Predicting Sub-Forest Type Transition Characteristics Using Canopy Density: An Analysis of the Ganjiang River Basin Case Study
4.1. Analysis of Land Use Change Characteristics from 2000 to 2020
Over the past two decades, the Ganjiang River basin has experienced significant changes in land use. From 2000 to 2020, the total forest land area in the basin decreased by 0.88%. Among the forest land subcategories, forest land type a decreased by 0.32%, type b by 0.87%, while type c increased by 0.30%. This indicates that types b and a are primarily responsible for the overall decrease in forest land area. The land use area transition matrix reveals that 8.16% of forest land type b converted to type a over this period. Additionally, 2.96% of type b transitioned to cultivated land, while construction land and forest land type c accounted for 1.73% and 1.14% of the total area of type b, respectively. Over the same period, the main transfers from forest land type a included cultivated land (1.65%), forest land type c (1.4%), type b (1.13%), and construction land (0.59%). In terms of area transfer, over 20 years, the land types most frequently converting into forest land type c were primarily types a and b, with other land types contributing less than 0.5%. The interval from 2005 to 2010 witnessed the most dramatic changes in the area of various forest land subcategories. During this period, the vast majority of forest land types c and b remained stable, with their area proportions being 90.42% and 91.84%, respectively.
To further analyze the transfer characteristics of forest land subcategories in the Ganjiang River basin over 20 years, we overlaid the 2000 land use data with that of 2020, identifying key areas of transfer for the three forest land types (Figure 10). The results show that transfers from forest land a to cultivated land were infrequent. Transfers from forest land a to type b were mainly concentrated in the southern part of Ganzhou City, especially in Longnan County, where over 170 km2 of land transferred in 20 years. Significant transfers to type b also occurred in the southern basin areas of Yudu County, Xingguo County, and Ningdu County, predominantly in areas with altitudes of 300–400 m. These transfers were characterized by large and concentrated plots, distributed along areas with high topographic relief. In the southern parts of Nankang District and Zhanggong District, where urbanization levels are higher, many transfers occurred around 200 m altitude, with relatively smaller and more scattered plots. Transfers from forest land a to type c were mainly concentrated in the northwestern basin areas of Lianhua County and Yongxin County and in the southern areas of Yudu County and Shicheng County, occurring at altitudes of around 400 m. Transfers from forest land a to construction land did not exhibit distinct characteristics, with this study recording only seven valid transfer plots, each less than 0.05 km2 in area.
Transfers from forest land type b to cultivated land were relatively infrequent, primarily occurring in Luxi County, located in the northwestern part of the basin. The transitions from forest land type b to type a were notably concentrated in Ningdu County, Anyuan County, and Dingnan County in the southern part of the basin, characterized by overall large plot sizes. Notably, eight transfer plots exceeded 5 km2 in area and were geographically concentrated. Several significant transfer plots appeared around major urban areas, including Xinjian District in Nanchang City, Yuanzhou District in Pingxiang City, and Ganzhou District, Zhanggong District, and Nankang District in Ganzhou City. Over 60% of the transfers from forest land type b to type c were focused in Guangchang County in the eastern part of the basin. Transfers from forest land type b to construction land were relatively rare, mainly occurring in Shangyou County in the southern part.
The transfer of forest land type c to arable land is concentrated in Dingyuan County in the southern part and Xingguo County in the central part, predominantly occurring in valley and basin areas. The transition of forest land type c to forest land type a is widespread. This transition occurs in all areas except for the northern plains of the basin and is concentrated in the central and southern regions, with the transfer patches often located in areas of relatively higher elevation. The transition from forest land type c to forest land type b is focused in Ganzhou District and Nankang District in the southern part of the basin. The transition of forest land type c to construction land occurs in the Xiangdong District area in the northwest.
From 2000 to 2020, the most significant phenomenon in the Ganjiang River basin was the transfer of land between various subcategories of forest land, with the main phase of these transfers occurring between 2005 and 2010. During this period, significant land activities were observed in the southern part of the basin, particularly in Yudu, Longnan, and Dingnan counties. In addition to forest land, construction land and arable land are notable for their significant changes. Over these 20 years, the proportion of construction land in the basin increased from 1.81% to 3.43%, while arable land decreased from 26.44% to 25.72%, with arable land primarily being converted into construction land.
The Ganjiang River basin is characterized by spatial heterogeneity in its terrain, featuring plains in the central and northern areas and hills and mountains in the southwest. Among the three subcategories of forest land focused on in this study, the spatial distribution of forest land type a is closely related to the terrain. Although forest land types b and c exhibit a weaker correlation with terrain, the changes in their area proportions across different elevational zones are significant. The mutual conversion between different forest land subcategories is more frequent in areas with high terrain undulation, particularly in the southern part of the basin where transfer patches are larger and more intact. In contrast, the central area features more fragmented transfer patches, and transfers are less frequent in the northern plains.
4.2. Land Use Transition Characteristics and Key Areas of Forest Land Subcategory Transitions in 2040
Based on the 2040 land use pattern prediction results, it was found that the growth of construction land is the main cause of land use changes. From 2000 to 2040, the area of construction land increased by 2188.8190 km2, the largest change among the seven land use types, while the largest decreases in area were in forest land a, forest land b, and cultivated land, in that order. Among the land use types converted to construction land, cultivated land had the largest area, reaching 2021.9622 km2. The area of converted forest land a was 691.6243 km2, forest land b was 673.7042 km2, and forest land c was 276.5588 km2. Overall, from 2020 to 2040, the land type changes showed a transfer pattern from forest land to cultivated land to construction land. Cultivated land around major urban areas was converted to construction land due to urban expansion, while nearby forest land further transformed into cultivated land to supplement the agricultural resources of the regions. This phenomenon aligns with the objective needs of urban expansion in the Ganjiang River basin and China’s strict policies on cultivated land protection, conforming to the objective laws of social development.
Overlaying the predicted 2040 land use data with the actual data from 2020, we analyzed the transfer distribution of different subcategories of forest land over 20 years (Figure 11). The largest transfer area from forest land a to cultivated land was mainly concentrated in the northern part of the basin and Zhanggong District in the south, all at the edges of urban areas. The transfer from forest land a to cultivated land was primarily distributed in the central and northern parts of the basin; the transfer to forest land b was in the southern part of the basin; the transfer to forest land c was more evenly distributed but mainly in plains or basin areas; the transfer to construction land was more concentrated in the urban areas of Nanchang, Yichun, and Ganzhou. The transfers from forest land b were more concentrated in the southern part of the basin, with the largest transfer area from forest land b to forest land a, mainly occurring in the south. Transfers from forest land b to cultivated land were distributed in the central and northern plains and the basin area of Longnan County in the south; transfers to forest land a were concentrated in Longnan County, Yudu County, Ningdu County, etc.; transfers to forest land c were more concentrated in the central area of Ji’an City and Yudu County; transfers to construction land were distributed around several major cities. The transfer phenomenon of forest land c, besides being concentrated around major urban areas, also had significant distribution in Guangchang County in the east and Lianhua County in the west. Transfers from forest land c to cultivated land were concentrated in the southern part of the basin; transfers to forest land a were concentrated in the high-altitude areas in the west; transfers to forest land b mainly occurred in Guangchang County in the west; transfers to construction land were more distributed around major cities.
Overall, transfers occur predominantly around major cities, with similar transfer characteristics between forest land a and forest land b. Transfers involving forest land c are relatively smaller in area and more scattered. Ji’an City, as the second largest city in the basin, experiences a much lower degree of land type changes around its urban area compared to Nanchang, Ganzhou, Yichun, and other places, which may be related to the flat terrain and relatively fewer transferable forest lands locally. The transfer of forest and cultivated lands to construction land is an inevitable phenomenon of social development, which may intensify in the next 20 years. This poses a severe challenge to the ecosystem. Strengthening the development from forest lands b and c to forest land a and scientifically enhancing the quality of existing forest lands might be an important approach to resolving land use conflicts [47].
4.3. Ganjiang River Basin Forest land Conservation Strategy
Our research highlights a concerning trend in the Ganjiang River basin of a continuous decline in forest land area over the past 20 years, with projections suggesting this decline may intensify by 2040. Moreover, studies have shown that the carbon storage in forest lands surrounding large cities in the northern part of the basin, such as Nanchang, will significantly decrease by 2040 [48]. This reduction in forest land not only poses a threat to biodiversity and ecosystem services [49,50], but also increases the risk of soil erosion. In China, stringent policies are in place to protect arable land [51]. Consequently, land that was reforested due to abandonment along with forest land in low mountain and gentle slope areas often revert to arable land following land consolidation. This process leads to a forest-to-cropland conversion. If the transition pattern of forest land-to-arable land-to-construction land becomes widespread, forest land indirectly contributes to the expansion of construction land. This trend complicates efforts to restore forest land area and poses a significant threat to the ecological security of the basin.
As of 2022, Jiangxi Province boasts a forest coverage rate of over 60%, demonstrating substantial potential for forest land conservation [52]. Ecological restoration projects [53] and policy management measures [54] have shown effectiveness in enhancing a region’s ecological environment. In the Ganjiang River basin, a core area of Jiangxi Province, there is a critical need to implement relevant ecological projects and guide forest land restoration through policy interventions [55]. When the primary objective of forest land restoration is to bolster ecosystem services rather than economic gains, the focus should be on cultivating native species. This approach involves carefully selecting suitable sites and restoration measures based on scientific planning [56,57,58]. In scenarios where timber production or agroforestry economy is considered, it is vital to strike a balance between environmental and economic outcomes [59]. This balance necessitates tailored restoration strategies for different plots [60] and rational land planning to prevent the encroachment of production of forest land on other forest areas [61]. Therefore, the initial focus of forest land restoration should be on determining the most appropriate locations for restoration efforts [62,63].
Our study reveals that, over the past 20 years, the transition from forest land type a to types b and c has predominantly occurred in the southern and northwestern regions of the basin. Key areas of transition include Longnan, Yudu, Xingguo, Ningdu, Lianhua, and Yongxin counties. This shift from type a to types b and c is indicative of a decrease in forest canopy density. Moreover, certain transitions from type a to type c represent a shift in land use from forest land to productive orchards. These areas, particularly those with significant land transitions in the past 20 years, have potential for forest land restoration and should be prioritized in restoration projects. In the next 20 years, the central part of the basin is also at risk of forest land type a transitioning to other subcategories of forest land, and we need to prevent similar occurrences in this area through planning and policy measures. Furthermore, in recent years, Jiangxi Province has focused on developing local specialty economic forest products, leading to the transformation of some low-yield forest lands into economic forests. Agricultural products such as navel oranges, pomelos, and oil tea have become significant industries in the southern part of Ganzhou city in the basin. In the land planning of the Ganjiang River basin, we should consider the space for agricultural and forestry economic development, guiding the overall societal progress in a stable and positive direction.
