1. Introduction
Land is regarded as a vital resource and material guarantee for humans. Land resources are at the basic position in the complex system of population–resource–environment–development (PRED). Land use reflects the closest and most direct interaction between humans and nature [
1]. Thus, humans generate economic development and material wealth as the result of land use and affect natural resources from the perspectives of their structure, ecology, and environment [
2]. In recent decades, China has experienced rapid economic development and unprecedented urbanization. Consequently, China is facing various sustainable problems related to land use, which cover population growth, resource depletion, and ecological and environmental deterioration. The urgency and the value of research on land use are determined by the polygenesis and structural complexity of land use [
3]. From 1990 to 2018, the proportion of the population in urban areas increased from 26.41% to 59.58% [
4]. With this accelerated urbanization, land-use patterns have changed dramatically, and the contradiction between human activity and the land has become increasingly prominent. Economic development and environmental protection have a complicated history in China, arising from the competing needs to boost rural economies and conserve natural resources. More recently, China has shifted from a “grow first, clean up later” approach toward an “ecological civilization” orientation, in which development respects environmental carrying capacity [
5]. Relatedly, cities are typical complex socioeconomic ecosystems [
6], and land-use changes result from multiple factors. Urbanization was inevitably accelerated in the GBA. In addition to the boost in the economy, the population clustering and change in the ecosystem also led to problems. The significance of land-use change is worth discussing.
The environmental effect of land-use/-cover change (LUCC) is an important research topic [
7]. Previous studies have used change detection to understand LUCC patterns and processes [
8]. Jin et al. started with the physical characteristics of LUCC and built a diffusion equation to stimulate and predict urban LUCC [
9]. In contrast, Gibas and Majorek took a further step to create an evaluation method on LUCC under the background of sustainable development, which evaluates LUCC in Europe on the scales of society, economy, and environment. The evaluation method created by Gibas and Majorek helps balance the reasonable usage of natural resources and sustainable development in urban land use [
10]. The spatial patterns of urban change in two Chinese urban megaregions, Beijing–Tianjin–Hebei and the Yangtze River Delta, have been examined in the context of national urbanization policies and the associated modes of economic growth. Davis [
11] summarized the amount of deforestation, reforestation, and persistent forestation for different time intervals (1938–1956, 1956–1975, 1975–1997, and 1997–2009) and then compared the trends by using a time interval capturing only the first and last periods. Studying the Beijing–Tianjin–Hebei and Yangtze River Delta urban megaregions, Yu et al. comparatively quantified urban expansion and change on the basis of developed land on both the regional and city scales during 1984–2010 [
12]. Yang (2021), meanwhile, found that changes in the thermal environment were closely related to variations in the urban form in the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) [
13].
After China’s reform and opening-up, rapid socioeconomic development occurred in the coastal areas, the urbanization rate continuously increased, and land-use types considerably changed [
14]. Many studies have investigated the spatial patterns of land-use change and changes in ecosystem service value in the GBA [
15,
16,
17,
18]. Few, however, have conducted hotspot analyses of land-use changes in the GBA using long time series. The land-use-change trajectory model is widely used in research on the spatiotemporal evolution of land-use changes [
19]. Also, the similarity/turnover/diversity indicators represent the similarity, change times, and diversity of the evolution process of each spatial land unit during the study period. The comprehensive dynamic degree of land use is also used as an analysis index of land-use-change hotspots [
20,
21]. Liu et al. used the number of land-use changes during the study period as an index to identify global land-use transformation patterns and hotspots [
22]. Watson et al. used the frequency of land-use change as an indicator to study the effect of land-use change on biological communities [
23]. Generally, depicting the characteristics of regional land-use changes and hotspots is vital for land-resource governance and ecosystem construction.
China initiated a land-consolidation project in the late 1990s that was implemented primarily after 2008. It was a far-reaching land-use-change project that had profound effects on ecosystems. Land consolidation has become essential to China’s landscape ecology practices [
24].
Along with the New York, San Francisco, and Tokyo Bay areas, the GBA is one of the four largest bay areas in the world [
25]. The GBA urban agglomeration has a high degree of openness and economic vitality, and its development has been an essential part of China’s strategic planning. The strategic position of the GBA places higher requirements on the GBA for top-level design, industrial complementarity, market integration, and resource allocation [
26]. The GBA’s advanced stage of development in terms of industrialization, urbanization, socioeconomic level, social systems, and market mechanisms has essential effects on land space. Therefore, the GBA is a suitable “experimental field” for studying land-space evolution. Urban construction in the GBA has occupied a great deal of forestland, farmland, and wetland in recent decades. As a result, the functioning of land and sea ecosystems, the retention rate of natural coastline, and biodiversity have all been significantly reduced, and environmental problems have become increasingly prominent [
27,
28,
29,
30]. Analyzing the spatial land structure and its evolution characteristics can therefore provide important scientific support for land use and spatial planning. The mentioned previous studies provide insightful methods to discuss the transformation on ecological patterns [
18,
31], LUCC efficiency [
32], and ecological system service [
33,
34] in the GBA, but these studies lack in covering LUCC frequency for spatiotemporal characteristics.
This study aimed to explore the spatial characteristics of LUCC in the GBA. Our research questions were as follows:
What are the spatiotemporal characteristics of construction land expansion in the GBA?
Which plots have a higher change rate than other areas?
Why was almost all this high-frequency change region finally transformed into ecological lands, such as forests, wetlands, and grassland?
To maintain land stability and reach sustainable development on future land use, what methods can reduce change frequency and disturbance?
To answer these questions, we quantified the spatiotemporal characteristics of urban expansion on the basis of thematic land-use and land-cover (LULC) maps. Then, we calculated the land-use-change transfer matrix of the study area for the period 1990–2018 to reveal the transformation relationships of each category. Finally, we further analyzed spatial clustering by using a hotspot analysis.
4. Discussion
According to the result of the previous section, it can be seen that there is an inevitable trend of land-use changes in the GBA. This trend is attributed to the development of urbanization. Meanwhile, this trend resulted in the expansion of the urban area. Because the foundation of LUCC was set in the past decades, population clustering and GDP gross were generated from LUCC but reversely promoted the further development of LUCC. Moreover, accompanying the LUCC, the local ecosystem suffered from detrition. Our research places extra emphasis on the functional impacts on the urban areas, nature, society, and economy from the expansion of urban land use. The following part will discuss the LUCC from three perspectives: urban planning, natural environment, and social and economic development.
4.1. Urban Planning
The change in land use was occurring with the development of cities. Under the context of LUCC, the research on the motivation mechanism of land use change in urban agglomeration is considered a hotspot direction in the field of urban agglomeration. According to the results of this paper, fast-growing cities such as Guangzhou, Shenzhen, Dongguan, and Foshan all created a massive rise in GDP. Meanwhile, the transformation of land types is also regarded as a hallmark of the GBA. The appearance of this growth represents the procession of urbanization growing in the GBA. From the urban planning perspective, this result was caused by the implementation and guidance of the government’s policy. The GBA has been in the strategic position of the reform and opening-up of China for decades. The requirement of urbanization and GDP increase has been the overarching goal in the GBA since the 20th century, leading to land-type conversions. However, an unbalance in the portion among different land types occurred. The unbalanced allocations were reflected in the urban patterns and the land-use transformations. The original surrounding area of the GBA was dominated by farmland, forestland, and wetland, but most of these land types were utilized as and transformed into construction land. The phenomenon has occurred since the 1990s, with urbanization in Guangzhou, Shenzhen, Dongguan, and Foshan, whereas this pace has decelerated since entering the 21st century. Areas with high-frequency changes in land use were all concentrated in the urban clusters centered in Guangzhou, Shenzhen, Foshan, and Dongguan, which was caused mainly by the faster urbanization in these areas. This phenomenon accelerated the mutual transformation between land-use types and led to the gathering of high-frequency land-use change. The unbalance in land-use types among cities in GBA still exists, but the suggested reason is the difference in urban development.
4.2. Natural Environment
During the research period, the various land types in the GBA experienced different levels of transformation, which resulted in the distinct expansion of construction lands. The portion of non-construction land types suffered from reduction, mainly farmland and forestland. Compared with the portion of non-construction land types between 1990 and 2018, the transformation rate is reasonable, which avoids disturbances to the local ecosystem. However, the land types in the GBA were frequently converted during the study period. The conversion of land types is not conducive to environmental conservation [
40]. The resilience of the ecosystem is limited; thus, the function of the ecosystem should be maintained while proceeding with land transformation. The land-use change could reduce the ability of soil water conservation and soil water–holding capacity [
41]. The lack of soil water conservation could lead to soil erosion and land degradation, and the following chain effect can directly increase pressure on the marine system [
42]. The sea space should not be overcrowded in land–sea coordination, to maintain the functional completeness of the marine system. In January 2021, the government of Guangdong Province issued the Three Lines and One List environmental zoning control program, which requires the strict implementation of the environmental protection red line, environmental quality bottom line, resource utilization upper boundary, and environmental access list [
43]. The result also indicates that nearly 80% of the plots in the high-frequency land-use-change area eventually transformed into ecological land, which reflects that the GBA’s environmental protection policies had reached specific achievements.
4.3. Social and Economic Development
From 1990 to 2018, the GBA benefited from rapid economic growth, and the pace of urbanization was forced to accelerate. It is estimated that over 10 trillion boosts to the GDP generated during this period, which indicates an astonishing 22 times growth, from 0.48 trillion in 1990 to 10.87 trillion in 2018 [
44,
45]. The GDP increase stimulates GBA urbanization and accelerates the requirement for construction land. The construction land expansion reversely attracts the population’s clustering in Guangzhou, Shenzhen, Foshan, Dongguan. The dense population in the GBA brings abundant labor resources, which are crucial to increasing GDP and inducing high-frequency land-use changes. It can be seen that a loop was established in the GBA, including the GDP gross, population clustering, and LUCC. These three elements are generated from each other and can also reversely motivate each other.
4.4. Limitations
This study was limited by the spatial resolution of the land-use data and the quality of the data. Thus, we explored only terrestrial land-use changes, with low precision in the coastal zone area and limited ability to identify the status of coastal reclamation during the study period. Future research should strengthen the analysis of land-use changes by including the GBA’s coastal zones, especially in coastal reclamation. In addition, the related change patterns were not entirely accurate, owing to the limited accuracy of the classification of remote-sensing images in different periods. Thus, only the overall trend of changes was discussed in this paper.
5. Conclusions
The paper explored and analyzed the high-frequency land-use changes in the GBA from 1990 to 2018. The land types in the GBA were dominantly transformed into construction land, which satisfied the economic growth requirement and urbanization development. Specifically, Guangzhou was the city with the most increased construction land area, followed by Dongguan and Foshan. The significant increase in construction land area in these three cities synchronously occurred around 2005. Nevertheless, LUCC often occurred in the cities with a solid economic increase, such as Guangzhou, Shenzhen, Foshan, and Dongguan. We can see the strong connection between economic development and land-use transformation. This phenomenon caused an unbalance in the distribution of land types and cities: the cities with less economic development tended to see less-frequent land-use changes, and the unipolar increase in construction lands also caused functional degradation in the ecosystem. Although nearly 80% of the land was finally transformed into ecological land under relative policy and regulation, the switch between land types still led to disability in ecosystem functions. Therefore, we recommend that land-use efficiency be improved more actively in the GBA to maintain ecological stability and reach sustainable development for land functions in the future. The comprehensive management of land is required in the GBA, which includes the control unit from the township scale. A stricter inspection process for territorial development plans needs to be implemented. In addition, the utilization rate of existing construction land should be in a rational range to avoid wasting land resources. A relevant increase in investing in consolidation is encouraged for the GBA [
46].
Moreover, the consolidation of the environmental zoning control program should focus on protecting ecological land while maintaining economic growth. Identifying low-utility urban land as the object of stock space tapping, quality improvement, and efficiency enhancement is also necessary. The land consolidation and ecological restoration projects should proceed under the conditions of ecological protection, restoration, and biodiversity conservation. In addition, the principles of whole-area planning, overall design, and comprehensive management are crucial in this process; the law of nature cannot be obeyed during land transformation and development [
46]. The linkage between increases and decreases in urban and rural construction land in Guangdong Province is also a favorable land-consolidation mechanism. This linkage can strengthen village planning and rural living environment governance while providing land for urban construction, especially for key construction projects. It is also conducive to accelerating the process of rural urbanization and modernization while protecting soil resources and implementing land-use planning. The arrangement of land consolidation and ecological restoration projects in rural areas can promote employment and then stabilize the land-change situation. The terrestrial ecosystem protection and integration of land–sea management should be considered. From the perspective of planning and management, we need to continue to improve the management system of reclamation and incorporate reclamation into overall land-use planning. Farmland should be strictly protected. Once the development has been related to permanent farmland, the quantity and quality of cultivated land should be guaranteed to maintain the ecology of farmland. The following indicators must be used to control the stability of farmland development [
46]:
- (1)
The area of newly added farmland shall not be less than 5% of the original area of farmland.
- (2)
If the adjustment involves permanent farmland, the area of newly added permanent farmland shall not be less than 5% of the adjusted area.
Meanwhile, the exploration of the value realization mechanism of ecological products cannot be separated from the realization of the ecological value conversion [
47]; this can be described as “Clear waters and green mountains are as good as mountains of gold and silver” [
48]. The availability has been proved by the success of the construction of ecological corridors on both sides of the Dongjiang River, Beijiang River, and Xijiang River. It is also necessary to consider the mobility and complexity of marine ecosystems, which differ from terrestrial ecosystems [
49]. Further, we need to take the connection of marine functional zoning and the marine environmental protection red line with terrestrial functional zoning and the terrestrial environmental protection red line as the starting point [
50] to incorporate ecosystem services into coastal and offshore spatial planning. This way, we can achieve the overall goal of “one map for land and sea” for ecosystem management and environmental protection.
This study was limited by the spatial resolution of the land-use data and the quality of the data. Thus, we explored only terrestrial land-use changes, with low precision in the coastal zone area and a limited ability to identify the status of coastal reclamation during the study period. Future research should strengthen the analysis of land-use changes by including the GBA’s coastal zone, especially in coastal reclamation. In addition, the related change patterns were not entirely accurate, owing to the limited accuracy of the classification of remote-sensing images in different periods. Thus, only the overall trend of changes was discussed in this paper.