constructed, but the spatial transmission process of ES is complicated. We have to omit some details and only study the ES radiation effects from a macro perspective. For example, the process of sand emission, the state of movement of sediment in rivers with different flow rates. Especially in the study of the radiation effect of water conservation services, since the process of water conservation is extremely complex, and there are many disputes about the process of water conservation in academic circles, we adopted the statistical model to research. In the following research, we will consider the use of higher-resolution meteorological and LULC data and more advanced atmospheric transport models to simulate the dust dispersion process and use the modified unsteady flow formula to study the state of sediment movement at different speeds. Additionally, we will learn from related research results in hydrology, ecology, and other fields, upgrades to the water conservation services radiation assessment model make it gradually become a processed model. Moreover, we are still exploring how to verify the radiation range of the water conservation service. Nevertheless, the results of this study are still statistically significant.
Third, we have only evaluated the radiation effects of the main ES in typical areas, but have not evaluated all ecosystem services and their radiation effects. The reason is that all types of ES have not yet reached a consensus, and the understanding of the connotation of some ES is still insufficient. Furthermore, the lack of relevant data is an important reason why this work cannot be carried out more comprehensively. For example, how does the ecosystem affect the precipitation patterns of the local and surrounding areas? This ecological process is very complex and involves multiple disciplines. In the future, we need to understand the connotation and radiation transmission effect of this ES based on ecological process through interdisciplinary learning. In addition, there may be some ES that we are unable to characterize at present, and their radiation effects will become clear only after more understanding of the ecosystem and its contribution to human well-being.
Finally, in China, the underdeveloped central and western regions provide excellent ES for the economic construction of the eastern region. Therefore, the eastern region should pay for the ecological interests of the central and western regions through ecological compensation fees, and guide the redistribution of ecological interests between regions with economic benefits, so as to achieve sustainable economic and environmental development between regions. However, EC is a complex issue, not just an ecological issue, involving many fields, such as environmental science, economics, management, sociology, etc. According to the research framework proposed in this paper, the ecological compensation scheme may not be in line with the reality of society. However, it is unreasonable to ignore radiation effects at all. We believe that effectively identifying the spatial scope and benefit level of SBA is one of the important references for determining EC standards.
5. Conclusions
To prevent large-scale loss of natural capital, the Chinese government’s investment in ecological environment construction and improvement is increasing. The ES radiation effect can play an important role in evaluating the effectiveness of government ecological projects and determining the area and amount of EC. However, there is a lack of research on the ES radiation effect, and the existing research results that can provide a basis for decision-making ecological information are still very few. Therefore, there is an urgent need for new methods to obtain this information and provide this well-understood ecological information to policymakers. Just as the research on the economic radiation effects of urban agglomerations gives policymakers information on how cities can promote the development of surrounding villages, the research on the ES radiation effect can provide policymakers with useful information on how the ecosystem services generated by SPA benefit the surrounding area, the scope, and extent of the benefits. Based on the theory of ES flow, this paper constructs an ESRAF to study the spatial scope, amount, and benefit level benefiting from ES provided by the ecosystem in typical areas in the NESZ in 2015. About 16 provinces and cities have received the sand-stabilization service provided by the Ordos Ecosystem, which has avoided huge investments in dust removal for these areas. Soil conservation services trap a lot of soil in the Loess Plateau Ecological Screen, which reduces the sediment concentration of the Yellow River and downstream sediment deposition. The water conservation service beneficiary area is primarily located near the Northeast Forest Belt, with 266 hydrological response units, covering an area of km2. and based on the above research results, a differentiated ECS was proposed. The ecological compensation fee paid by each beneficiary area is inversely proportional to the radiation effect, that is, the ecological compensation fee with the distance from SPA. The results of typical areas show that it is feasible to use the ESRAF constructed in this paper and the existing data to evaluate the radiation effects of sand-stabilization service, soil conservation service, and water conservation service. As data and methods improve, this research framework can be improved over time.