The Siberian Railway is an important trunk railway traversing across and connecting China, Mongolia, and Russia and passing through Continental Europe [1
]. The China-Mongolia Railway (Mongolian section, hereinafter “the Railway”) linking China and Mongolia was built between 1947 and 1955. The Railway bears 80% of the freight volume and 30% of the passenger volume within the Mongolian territory. In September and October 2013, Chinese President Jinping Xi put forward the cooperation initiatives of building “the Silk Road Economic Belt” and “21st-Century Maritime Silk Road”, which are collectively known as the “Belt and Road Initiative” [2
]. In June 2016, the heads of China, Russia, and Mongolia witnessed the signing of the Outline for the Construction of China-Mongolia-Russia Economic Corridor in Tashkent, which was the first multilateral cooperation outline of the “Belt and Road Initiative” [3
]. The Railway is located at the intersection of the China-Mongolia-Russia economic corridor and Mongolia’s Steppe Road [4
]. It has historically been the main traffic artery among China, Mongolia, and Russia, and with the implementation of the Belt and Road Initiative, will become the core of traffic infrastructure in the China–Mongolia–Russia Economic Corridor.
Mongolia is a hotspot of global land degradation [5
]. Land degradation leads to loss of available grassland resources, decreased biological production, and deterioration of the ecological environment [6
]. During 1961–2006, the numbers of plant species in Mongolia’s forest steppe, real grassland, mountain meadow, desert steppe, and desert decreased by 50.0%, 44.7%, 30.3%, 23.8%, and 26.7%, respectively. The numbers of plant species continued to decrease annually, while grassland also continued to be severely degraded [7
]. By 2009, at least 72% of Mongolia’s land had been degraded, with continued expansion of the scope of desertification [8
]. With the continuous aggravation of land degradation and the steady advances made in the China–Mongolia–Russia Economic Corridor construction, it is urgent to monitor, analyze, and diagnose patterns and dynamic variations of land degradation along the Railway.
To date, limited data monitoring of land degradation has been conducted in the China–Mongolia–Russia Economic Corridor regions. Some scholars have conducted large-scale studies with coarse resolution data on the Mongolian Plateau and its near regions. For example, Liu et al. determined the fractional vegetation cover (FVC), modified soil adjusted vegetation index (MSAVI), Albedo, land surface temperature (LST), and temperature vegetation dryness index (TVDI) through inversion of national oceanic and atmospheric administration (NOAA) data and moderate-resolution imaging spectroradiometer (MODIS) data, and accordingly, built a quantitative desertification remote-sensing monitoring index system. Furthermore, they provided data on the distribution of desertification land in China and Central Asia for 1995–2001 at a spatial resolution of 1 km [9
]. Unulbart et al. extracted information about desertification on the Mongolian Plateau from MODIS data, and analyzed the spatial distribution pattern of desertification and its variation trend on the Mongolian Plateau for the years 2001–2010 [10
]. He found that moderate desertification land was expanding from northwestern Mongolia to Dundgovi and Dornogovi in southeastern Mongolia, and that severe and extremely severe desertification lands were mainly distributed in southern Dundgovi, Omnogovi, and Dornogovi. Through inversion of MODIS data, Zhuo determined the normalized difference vegetation index (NDVI), MSAVI, FVC, LST, and drought index to generate a status map of desertification in the Mongolia Plateau for 2006 at a spatial resolution of 1 km [11
]. He found that moderate desertification land was distributed mainly in sandy regions (e.g., Khorchin sand land), severe desertification land was distributed mainly in the outer Altai Gobi and Tengger deserts, and extremely severe desertification land was concentrated in the Central Gobi and Badain Jaran deserts. In such studies, data were acquired with coarse spatial resolution and very little data were acquired after 2010. With the continuous improvement in spectral resolution, time and spatial resolution of remote sensing data, people began to utilize more sensors to discover more accurate land degradation monitoring results [12
]. In particular, the Landsat series data have been widely used [15
]. Wang et al. used Landsat TM and Landsat 8 images to monitor the recent trend of aeolian desertification in the Qinghai Lake basin in the north-east Qinghai–Tibet Plateau [18
]. They discussed the spatio-temporal evolutions of the landscape patterns of regional aeolian desertification land (ADL). Munkhnasan et al. extracted desertification information on the Hogno Khaan Nature Reserve in Mongolia based on Landsat TM/ETM + data, obtained a status map of desertification with a resolution of 30 m in the region, and completed the desertification dynamic analysis between 1990 and 2011 [19
]. Noyola et al. assessed the progress of desertification of the southern edge of the Chihuahuan Desert based on Landsat TM images [20
]. They found that desertification in the region has a slightly increasing trend. From this, we can determine that there are few studies on land degradation and desertification in Mongolia based on Landsat series data.
With the implementation of the Belt and Road Initiative and the China–Mongolia–Russia Economic Corridor program, it is urgently necessary to analyze the land degradation patterns within this region at a higher spatial resolution and to employ a longer time sequence. This is to provide the necessary data and decision support for the sustainable development of the China-Mongolia-Russia transportation area. Through object-oriented remote-sensing image interpretation, in this study, we acquire data for different types of land covers in the regions along the Railway for 1990, 2010, and 2015 at a resolution of 30 m. We then analyze the change in land cover types for these years by using the GIS spatial analysis technology, and acquire land degradation patterns occurring over the most recent 25 years. Finally, we aim to ascertain the driving factors behind land degradation in these regions and present corresponding suggestions.
4.1. Spatiotemporal Distribution Characteristics of Land Cover and Newly-Increased Land Degradation Along the Railway
According to the spatial distribution pattern of different land types in regions along the Railway in 1990, 2010, and 2015, sand and desert were concentratively distributed in the middle part Dornogovi, barren was concentratively distributed in southern regions, and desert steppe was concentratively distributed in central and southeastern regions. This reflects obvious transitional characteristics along the Railway. During 1990–2015, the total area of desert steppe, barren, sand and desert had been increasing. The overall distribution of the newly-increased degraded land demonstrated an obvious zonality, and was mainly distributed in the middle region along the Railway. Due to the influence of natural and human factors, there was less newly-increased land degradation area in the south. The land cover type in this area was dominated by barren, for example, and the Gobi was widely distributed here. In addition, previous land degradation monitoring results showed that most of southern Mongolian regions (including Dundgovi, Dornogovi, and Omnogovi) had been mostly covered by severe desertification and extremely severe desertification land for a long period. In other words, the land in southern regions was unlikely to be further degraded within the short term. In this study area, the distribution of the newly-increased land degradation areas presented transitional characteristics. The land degradation degree gradually increased from north to south, and the land degradation from non-degraded land to desert steppe was the most dominant type observed.
Although continuing land degradation was occurring, some areas in regions along the Railway were being restored. The areas of persistent restored land were mainly distributed in the southeastern part of Hentiy, the southwestern part of Suhbaatar, and the northeastern parts of Bulgan. Land was mainly being restored from desert steppe to non-degraded land, which implies that desert steppe had a relatively high restorability capacity. Although there had been an increase in the amount of land being restored, and the ability to restore land had been improved, land restorability lags behind the land degradation process. Furthermore, the gap between the speed at which land is restored and that at which land degradation occurs continues to widen. The amount of degraded land also continues to expand toward northern provinces such as Tov, Hentiy, and Selenge. Overall, there has been an increased trend in land degradation along the Railway.
The results showed that the degree and speed of land degradation vary somewhat from region to region along the Railway. During 1990–2015, the middle regions were the most responsive areas to the land degradation process, and land degradation proceeded from non-degraded land to desert steppe. Desert steppe was characterized by a relatively low degree of land degradation, was unstable land, but had a high restorability potential. Therefore, to prevent further land degradation and suppress aggravation of desertification along the Railway, it is necessary to focus monitoring and prevention of land degradation on these newly-increased land degradation regions, which have a low degree of land degradation.
We found that the phenological state of vegetation can affect remote sensing classification and manual interpretation. In the interpretation of land cover, the transition zone between vegetation and non-vegetation or between steppe and barren is easily misclassified. The influence of the phenological state of vegetation on these transition areas is greater. In the future, strengthening the study on the phenological effects of vegetation may be the key to solve this problem and improve the classification accuracy in Mongolia. Moreover, although regions within 200 km along both sides of the Railway were selected as the study area, it remains to be verified whether the variation trends and laws relating to land degradation throughout Mongolia are consistent with the conclusions obtained in this study. To explore the distribution pattern, variation, and mechanisms involved in land degradation along the Railway in greater depth, this study will be extended in the future to cover the entire Mongolian territory, with the aim of discussing these issues in a broader context.
4.2. Analysis of Driving Forces Behind Land Degradation in Regions Along the Railway
4.2.1. Natural Factors
Mongolia is dominated by a temperate continental climate. The average temperature varies very significantly. According to a statistical analysis of meteorological data acquired from 12 meteorological stations along the Railway, air temperature showed a slow rising trend and significant fluctuation during 2000–2015 (as shown in Figure 5
), and the difference between the maximum and minimum annual average temperature was 3.14 °C. The temperature fluctuations create an adverse effect on the normal growth and succession of vegetation, and can cause a reduction in vegetation coverage and productivity, severe grassland degradation, and can thereby accelerate the land degradation process.
Mongolia falls within arid and semiarid regions, and the precipitation in the country has a significant influence on vegetation growth. There is a very low level of annual precipitation in Mongolia. We obtained the average annual precipitation data of 12 meteorological stations covered by the study area through the website of Mongolia Statistical Information Service. After calculating these data, we found that the average annual precipitation from 2000 to 2010 was 2494.20 mm and from 2011 to 2015 was 2361.42 mm [24
], thereby declining overall. In addition, the precipitation distribution has obvious zonal characteristics in the Mongolian Plateau: It increases gradually from south to north [38
]. Vegetation growth is suppressed with decreased precipitation along the Railway, where precipitation mainly occurs from June to September. There is minimal precipitation in spring and winter, which aggravates the spring and winter droughts and prevents vegetation growth, thus accelerating the land degradation process. However, frequent summer and autumn rainstorms also aggravate water and soil loss, erosion deposition, and surface crushing, which could accelerate the land degradation process and increases the risk of floods in the rainy season. In the early morning of 12 August 2018, the No. 286 passenger train running along the China–Mongolia railway (Ulaanbaatar–Sainshand section) derailed in Airag Soum of Dornogovi, overturning it and injuring passengers in the process [39
]. Prior to this accident, Airag Soum had suffered an accumulated precipitation of 82 mm during incessant rains, and most of the rainwater remained within the surface soil. Flooding caused by the burst of heavy rainfall destroyed the railroad bed, which ultimately caused the accident.
4.2.2. Socioeconomic Factors
Overgrazing and population migration can accelerate the land degradation process in regions along the Railway. The animal husbandry and mining industries are Mongolia’s two mainstay industries. As shown in Figure 6
a, with the increased demand for cashmere at home and abroad during 2006–2015, there was an increase of 2,876,827 in the breeding quantity of goats in most provinces (Bulgan, Selenge, Orkhon, Darkhan, Tov, Ulaanbaatar, Hentiy, Govisumber, Dundgovi, and Dornogovi) of this study area (an increase of 73.66%) [24
]. Due to their great foraging capacity, goats chew grass roots when there is insufficient grass, which damages pastures and directly degrades the land. Mining enterprises in southwestern Mongolia have also encroached on pasture grounds, and herdsmen have been forced to migrate toward northeastern Mongolia across the Railway to find pastures. As a result, the population in eastern Mongolia has increased. Ulaanbaatar, Darhan and Tov are major political and economic centers of Mongolia, as well as important destinations or transit stations for the migration of herdsmen. During 2006–2015, the population of most provinces (Bulgan, Selenge, Orkhon, Darkhan, Tov, Ulaanbaatar, Hentiy, Govisumber, Dundgovi, and Dornogovi) in the study area increased by 417,208, representing an increase of 25.50% (as shown in Figure 6
b). After being displaced by mining areas, pasture grounds have migrated eastward and northward, which has also aggravated overgrazing in adjacent regions of the Railway [40
To promote economic development, Mongolia has actively constructed railway and road infrastructure, which can accelerate land degradation. Along with this growing trend, there has been an increasing demand for engineering construction land. To reduce construction costs, many construction companies have acquired soil directly from adjacent infrastructure regions without undertaking reclamation measures. Consequently, the ground’s surface is bare and severely crushed in these areas, which accelerates the land degradation process. Southern Mongolia has abundant mineral resources such as coal, fluorite, tungsten, gold, iron, and tin, and several strategic mineral bases [41
]. With the development of the mining industry, mineral deposits have been abandoned and piled up together with mined soil, causing an intensified flow of sand dust, thus increasing the risk of land degradation [42
]. Increased infrastructure construction for both transport and mining industries has deepened land disturbance in southern Mongolia, which has subsequently increased the degraded land area.
Rapid urbanization brings an increased risk of land degradation, and in response, Mongolia enacted a “civil freedom” policy in the 1990s with respect to citizens’ residential location choices [7
]. This policy stimulated the massive migration of herdsmen to resource-intensive regions, including the capital peripheral regions of important traffic arteries, regions near water resources (such as rivers, lakes, and wells), to central cities of provinces, and to Ulaanbaatar. This has drastically accelerated the urbanization process along the Railway. During 1990–2015, the urban population of regions adjacent to the Railway (including Selenge, Darhan, Orhon, Nov, Ulaanbaatar, Govisumber, and Dornogovi) increased by approximately 883,800, an increase of approximately 108% [24
]. In the absence of reasonable planning and administration, land available for urban construction has been underutilized, many land resources have been squandered, and the vegetation coverage of local regions has been reduced. All these factors have produced a higher risk of land degradation. As the area of land degradation expands and available land continues to decrease, the sustainable development potential in this area will also decrease.
Through remote-sensing interpretation, we acquired refined data for 1990, 2010, and 2015 with respect to different land cover types in regions along the Railway. We then analyzed the change of land cover types in 1990, 2010, and 2015 by using the GIS spatial analysis technology, and acquired land degradation patterns occurring over the 25 most recent years. Based on the analyses, we found that land cover types in this region have obvious transitional characteristics. In particular, barren, desert steppe, and non-degraded land cover types were distributed in a particular sequence from south to north. In the 25-year period, there had been obvious zonation of the newly-increased areas of degraded land in the region: They were mostly concentrated in the middle part and the degree of land degradation gradually worsened from north to south. Desert steppe, barren, sand, and desert in regions along the Railway were mainly distributed in the southern part but have tended to expand northward, showing a trend of land degradation expansion. However, some land has been gradually restored, but the restoration ability lags far behind the land degradation process. The joint effect of natural and socioeconomic factors has resulted in land degradation in such regions. Specifically, we speculate that land degradation along the Railway region is induced by significant temperature fluctuations and decreased rainfall and has been aggravated by population migration, overgrazing, infrastructure construction, unreasonable mineral exploitation, and rapid urbanization. This study suggests that monitoring and prevention of land degradation in the regions along the Railway should focus on the particular regions where slight land degradation is newly increasing. To promote the sustainable development of the China–Mongolia–Russia Economic Corridor regions, it is necessary to make reasonable plans related to human activities in these regions, to enhance their ability to tackle climate and environmental changes, and to prevent and control ecological risks. To further discover a more comprehensive and detailed process of land degradation in the regions along the Railway, we will conduct an encryption analysis at intervals of 5 years or 10 years, and strengthen the dynamic study of land degradation at different internal distances within the study area or throughout Mongolia. We will also analyze the land degradation process and development trend from a wider perspective throughout Mongolia in the future.