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Article

Estimation of Grain Crop Yields after Returning the Illegal Nurseries and Orchards to Cultivated Land in the Yangtze River Delta Region

by
Yirui Han
1,†,
Qinqin Pan
1,†,
Yuee Cao
2,†,
Jianhong Zhang
3,
Jiaxuan Yuan
1,
Borui Li
4,
Saiqiang Li
5,
Renfeng Ma
1,5,
Xu Luo
1,5,
Longbin Sha
1,5 and
Xiaodong Yang
1,4,5,*
1
Department of Geography and Spatial Information Techniques/Center for Land and Marine Spatial Utilization and Governance Research, Ningbo University, Ningbo 315211, China
2
School of Environmental and Geographical Science, Shanghai Normal University, Shanghai 200234, China
3
Ningbo Academy of Agricultural Sciences, Ningbo 315040, China
4
College of Ecology and Environment, Xinjiang University, Urumqi 830046, China
5
Institute of East China Sea, Ningbo University, Ningbo 315211, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Land 2022, 11(11), 1993; https://doi.org/10.3390/land11111993
Submission received: 27 October 2022 / Revised: 3 November 2022 / Accepted: 4 November 2022 / Published: 7 November 2022
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Abstract

:
The purpose of this study is to develop a new remote sensing method to assess the area of cultivated land illegally converted to nurseries and orchards in the Yangtze River Delta region of China over the past 40 years (1980–2020), and then estimate the increase in grain yield based on this area. Our result showed that 2.23 × 104 km2 of cultivated land was reduced by 11.8% over the past 40 years. About 14,521.40 km2 of cultivated land was converted to nursery and orchard from 1980 to 2020. The conversion area was unevenly distributed among different administrative regions. Zhejiang and Anhui Provinces had higher conversion rate and area than Jiangsu Province and Shanghai Municipalities. After the illegal nurseries and orchards return to cultivated land, if rotation farming mode is adopted, in which each grain crop is planted only once a year, the increased production of rice, wheat, corn, beans and tubers will reach (632.57 ± 13.08) × 104, (0~531.05 ± 33.25) × 104, (0~556.19 ± 30.36) × 104, (0~249.85 ± 13.14) × 104 and (0~489.11 ± 28.14) × 104 tons at most in each year, respectively. Our results provide theoretical guidance for implementing food security policies and alleviating contradiction between grain production and rural labor shortages in the Yangtze River Delta region.

1. Introduction

The concept of food security was first proposed in the mid-1970s. It is defined by the Food and Agriculture Organization of the United Nations (FAO) as “when all people, at all times, have physical, social and economic access to sufficient, safe and nutritious food that meets their dietary needs and food preferences for an active and healthy life” [1]. Since it is closely related to social harmony, political stability and sustainable economic development, food security has been regarded as a fundamental principle and national policy in many countries [2,3,4]. Cultivated land is the foundation for grain production. In a country or region, maintaining the area and improving the quality of cultivated land are considered to be the most effective way to solve the food security issues. In contrast, the changes in these two attributes will tip the balance between grain production and supply, causing death, warfare, famine and disease [3,4,5]. Many countries regard the amounts of cultivated land as important foundations of grain production and national securities [6,7]. However, over the past half century, imbalance between grain production and demand among countries has increased yearly with the trade globalization, rapid urbanization and development of agricultural science and technology (e.g., ammonia synthesis and crossbreeding) [8,9]. It is reported that more than 50% of the world’s countries, especially concentrated in East Asia, Southeast Asia and Africa, do not produce enough grain to meet the demand, needing to import a large amount of grain from the United States, Canada, Russia, Ukraine and other grain-producing countries [10,11].
Driven by geopolitical conflicts, the COVID-19 pandemic and extreme weather events, global grain prices have been rising over the past few years, now reaching the peak since 2012 [12,13,14]. At present, the armed conflict between Russia and Ukraine, which started in February 2022, has increased worldwide food prices, further exacerbating the food security issue. In order to ensure domestic food security or obtain high profits, trade restrictions have been gradually upgraded by grain-producing countries while the rise of prices has further unbalanced the global grain supply–production relationship [15,16]. According to the UN World Food Programme (WFP), 44 million people in 38 countries were on the brink of famine in early 2022 [17]. To cope with grain shortages and ensure national food security, various countries have taken many measures, such as increasing cropland area, improving agricultural technology and efficient management, to increase grain output and enhance food supply capacity [18,19,20].
China is the world’s largest consumer of grain [5]. Although grain output and the total area of cultivated land rank first in the world, there are also many problems, such as less per capita cultivated land area, the shortages of high-quality cultivated land and small reserve resources of cultivated land due to geographical constraints and large population [21,22]. Producing enough grain to feed its people has long been a major security strategy in China. In recent decades, however, the rapid economy developments and expansion of urbanization have led to a continuous reduction of rural cultivated land [5,23]. A large amount of cultivated land has been converted into urban construction land [24,25]. At the same time, the benefits of producing grain in rural regions are much lower than those of working in cities [26]. China’s population outflow from rural to urban areas in employing non-agricultural industries is glaringly obvious [27]. It is reported that about 250 million young and middle-aged people in rural regions have left agricultural production to enter urban employment [28]. At present, most of the elderly are engaged in agricultural production in China, resulting in a very serious shortage of rural labor, which inevitably leads to the abandonment of cultivated land [29]. On the contrary, some industrial and commercial capitals lease land from farmers, and change the use of the cultivated land to growing non-food crops without permission of the local and administration departments [30]. For example, planting green plants and non-edible agricultural products on cultivated land further reduces grain-producing land area [31].
In China, land is owned by the state and collectives, not individuals. The farmers have only the right to use the land but no ownership. According to the geographical characteristics and economic development conditions, land types are divided into many types (i.e., cultivated land, forest, grassland, construction land, water and unused land) by the administrative departments (mainly the Department of Natural Planning and Land Resources) [32]. As we all know, land is not only a place to grow crops, it also participates in economic activities as an adjustable resource [5,23]. For example, local governments can change cultivated land into construction land, and then take advantage of lower selling prices to attract external funds to buy land use rights and stimulate development of industry and commerce [26,28]. However, cultivated land cannot be incontinently converted to other land use types because of China’s large population and food security issues. To meet the basic grain production needs, the government defines the land allocated by the state to farmers for management as permanent cultivated land, which is only allowed for food production and cannot be privately modified, for example, by turning it into nurseries [5]. Of course, the government can reclaim the right to use the cultivated land from farmers and change the attribute of land use by economic compensation. In the past few decades, China has been able to import large amounts of grain from abroad to meet the needs of its residents, as the food security issue was not as serious as it is now. In order to increase farmers’ income, the land administrative departments did not take the initiative to prohibit farmers from privately converting cultivated land into non-agricultural land such as nurseries, but only encouraged farmers to grow grain crops by using the economic compensation [21]. However, geographical conflicts and trade frictions in recent years may make it more difficult for China to import grain from abroad. Therefore, the Chinese government wants to increase the cultivated land area and grain output through the adjustment of land management policy [33]. Since September 2020, the General Office of the State Council has issued a series of policies, including the “Notice on Resolutely Stopping the Conversion of Cultivated Land into Non-Agricultural Land” and the “Opinions on Preventing the Conversion of Cultivated Land into Non-Agricultural Land to Stabilize Grain Production”, calling for effective protection of cultivated land and grain output to ensure grain production and national food security [34,35]. In response to China’s national policy of protecting cultivated land and boosting grain production, local governments at all levels have started to survey the area and location of the non-agricultural land that was transformed from cultivated land without permission, and implement a policy of “returning illegal nurseries and orchards to cultivated land (abbreviated as land returning policy for brevity)” since 2020.
The “land returning policy” is described as the process of restoring illegal non-agricultural land to cultivated land through administrative and economic compensation measures in order to increase grain production. Illegal non-agricultural land is classified as cultivated land by the land administrative departments, on which only food crops are allowed to grow. However, it is converted into nurseries or orchards driven by commercial interests and lack of a rural labor force, without the permission of land administrative departments [5,26]. The Yangtze River Delta region, located on China’s southeast coast, has suffered the most from farmland abandonment or illegal conversion of cultivated land to non-agricultural land in China over the past 40 years [36,37]. The most fundamental reason is that the local industry and commerce are highly developed and the degree of urbanization is very high [38,39], which made about 60% of the farmers become urban residents [40,41]. In the context of global food security, although the local government in the Yangtze River Delta region began to implement the land returning policy to increase grain production in 2020, no study has assessed the amount of cultivated land illegally converted to nurseries and orchards, or the increase in local grain production after the implementation of the land returning policy.
There are two main methods to estimate the area and location of conversion between cultivated land and other land use types [32,41]. One is to quantify the land conversion area and location based on the historical data of land use on the basis of a large number of field surveys. The other is mainly achieved by using the classification of remote sensing satellite data. Specifically, according to the spectral reflectance and shape characteristics of different ground features (i.e., forest, water, cultivated land, construction land and unused land), remote sensing images are used to classify land use types. Then, the changes in the area and location of these land uses are compared between different years to determine the conversion relationship between cultivated land and other land use types [32]. Compared with field survey methods, this method can be used on a large geographic scale because it saves time and labor costs. To date, many scholars have carried out a lot of work using remote sensing satellite images [32,36,37]. For example, Niu et al. (2022) analyzed land use changes in the Yangtze River Delta from the perspective of urban–rural transformation development [37]. Weng et al. (2021) estimated changes among different land use types in cities along China’s eastern coast [32]. Based on Landsat-TM/ETM remote sensing data, Yang et al. (2018) found that the cultivated land area in the Yangtze River Delta has decreased constantly, most of which has been converted to construction land [36]. However, none of these studies specifically looked at the conversion between cultivated land and forests (especially for nurseries and orchards). Under the influence of the urbanization process, the lack of labor force leads to the conversion of a large amount of cultivated land into forests [21,31]. To increase crop production, the government has tried to take administrative measures to return forests to cultivated land [34,35]. However, this process is not to return all the forests to cultivated land, because some of the cultivated land has been transformed into necessary forest in the social–natural system, such as ecological public-welfare forest and disaster prevention forest [42]. Given their role in biodiversity maintenance, environmental protection and natural disaster control, they cannot be returned to cultivated land. At present, however, we do not have a suitable method to calculate the area and location of illegal conversion of cultivated land to forests, and further predict the increase in grain crop yields based on them.
The purpose of this study is to find a reasonable method to estimate the area of nurseries and orchards that were transformed illegally from cultivated land in the Yangtze River Delta region over the past 40 years, and to assess the new increase in grain production after the implementation of the land returning policy. Our results will provide data and theoretical guidance for the formulation and implementation of local agricultural policies, and also provide a policy proposal to achieve the balance between local grain supply and demand.

2. Materials and Methods

2.1. Geographical Attributes and Farming History of the Study Area

The Yangtze River Delta region includes Jiangsu, Anhui, and Zhejiang Provinces, and Shanghai Municipalities (27°01′~35°20′ N, 114°54′~123°08′ E) (Figure 1). It is located in the plain area of the middle and lower reaches of the Yangtze River, and controlled by the subtropical monsoon climate. The average annual temperature ranges from 14 to 18 °C, while the average annual precipitation ranges from 1000 to 1400 mm [36]. Benefiting from abundant light, heat and water resources, the Yangtze River Delta region has been well known as “a land of fish and rice” and the most important production area of various agricultural products in China [43]. The local crops can be harvested two or three times per year. Jiangsu and Anhui Provinces are the main grain-producing areas in China.
Agriculture has been practiced in the Yangtze River Delta region for more than 7000 years, making the land highly agrarian [44]. Although a lot of cultivated land was abandoned due to the influences of civil war and the anti-fascist war before the founding of the People’s Republic of China [45], under the guidance of the government’s policy of advocating independent agricultural supply after the birth of New China, all the land suitable for planting crops in this region was reclaimed as cultivated land in a short period [46].

2.2. Data Collection and Analysis

We take the implementation of the “reform and opening-up” policy in China (1978) as the initial time for the conversion of cultivated land to other land types. In other words, we assume that the Yangtze River Delta region had the largest area of cultivated land in 1978 due to highly agricultural cultivation in this region. After that, with the progress of urbanization and industrialization, the cultivated land began to transform into other non-agricultural land, and the area kept decreasing [37,47]. Since complete satellite remote sensing data did not exist in 1978, we substituted them with adjacent satellite remote land use classification data (1980). At that time, China’s urbanization and industrialization processes were just beginning, and the interval was only two years, so there was little difference in the area of cultivated land between 1978 and 1980.
In this study, 3-year (1980, 2000 and 2020) data product of China’s land use and land cover remote sensing monitoring datasets (CNLUCC) with a resolution of 1 km were downloaded from the Data Center for Resources and Environmental Sciences of the Chinese Academy of Sciences (https://www.resdc.cn/Default.aspx; accessed on 2 March 2022) (Table 1). According to the classification standard of China’s land use status (GB/T21010-2007), the areas of six land use types (cultivated land, forest, grassland, water area, construction land and unused land) were collected in three years (Table 1). Then, we divided the cultivated land into paddy field and dry farmland, and counted their areas. Visual verification and kappa coefficient were conducted to test whether the classification of the six land use types in the CNLUCC products is consistent with their real status. To be specific, 400 points in the CNLUCC products of 2020 and 2020 were randomly selected, then the attributes of their land use types were visually validated with Google Earth, and the kappa coefficients were calculated. The results showed that the overall classification accuracy in 2000 and 2020 was 83% and 87%, and the kappa coefficient was 0.75 and 0.78, respectively. This indicated that it was feasible to use the CNLUCC products to extract the potential area for illegal nurseries and orchards.
The Markov transition matrix was used to analyze the conversion relationship between cultivated land and the other land use types. Area of cultivated land converted to forest from 1980 to 2020 was defined as the potential area for the implementation of the land returning policy. This was because the local zonal vegetation is evergreen broad-leaved forest due to control by the subtropical monsoon climate. Cultivated land is rarely converted to grassland [48]. Additionally, the non-agricultural conversion of cultivated land is mainly used for planting green seedlings or fruit trees [40,41]. Both nurseries and orchards are considered to be forests with medium to high coverage in the classification of satellite remote sensing monitoring data [32].
There are eight large nature reserves and public-welfare forests in the Yangtze River Delta region: Poyang Lake National Nature Reserve, Ecological Function Reserve of Water Conservation in the Upper Reaches of Xin’an River, Ecological Function Reserve of Zhoushan Islands, Ecological Function Reserve of the Yangtze River Estuary, Ecological Function Protection Area for Flood Storage along Huaihe River, Water Conservation District of the South-to-North Water Diversion Project, Ecological Function Reserve of Huaihe River Source and the Ecological Function Reserve of Dabie Mountain. Since most of them were built after 1980, it is possible that they were also constructed on the original cultivated land. According to some important and obligatory laws, e.g., “Regulations of the People’s Republic of China on Nature Reserves” and “Environmental Protection Law”, these forests cannot be converted into cultivated land in the process of “returning illegal nurseries and orchards to cultivated land”. The forest in the ecological function reserves and the public-welfare forests should be removed. In this study, we obtained the distributions of the ecological function reserves and the public-welfare forests in the Yangtze River Delta region from the Chinese Data Center for Resources and Environmental Science (http://www.resdc.cn; accessed on 10 March 2022) and the local forest management departments (Table 1). After that, we superimposed them onto the potential area of “returning illegal nurseries and orchards to cultivated land” and then removed repetitive parts by using ArcGIS software.
Forest with a slope >25° was also removed from the potential returning area. This was because farmers reclaimed some land with a slope greater than 25° as cultivated land before 1980, as there was little consideration of soil quality and environmental damage, while only grain yield was pursued at that time [45]. After that, a large amount of the rural population migrated to cities in the process of economic development and urbanization, resulting in the lack of labor for agricultural production, thus part of this land was abandoned, and then naturally restored to forest [34,41,49]; or, affected by soil erosion, difficult tillage and poor soil quality, the land has been converted by local farmers into nurseries or orchards due to lower farming requirements and high profits [31]. According to the “Law on Soil and Water Conservation” issued by the Chinese government, land with a slope >25° cannot be used for food production, but can be cultivated into many non-grain types such as nurseries, orchards and natural forest. Therefore, forest with a slope >25° was removed from the potential returning area. Forest with slope above 25° was judged according to DEM data, which were downloaded from the Chinese National Earth System Science Data Center (http://www.geodata.cn; accessed on 13 March 2022) (Table 1).
After the final potential returning area was confirmed, we combined it with the grain yield per unit area of different administrative regions (Zhejiang, Jiangsu, Anhui and Shanghai) of the Yangtze River Delta region from 2012 to 2021 to predict the increase in grain yield by using Equation (1). Y refers to the grain output after the implementation of the land returning policy. A is the potential returning area. GYPUA is the annual yield of a certain grain grown per hectare of cultivated land in different administrative regions. ε is the estimation error. Data on grain yield per unit area in the Yangtze River Delta region were collected from statistical yearbooks published by the National Bureau of Statistics (NBS) or local governments (http://www.stats.gov.cn; accessed on 21 March 2022) (Table 2). All processing of the data analysis can be seen in Figure 2.
Y = A × G Y P U A + ε

3. Results and Discussion

3.1. Changes of Cultivated Land over the Past 40 Years

The area of cultivated land in 1980, 2000 and 2020 was 189,447.24 km2, 183,832.00 km2 and 167,123.89 km2 in the Yangtze River Delta, respectively. The total area of cultivated land decreased continuously 2.23 × 104 km2 to and reduced by 11.8% over the past 40 years (Figure 2 and Figure 3). The main reason for the decrease was China’s rapid urbanization and industrialization processes [50]. It was reported that urban area of the Yangtze River Delta had increased by 2.5 times over the past 40 years. The urbanization rate is more than 70% at present [51]. Throughout the urbanization process, a large number of rural young people swarmed into cities, requiring a large number of residential, industrial and commercial housing to meet the needs of life and work. This in turn promoted the conversion of cultivated land into urban construction land [28,37]. Another reason for the decrease may be the influences of local topography and climate. The Yangtze River Delta is located on the alluvial plain of the lower reaches of the Yangtze River, meaning that the local terrain is very gentle. Moreover, it has favorable hydrothermal conditions due to the close proximity with the ocean. These make the local soil suitable for farming [43,50]. Before urbanization, the vast majority of land was devoted to growing crops. On the other hand, superior terrain and climate conditions make it easier for the local urbanization process to occur. The continuous construction and expansion of cities have taken up a large amount of cultivated land, making the area of cultivated land decrease more than in other parts of China.
After dividing cultivated land into paddy fields and dry farmland, our results showed that paddy fields (15,439.89 km2) decreased more than dry farmland (6883.46 km2) over the past 40 years (Figure 3 and Figure 4). The reduction proportion was 12.8% and 10.0%, respectively (Figure 4). Affected by the hot and humid climate, the local river network was highly developed. In addition, sedimentation has made valley basins and river network plains in the region more than 50% of the total area because it is located at the estuary of many tributaries of the Yangtze River basin [52]. Due to the flat terrain and abundant agricultural water resources, the cultivated land was suitable for irrigation [36]. The paddy fields were mainly distributed in flat terrain, and their area is much higher than that of dry farmland. On the contrary, dry farmland is mainly located in hilly areas. In the process of urbanization, residential, industrial and commercial housing are preferentially built on plain areas with developed transportation potential. As a result, the decline in paddy fields over the past 40 years has been much higher than in dry farmlands [46]. As the productivity potential of paddy fields was much higher than that of dry farmland, the higher conversion resulted in a larger proportion of decline in local grain productivity potential. Our results also showed that the decrease rate of the cultivated land in the first 20 years (1980–2000; 3%) was much lower than that in the later 20 years (2000–2020; 9.1%) (Figure 4). This change was consistent with the rate of local economic development and urbanization. In the past 40 years, as China’s economic development and urbanization continued to accelerate, more cultivated land was converted into construction land [52,53,54]. Similar results were also found in the Niu et al. (2022) and Yang et al. (2018) studies [36,37]. They confirmed that the rate of urbanization in eastern China has been significantly higher in the last 20 years than before this period. The year 2000 was considered the beginning of an explosive acceleration of China’s urbanization process that has continued to the present. In this process, a large amount of cultivated land had been converted to construction land.
The results of the Markov transition matrix showed that the conversion ratios from cultivated land to construction land, forest, water area, grassland, ocean and unused land were 42.4%, 37.1%, 14.0%, 6.4%, 0.08% and 0.05%, respectively (Figure 5a and Table 3). Construction land has the highest proportion among all converted land. These results were consistent with the studies of Liu et al. [46] and Miao et al. [52]. Urbanization was the main reason for the decrease in cultivated land area in all of China, including the Yangtze River Delta, in the past 40 years [37,55]. A large amount of cultivated land (about 40%) has been converted into construction land in order to meet the rapid increase in the needs of the urban population for places to live, work and consume [56].
The conversion ratio of cultivated land to forest ranked second among all land use types (Figure 5a), which was much higher in the Yangtze River Delta region than that in northwest, southwest and northeast China. According to the studies of Xie et al. [49] and Zhang et al. [57], the conversion ratio of cultivated land to forest in northeast, southwest and east China was only 21.4%, 25.72% and 19.09%, respectively. The reason for this difference may be related to the scale and degree of local urbanization. The Yangtze River Delta region has witnessed one of the highest urbanization rates in China over the past decades [58]. In the process of rapid urban expansion, a large number of green plants were needed. At the same time, due to the rise of township and private enterprises, a large proportion of local farmers became hired workers in factories, resulting in a large amount of idle cultivated land due to a lack of labor [37,59]. In the case of this scenario, the huge demand for green plants in the urban construction has led to much cultivated land being leased by businessmen or companies and then turned into nurseries, driven by interests [55,60]. It is estimated that the current nursery production in the Yangtze River Delta region accounts for about 1/3 of China [61]. In addition, the higher conversion ratio of cultivated land to forest may also be related to local industrial adjustment [40,41]. The development of local industries and services has led the local government to downplay or even reduce grain production, instead mobilizing farmers to grow more valuable green plants and fruits [58,62]. According to the “Yearbook on China’s Agricultural Products of 2021”, the benefits of planting a hectare nursery were roughly 4~5 times those of growing grain [63]. The benefits of growing fruit, such as oranges, peaches and macaque heads, were roughly 4~9 times as high per mu as they were from growing grain in an acre of farmland. These government restructurings of the industry have also resulted in the conversion of much of the cultivated land to forest.
The remaining land use types (water, grassland, ocean and unused land) converted from cultivated land accounted for only 20.53% of the total conversion area, of which the largest proportion was grassland (14.0%) (Figure 5a). According to the research of Weng et al. [32], grassland in eastern China was mainly lawns for greening in residential quarters, schools, parks and other public places in cities. This was because the zonal vegetation in the Yangtze River Delta region is subtropical evergreen broad-leaved forest [48]. The grasslands will naturally succeed into forests within 3–5 years in the absence of human interference [64]. This resulted in the conversion of cultivated land to grassland being only a small part of the total conversion area.

3.2. Forest Area That Can Be Returned to Cultivated Land

In this study, we defined the total forest area converted from cultivated land after 1980 as the potential area of “returning illegal nurseries and orchards to cultivated land”. Our results showed that a total of 14,521.40 km2 of cultivated land was converted to forest from 1980 to 2020. After dividing it into two types of cultivated land, our results showed that the converted areas of paddy fields and dry farmland were 3365.5 and 11,155.9 km2, respectively (Figure 5b). Then, the elevation was used to remove the land unsuitable for grain production with a slope > 25°, and the cultivated land area was reduced to 13,212.2 km². The areas of paddy fields and dry farmland decreased to 10,325.5 and 2886.7 km2, respectively. Similarly, after further removing the areas of ecological function reserves and public-welfare forests, the areas of cultivated land, paddy fields and dry farmland were reduced to 11,675.6 km2, 9127.3 km2 and 2548.3 km2, respectively (Figure 6). These were determined as the final increased areas of cultivated land, paddy fields and dry farmland after the implementation of the land returning policy, respectively.
At present, there is no existing research to judge whether the area returning from forest to cultivated land is accurate or not. However, compared with the regular methods for local governments to obtain the returning area by using historical land use inventory data [41], our remote sensing estimation method can not only save more labor costs, but also obtain the returning area and location in a short time. In the process of estimation, our study removed the forests that cannot be returned to cultivated land according to the terrain and the spatial distribution of natural protection regions. This consideration made up for the deficiency of using only the classification results based on remote sensing. As many studies have pointed out, the conversion relationship between cultivated land and other land use types in a certain period can be accurately identified by many remote sensing methods, such as by improving the resolution of satellite images, fusing different images and enhancing spectral information [32,36,37]. However, the conversion of land use was complex and changed with the development of the social–environmental system, thus it was not possible to confirm the returning area only by using the conversion relationship of land use in a period of time based on the computer remote sensing (RS) classification. According to the needs of the social–environmental system, some land that was historically used for farming is now being converted into public-welfare forests, nature conservation forests and disaster control forests [42]. The role of these in slowing down biodiversity loss and protecting ecological security cannot be ignored. Therefore, the returning area obtained by only using remote sensing images without combining it with the needs of the local social–environmental system was extremely unreasonable. If the government implements the land returning policy based on such unreasonable data, it may lead to a series of ecological risks. From this aspect, our method was more reasonable to estimate the returning area than traditional remote sensing technology. Although China issued the land returning policy in 2020, it is difficult for frontline departments of land management to implement it at present [33,34]. The main reason is the lack of effective methods in estimating the area and location of forest to be returned. In this study, we propose a method to solve this dilemma. Our method can help the frontline departments of land management accurately obtain the returning area and location of the forests, and further promote the implementation of the policy.
After locating the potential returning area in each administrative region, we found that they followed the order: Zhejiang Province (6932.72 km2) > Anhui Province (3930.07 km2) > Jiangsu Province (773.18 km2) > Shanghai Municipalities (38.28 km2) (Table 4). The area of paddy fields and dry farmland also presented the same ranking relationship. Among these administrative regions, Zhejiang Province has the largest potential returning area. This was because Zhejiang has the most developed nursery industry compared with the other regions [65]. In addition, the main body of industry and commerce in Zhejiang Province is individual private economy. More than 70% of the local residents are self-employed farmers, and there are basically no young people to grow grain for a living [66]. This made further cultivated land easy to convert to nursery or orchard after leasing by businessmen or companies. Anhui Province also has a large potential returning area. This may be a combination of government land regulation and geographical constraints. In order to ensure self-sufficiency of grain, China has legislated 12 × 105 km2 (1.8 billion Mu) of cultivated land as the red line throughout the country in accordance with the minimum grain demand since 2006. In the process of national spatial planning for land use, the red line of 1.8 billion Mu of cultivated land was not allowed to be reduced. However, because land is one of the important foundations of economic development, the government usually used it as a means to regulate the economy, such as changing land use types, or using land prices to attract investment [47,67]. Accordingly, cultivated land protection and economic development became a pair of mutually opposite relations. Due to the differences of cultivated land quality, economic development level and development mode, the protection will be varied among regions [40,45]. In the Yangtze River Delta region, high-quality cultivated land was mainly distributed in central Zhejiang Province, Jiangsu Province, Shanghai and eastern Anhui Province. In the implementation of supervision and protection of cultivated land, these lands were more likely to receive the attention and stricter supervision from the central land department, resulting in fewer cases of illegal conversion of cultivated land [68,69]. However, due to the hilly terrain in eastern Zhejiang Province and northern Anhui Province, cultivated land was mainly distributed in river valleys, basins or mountain terraces, and rarely distributed in a large area. Mechanized farming was relatively difficult to implement. Additionally, the poor quality of the cultivated land, coupled with geographical constraints, meant that local farmers did not benefit much from growing grain. Therefore, the implementation intensity of land supervision and protection policies in this region was relatively lower than that in the region where high-quality cultivated land is located. To improve the incomes of farmers, local governments often adopt a vague management attitude toward the illegal use of cultivated land, resulting in large amounts of it being converted into nurseries and orchards [34,70]. The typical evidence was that many prefecture-level cities in western Anhui Province, such as Feixi, and Changfeng were the most well-known producing areas of green seedlings in China except eastern Zhejiang Province.

3.3. Estimation of Grain Yield in the Yangtze River Delta Region after the Implementation of the “Returning Policy”

Based on the annual grain output of the China Statistical Yearbook from 2012 to 2021 (Table 2), we estimated the yield of five grain crops after the implementation of the land returning policy in each administrative region. In the estimation process, according to local planting modes, we assumed that paddy fields are basically used to grow rice, which is planted once a year, usually in April, and ripens in late October or early November. After that, another grain crop (such as winter wheat, corn, beans or tubers) or another cash crop (such as canola) is planted and harvested in April. Dry farmlands are used locally to grow wheat, corn, beans or tubers in a rotation of two or three crops a year. Each crop is usually planted only once. The specific planting modes of paddy fields and dry farmlands are not detailed in the statistical yearbook, while only the total annual output of each region (such as province or city) and the output per unit area for each crop are counted [71]. Thus, the yield per unit area is an average calculated based on local planting area and annual yield in the statistical yearbook. In this study, since grain crops are assumed to be planted once a year in the Yangtze River Delta region, the increased yield after the implementation of the land returning policy is roughly equal to the product of its area and the yield per unit area.
The estimated results showed that if paddy fields are used exclusively for rice production, the annual increase will be (6.32 ± 0.1) ×106 tons. After the rice harvest, if growing wheat, corn, beans or tubers separately, it can increase (0~4.21 ± 0.26) × 106, (0~4.36 ± 0.24) × 106, (0~1.90 ± 0.10) × 106 and (0~3.6 ± 0.23) × 106 tons in paddy fields, respectively (Table 5). The “0” refers to a paddy field not planted with a given corresponding grain crop after rice was harvested. In contrast, “Mean” and ”SD” refer to the average yield and its variance after rice was harvested to grow that crop, respectively. For dry farmland, since no rice is grown, its yield is equal to zero. According to the grain planting in a rotation pattern, the increase in the output of wheat, corn, beans or tubers can be up to (0~1.10 ± 0.07) × 106, (0~1.20 ± 0.06) × 106, (0~0.59 ± 0.03) × 106 and (0~1.22 ± 0.06) × 106 tons in each year, respectively. Here, we assume that each crop is planted only once during the year. As there are four kinds of grain crops, even if three crops are planted in a year, while the remaining one is not planted in the year, its yield will be equal to zero.
After adding up the increased yields from paddy and dry fields, our results showed that the increase in the output of rice, wheat, corn, beans or tubers can be up to (632.57 ± 13.08) × 104, (0~531.05 ± 33.25) × 104, (0~556.19 ± 30.36) × 104, (0~249.85 ± 13.14) × 104 and (0~489.11 ± 28.14) × 104 tons, respectively (Table 5). According to the statistical yearbook, the average yields of rice, wheat, corn, beans and tubers in the Yangtze River Delta region from 2012 to 2021 were 4048.62 × 104, 2694.96 × 104, 807.16 × 104, 218.91 × 104 and 108.27 × 104 tons, respectively [71]. After the implementation of the land returning policy, their yields can be increased by up to 15.30~15.95%, 18.47~20.94%, 65.15~72.67%, 108.13~120.14% and 425.78~477.76%, respectively. According to The Dietary Guidelines for Chinese Residents of 2022, an adult consumes 0.6 kg of grain per day, for a total of 219 kg per year [72]. The implementation of the land returning policy in the Yangtze River Delta region can address the grain demand of 28–29 million adults. However, it should be noted that the percentages of beans and tubers increased 108.13~120.14% and 425.78~477.76%, respectively, in our study, which are obviously overestimated. Although beans and tubers are classified as a grain crop in China, they are rarely used as the daily grain food because rice and wheat are the main grains eaten in the Chinese diet. As a result, beans and tubers are rarely planted in the cultivated land of Chinese farmers.

3.4. Suggestion for the Implementation of the Land Returning Policy

It is important to note that the grain yields estimated in this study were the ideal values for the illegal nurseries and orchards to be fully returned to cultivated land. They do not consider the influences of specific operations of the land returning process on the grain yields. The Chinese government issued a series of policies and used administrative measures to return illegal nurseries and orchards to cultivated land [33,34], but even if all the irregular nurseries and orchards were returned, there would still be no people to carry out grain cultivation due to the shortage of rural labor. Thus, the object of increasing grain production was still not achieved. In this regard, we believe that the Chinese government can increase the rural labor force through the following aspects: (1) the government could use policies to consolidate small farmers scattered in the countryside into agricultural cooperatives. Due to the increases in cultivated land area and collective financial resources, the cooperatives can implement intensive agriculture. The use of farm machinery and scientific planting will solve the shortage of rural labor due to the aging problem of grain growers; (2) the government and social organization could guide college graduates and other young people to rural areas to grow grain via support with start-up capital and preferential tax policies. The participation of young people will increase the rural labor force and increase crop yields; (3) the government could provide policy and economic support to commercial entities in converting their owned nurseries and orchards to cultivated land, and growing grain crops on them; (4) the use of compensation and tax policies to encourage commercial companies to rent land from farmers to engage in the conversion of non-agricultural land to cultivated land, and in large-scale and intensive agricultural production. This process can not only solve the dilemma of rural labor shortages, but also allow farmers to increase their income through land rent.

3.5. Deficiencies and Prospects

Remote sensing data products were used for land use classification and estimation of returning areas in this study. Since its resolution was 1 km and the accuracy of visual verification was less than 90%, our obtained area returning from nurseries and orchards to cultivated land and subsequent grain estimation deviate from the real scenario. In recent years, in addition to the land use classification data products with a low resolution produced by multispectral satellite images (i.e., Landsat and MODIS), high-resolution and hyperspectral remote satellite images, such as Sentinel, Chinese Gaofen series satellites and EO-1 Hyperion, have also been adopted by many researchers [32,37,60,73]. Compared with traditional multispectral satellite data, their advantages in spatial monitoring and spectral information abundance make them more accurate in estimating the area of different land use types and monitoring the conversion of cultivated land to non-cultivated land in real time. The use of these images can help to improve the estimation accuracy of the area returning from the illegal nurseries and orchards to cultivated land, as well as evaluate and supervise the implementation process of the land returning policy. Therefore, we suggest that high-resolution and hyperspectral remote sensing satellite images should be used to carry out land use classification and estimate the returning area in subsequent studies.
In addition to sown area, grain yield is also related to soil quality, planting mode, crop management practices, climate, topography and other factors [74,75,76]. In this study, we only used the annual average grain yield and area to estimate the increased grain yields without including the above factors, thus the accuracy of estimation may not be high. In addition, we do not distinguish between paddy fields and dry farmland when estimating grain yields. According to Qi et al. [64], the difference in soil quality between them resulted in higher yields in paddy fields than dry farmland. In our estimation, each grain crop in paddy fields and dry farmland was assumed to be subject to rotation planting, and only planted once per species in a year. However, it is possible to plant rice or wheat twice in the year in the actual agricultural production. All of these factors further reduce the accuracy of estimation in our study. Moreover, the area used to estimate grain yield is the potential optimal area of “returning illegal nurseries and orchards to cultivated land”, which is much larger than the actual area that can be realized locally in the future. This is because many factors, including compensation price, the intensity of policy implementation and the cooperation of planters and farmers, in the process of policy implementation may lead to the area of “returning illegal nurseries and orchards to cultivated land” being smaller than our estimate [33,35]. Therefore, more research is needed to accurately estimate the increased grain yield due to the implementation of the land returning policy in the future.
In conclusion, although our results have many problems, the study provides an innovative estimation method and basic data support for the implementation of the land returning policy in the Yangtze River Delta region, and can guide local governments to formulate grain production policies. In addition, our study provides a good example for the use of satellite remote sensing data to dynamically assess the conversion between cultivated land and other land use types, and to evaluate and monitor the increased crop yield after the implementation of the returning policy.

4. Conclusions and Remarks

We developed a new method for estimating the area and location of forest that is to be returned to cultivated land. Compared with field survey and traditional remote sensing estimation methods, our method took into account the complexity of land use transformation and the needs of environmental–social systems. It can help solve the current dilemma of the frontline departments of land management in estimating the returning area due to the lack of estimation methods. Based on this method, we estimated that the area of cultivated land in the Yangtze River Delta has decreased by 11.8% in the past 40 years. Of this land, 37.1% was converted illegally to nursery and orchard. After the implementation of the land returning policy, the Yangtze River Delta region can increase newly cultivated land 11,675.6 km2, including in 9127.3 km2 of paddy fields and 2548.3 km2 of dry farmland. If grain crops were planted on these newly increased cultivated lands, they would produce 6.33, 5.31, 5.56, 2.50 and 4.89 million tons of rice, wheat, corn, beans and tubers at most in each year, respectively. These large increases in area and yield indicated that the implementation of the land returning policy was necessary and can effectively alleviate local food security problems. Additionally, we found that the area returning from cultivated land to forests varied among different administrative regions. The areas of Zhejiang and Anhui Provinces were much larger than that of Jiangsu Provinces and Shanghai Municipalities. This indicated that Zhejiang and Anhui Provinces may encounter more problems when implementing the land returning policy. These two provinces need to prepare more compensation funds for land use changes and more communication with nursery owners to push the land conversion.

Author Contributions

Conceptualization, X.Y., R.M. and J.Z.; Funding acquisition, X.Y.; Investigation, Y.H., Q.P., Y.C., B.L. and J.Y.; Methodology, X.Y., X.L. and L.S.; Supervision, X.Y., S.L., B.L. and J.Z.; Writing—original draft, Y.H., Q.P., Y.C., S.L. and X.Y.; Writing—review and editing, X.Y., R.M., X.L. and L.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41871031 and 31860111), and Ningbo Natural Science Foundation (202003N4133).

Data Availability Statement

All data used in this study are issued by the Data Center for Resources and Environmental Sciences of the Chinese Academy of Sciences, Chinese National Earth System Science Data Center, and China Statistical Yearbooks and regional statistical bulletins. These data can be found here: https://www.resdc.cn/Default.aspx (accessed on 2 and 10 March 2022); http://www.geodata.cn (accessed on 13 March 2022); http://www.stats.gov.cn/tjsj./ndsj (accessed on 21 March 2022).

Conflicts of Interest

The authors declare no conflict of interest.

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Land 11 01993 g001 550
Figure 1. Location of the Yangtze River Delta region. This map was made based on the standard map No. GS (2019) 1822, which was downloaded from the National Administration of Surveying, Mapping and Geoinformation (NASG) of China. The standard map is free to use and does not require copyright instructions. The base map is unchanged, and the geographical coordinate is WGS84.
Figure 1. Location of the Yangtze River Delta region. This map was made based on the standard map No. GS (2019) 1822, which was downloaded from the National Administration of Surveying, Mapping and Geoinformation (NASG) of China. The standard map is free to use and does not require copyright instructions. The base map is unchanged, and the geographical coordinate is WGS84.
Land 11 01993 g001
Land 11 01993 g002 550
Figure 2. A flowchart of data analysis.
Figure 2. A flowchart of data analysis.
Land 11 01993 g002
Land 11 01993 g003 550
Figure 3. The decreases in the cultivated land area over the past 40 years.
Figure 3. The decreases in the cultivated land area over the past 40 years.
Land 11 01993 g003
Land 11 01993 g004 550
Figure 4. Changes of cultivated land in the Yangtze River Delta over the past 40 years. (a–c) Distributions of cultivated land in 1980, 2000 and 2020, respectively; (d) conversion of cultivated land to other land types from 1980 to 2022. For the instructions of copyright for the base map, see Figure 1.
Figure 4. Changes of cultivated land in the Yangtze River Delta over the past 40 years. (a–c) Distributions of cultivated land in 1980, 2000 and 2020, respectively; (d) conversion of cultivated land to other land types from 1980 to 2022. For the instructions of copyright for the base map, see Figure 1.
Land 11 01993 g004
Land 11 01993 g005 550
Figure 5. Conversion of cultivated land to other land use types in the Yangtze River Delta from 1980 to 2020. (a) Conversion rate of cultivated land to other land use types; (b) conversion of cultivated land to forest. For the instructions of copyright for the base map, see Figure 1.
Figure 5. Conversion of cultivated land to other land use types in the Yangtze River Delta from 1980 to 2020. (a) Conversion rate of cultivated land to other land use types; (b) conversion of cultivated land to forest. For the instructions of copyright for the base map, see Figure 1.
Land 11 01993 g005
Land 11 01993 g006 550
Figure 6. The potential area of forest that can be returned to cultivated land in the Yangtze River Delta region. (a) Elevation; (b) conversion from cultivated land to forest, which removes forest with slope ≥ 25°; (c) distributions of ecological function reserves (EFRs) and public-welfare forests (PWFs); (d) the final cultivated land (including dry farmland and paddy field) returned from forest after the removal of the forest with slope ≥ 25°and ecological function reserves and public-welfare forests. FCC is the abbreviation of forest converted from cultivated land. For the instructions of copyright for the base map, see Figure 1.
Figure 6. The potential area of forest that can be returned to cultivated land in the Yangtze River Delta region. (a) Elevation; (b) conversion from cultivated land to forest, which removes forest with slope ≥ 25°; (c) distributions of ecological function reserves (EFRs) and public-welfare forests (PWFs); (d) the final cultivated land (including dry farmland and paddy field) returned from forest after the removal of the forest with slope ≥ 25°and ecological function reserves and public-welfare forests. FCC is the abbreviation of forest converted from cultivated land. For the instructions of copyright for the base map, see Figure 1.
Land 11 01993 g006
Table
Table 1. Introduction and sources of research data.
Table 1. Introduction and sources of research data.
Data TypeData IntroductionData Sources
Land use classification1. Land use and land cover remote sensing monitoring datasets (CNLUCC) with a resolution of 1 km for 1980, 2020 and 2020.
2. Land use types included the cultivated land, forest, grassland, water area, construction land and unused land.
3. The cultivated land consists of paddy and dry farmland.		Data Center for Resources and Environmental Sciences of the Chinese Academy of Sciences (https://www.resdc.cn; accessed on 2 March 2022)
Ecological function reserves and ecological public-welfare forests/Data Center for Resources and Environmental Sciences of the Chinese Academy of Sciences (https://www.resdc.cn; accessed on 10 March 2022), and the local forest management departments
DEMThe resolution is 90 m, which is generated by the splicing from the latest SRTM V4.1 data.Chinese National Earth System Science Data Center
(http://www.geodata.cn; accessed on 13 March 2022)
Grain yield per unit area and sown area1. Annual grain output and sown area in the China Statistical Yearbook from 2012 to 2021.
2. The China Statistical Yearbook divides grain crops into five categories: rice, wheat, corn, beans and tubers.		National Bureau of Statistics of China (NBS)
(http://www.stats.gov.cn; accessed on 21 March 2022)
Table
Table 2. Grain yield per unit area in the Yangtze River Delta region over the past 10 years (2010–2020) in the China Statistical Yearbook (kg/ha).
Table 2. Grain yield per unit area in the Yangtze River Delta region over the past 10 years (2010–2020) in the China Statistical Yearbook (kg/ha).
RegionStatisticRiceWheatCorn BeansTubers
ShanghaiAverage yield8447.004814.00 6825.00 2448.00 6038.00
SD155.91 1128.32 380.96 416.52 2062.17
Maximum8602.70 5942.30 7205.49 2864.15 8099.95
Maximum8290.88 3685.66 6443.57 2031.10 3975.61
ZhejiangAverage yield7218.003856.00 4396.00 2525.004868.00
SD125.91 299.87 215.62 95.41 203.16
Maximum7344.12 4156.10 4611.23 2620.89 5070.75
Maximum7092.30 3556.36 4180.00 2430.07 4664.43
JiangsuAverage yield8575.005281.005625.002456.006373.00
SD241.97 278.44 343.71 129.39 201.85
Maximum8817.01 5559.72 5968.64 2585.20 6574.71
Maximum8333.06 5002.84 5281.22 2326.43 6171.01
AnhuiAverage yield6331.005624.005227.001395.002546.00
SD158.24 251.23 322.48 136.65 297.82
Maximum6489.01 5875.26 5549.29 1531.98 2844.16
Maximum6172.52 5372.81 4904.32 1258.67 2248.52
Table
Table 3. The conversion from cultivated land to other land use types in the Yangtze River Delta from 1980 to 2020. Numbers in the table are areas of the subtypes of land use (km2).
Table 3. The conversion from cultivated land to other land use types in the Yangtze River Delta from 1980 to 2020. Numbers in the table are areas of the subtypes of land use (km2).
1980Dry FarmlandPaddy FieldForestConstruction LandGrasslandWater AreaOceanUnused LandSum
2020
Dry Farmland46,120.805120.543365.5 11,555.82514.321676.600.2430.3368,384.18
Paddy Field3638.8077,648.7511,155.8822,093.961424.374803.58\46.37120,811.71
Forest2477.4510,160.7877,821.152267.723956.01989.57\64.3797,737.05
Construction Land6947.137500.46 711.859441.81139.931159.16\12.6025,912.93
Grassland739.121449.04 5763.17431.534998.14766.7126.6722.8914,170.87
Water Area1424.593365.69 899.361703.75590.9517,212.8253.73186.8525,437.72
Ocean22.166.47 3.2032.6015.7517.46\14.11111.75
Unused Land5.3710.24 46.394.932.18 2.81\7.24 79.16
Sum61,375.42105,26299,766.5247,532.1211,641.6526,628.7180.64384.76352,645.4
Table
Table 4. The increased area of cultivated land in the Yangtze River Delta after the implementation of the land returning policy.
Table 4. The increased area of cultivated land in the Yangtze River Delta after the implementation of the land returning policy.
Area (km2)Dry FarmlandPaddy FieldsCultivated Land
Regions
Shanghai4.2134.0738.28
Jiangsu406.69366.48773.18
Zhejiang1772.335160.406932.72
Anhui363.763566.313930.07
Table
Table 5. Annual increment of grain crops after the implementation of the land returning policy (×104 tons).
Table 5. Annual increment of grain crops after the implementation of the land returning policy (×104 tons).
RegionsRiceOther Grain Crops
WheatCornBeansTubers
Paddy fieldShanghai2.88 ± 0.050~1.64 ± 0.380~2.26 ± 0.060~0.83 ± 0.140~2.06 ± 0.70
Jiangsu31.43 ± 0.890~19.36 ± 1.020~20.61 ± 1.260~9.00 ± 0.470~23.36 ± 0.74
Zhejiang372.49 ± 6.500~199.00 ± 15.470~226.83 ± 11.130~130.32 ± 4.920~251.19 ± 10.48
Anhui225.77 ± 5.640~200.57 ± 8.960~186.40 ± 11.500~49.76 ± 4.870~90.81 ± 10.62
Dry farmlandShanghai/0~0.20 ± 0.050~0.29 ± 0.020~0.10 ± 0.020~0.25 ± 0.09
Jiangsu/0~21.48 ± 1.130~22.88 ± 1.400~9.99 ± 0.530~25.92 ± 0.82
Zhejiang/0~68.35 ± 5.310~77.90 ± 3.820~44.76 ± 1.690~86.27 ± 3.60
Anhui/0~20.46 ± 0.910~19.01 ± 1.170~5.08 ± 0.500~9.26 ± 1.08
Sum/632.57 ± 13.080~531.05 ± 33.250~556.19 ± 30.360~249.85 ± 13.140~489.11 ± 28.14
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MDPI and ACS Style

Han, Y.; Pan, Q.; Cao, Y.; Zhang, J.; Yuan, J.; Li, B.; Li, S.; Ma, R.; Luo, X.; Sha, L.; et al. Estimation of Grain Crop Yields after Returning the Illegal Nurseries and Orchards to Cultivated Land in the Yangtze River Delta Region. Land 2022, 11, 1993. https://doi.org/10.3390/land11111993

AMA Style

Han Y, Pan Q, Cao Y, Zhang J, Yuan J, Li B, Li S, Ma R, Luo X, Sha L, et al. Estimation of Grain Crop Yields after Returning the Illegal Nurseries and Orchards to Cultivated Land in the Yangtze River Delta Region. Land. 2022; 11(11):1993. https://doi.org/10.3390/land11111993

Chicago/Turabian Style

Han, Yirui, Qinqin Pan, Yuee Cao, Jianhong Zhang, Jiaxuan Yuan, Borui Li, Saiqiang Li, Renfeng Ma, Xu Luo, Longbin Sha, and et al. 2022. "Estimation of Grain Crop Yields after Returning the Illegal Nurseries and Orchards to Cultivated Land in the Yangtze River Delta Region" Land 11, no. 11: 1993. https://doi.org/10.3390/land11111993

APA Style

Han, Y., Pan, Q., Cao, Y., Zhang, J., Yuan, J., Li, B., Li, S., Ma, R., Luo, X., Sha, L., & Yang, X. (2022). Estimation of Grain Crop Yields after Returning the Illegal Nurseries and Orchards to Cultivated Land in the Yangtze River Delta Region. Land, 11(11), 1993. https://doi.org/10.3390/land11111993

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