1. Introduction
In this century, a large extent of road expansion is projected to meet increasing demands for transportation, energy and agriculture, primarily in developing countries [
1]. Constructing roads is often a proxy for environmental degradation in arid ecosystems [
2]. In contrast, in sparsely populated countries, for example, in Mongolia, not having paved roads causes more environmental degradation than building roads [
3].
When there is a lack of paved roads in Mongolia’s rangelands, people create their own tracks. A created dirt track shortly becomes unusable and impractical for driving because of corrugations in the soil, and the formation of potholes, ruts, and washboards from repeated vehicle usage. This leads people to create a new dirt track nearby. Eventually, a corridor of quasi-parallel roads covering a large area of pastureland is created. For example, in some places in central Mongolia, these dirt-track corridors are 164 m wide on average, but the widest is 6200 m [
3].
Over the last 30 years, vehicle ownership has increased immensely in Mongolia; 43,700 vehicles were tested for annual inspection in 1990, increasing to 723,200 by 2021 [
4] (
Figure 1). As of 1990, Mongolia only had 1250 km of paved roads [
5] for its 1.566 million km
2 land area. A high need for having paved roads emerged, and Mongolia has currently extended its paved road routes to 9055.1 km [
4], connecting only the centers and cities of the 21 aimags. The paved roads are capable of meeting 16% of the country’s overall transportation demands [
6]. Therefore, dirt-road corridors are still utilized, extended and created. The spatial extent of the dirt roads and their adjacent corridors, as well as their effects on the nearby vegetation, remains unknown.
The southern part of Mongolia, the Gobi Desert, which constitutes about 30% of the total area of Mongolia, is experiencing a major change in land use as a consequence of an upsurge in coal, gold and copper mining activities, which account for about 40% of Mongolia’s GDP [
7]. The livestock population quadrupled over the last 30 years. Until the present, the desert was used only by nomadic pastoralists since their first appearance in the ecosystem during the Late Holocene [
8] and was one of the most intact ecosystems in the world and a refugee for many endangered and threatened plant and animal species [
9,
10,
11]. Not only significant land (rangeland) degradation but also air pollution contamination, caused mainly by anthropogenic aerosols and frequent dust storms, have been reported in the desert [
12,
13]. This land degradation in the Gobi Desert is not only a problem for Mongolia; the desert is one of the largest emitters of airborne dusts reaching other parts of Asia on prevailing winds [
14].
Due to the absence of railroads and paved roads, and as a result of the mining boom in the desert, many dirt roads were created between 2000 and 2011 to transport mining products to China. In 2012, Oyu Tolgoi, the largest mine in the desert, located in Khanbogd soum in Mongolia’s Umnugovi province, was connected to China’s border by a paved road (100 km long) that was built along the existing dirt-road corridor. Mongolia’s economy relied heavily on this road, making it an important political issue. Although the population of this region has grown considerably, only a limited number of roads have been paved since 2012. The absence of paved roads to other destinations leads to the creation of new dirt roads and the extension of old dirt-road corridors, resulting in rangeland degradation.
No attempts have been made to reveal the effects of dirt roads on the vegetation of desert ecosystems in Mongolia. Studies in other arid systems of the world have shown that the passage of vehicles removes vegetation and leads to a loss of surface soil and soil compaction [
15,
16]. The soil of the roads loses the ability to absorb and retain moisture and nutrients [
17]. Plant communities adapt to the environment on the edges of the road, but the majority of them are invasive species that dominate the vegetation along roads [
18]. Soil along the roads reduces the soil’s water-holding capacity, causing nutrient losses and erosion. The repeated passage of vehicles increases the levels of heavy metals, such as lead, zinc, cadmium, and nickel in road and roadside soil, and the intake of those heavy metals has a harmful impact on plants [
19,
20] Dirt roads generate a large amount of dust, which inhibits the key physiological activities of adjacent plants (respiration, evaporation, and photosynthesis), lowering the ecosystem’s primary production [
21]. Dust falling on plants, for example, smothers the leaves and blocks stomata [
22]. Furthermore, roads have numerous ecological consequences, not just on vegetation but also on animal habitats, slowing animal movement, isolating populations and communities, and causing mortality [
23,
24].
The construction of the new paved road in the desert in 2012 could release pressures on areas used for dirt-road corridors, but follow-up restoration and research on the corridors’ vegetation have not yet been conducted. It takes about 10 to 15 years for vegetation to recover its native state after the disturbance of vehicle passage [
3]. Annuals predominate near recently abandoned roads, and permanent forbs and grasses eventually take their place. On the other hand, if proper restoration is not implemented after paving a road, the new construction could negatively impact the vegetation. According to the study of vegetation succession along newly paved roads in Yemen, road construction disturbed the vegetation along the road, and soil erosion often occurred shortly after construction [
25]. Disturbance-sensitive and less-resistant plant species were found to go locally extinct as a response to road construction [
26], and invasive species colonized the vegetation, and their cover was greater within 100 m of the highways in central California [
27].
The effects of the paved road on nearby vegetation as well as the spatial extent of dirt-road corridors and their ecological impacts in the Gobi Desert remain unknown. Because the paved road was built over the previous dirt-road corridors, the vegetation along the road was impacted by a sequence of disturbances: (i) usage as a dirt road until 2010, (ii) paved road construction from 2010 to 2012, and (iii) the absence of reclamation following construction from 2012 onwards. Therefore, our research aimed (i) to quantify the spatial extent of the dirt-road corridors in the case of the research area, (ii) to describe the vegetation along the paved road and dirt-road corridors, and (iii) to characterize the natural recovery of vegetation along the paved road since the road’s construction in 2010.
Since 2000, the Oyu Tolgoi copper–gold mine has been under development in Mongolia’s South Gobi region. Mining has promised massive economic growth, minimal environmental impact, and the capacity to reconstruct the country and end the social and economic instability that Mongolia has faced for the past 30 years. Despite a 2014 contract between the Mongolian government and Oyu Tolgoi mining, the public remains wary of how mining is carried out and how it impacts the environment. The repercussions of mining on the country have given rise to a new neologism, “Mine-golia” [
28]. Local herders primarily protested against the mining in its early stages, before 2015, due to dust from unpaved mining routes. Dust from unpaved roads creates distances and disconnections between people, livelihoods, and landscapes, amounting to a pasture enclosure, causing cattle diseases and threatening herders’ livelihoods in the region [
13]. Protesters forced the construction of infrastructure, such as the paved road. The impact of mining product transportation on desert vegetation prior to the construction of the paved road was disregarded, while the paved road’s economic implications were applauded.
4. Discussion
The species diversity, canopy cover, and basal gap between perennial plants and biomass were all lower for the paved road’s vegetation than they were for the dirt road. However, dirt roads have a less negative impact on vegetation than paved roads do. The area covered by the dirt-road corridors is a large portion of the region’s pastureland. The mining corporations have suggested exploring the region until at least 2070, which means that both kinds of roads would likely grow in the next 50 years as a result of increased mining activity.
Before the mining boom, the roadless and low-traffic Gobi Desert was a very important refuge for many endangered and rare species of plants and animals, such as the Gobi bear (Ursus arctos gobiensis, Sokolov and Orlov, 1992), Mongolian wild ass (Equus hemionus hemionus Pallas), wild Bactrian camels (Camelus ferus Przewalski), cottonwood (Populus euphratica Oliv.), spotted arnebia (Arnebia guttata Bunge) and savin juniper (Juniperus sabina L.). Vegetation along both types of roads did not include plants with conservation status at the national or international level. However, rare and endangered plant species remain highly susceptible to habitat change or destruction, and every road could affect their abundance. Rare and endangered animal species’ grazing pastures could also be destroyed, fragmented, and degraded by road development.
Changes in plant communities along highways can provide high-quality pasture for herbivores. The growth of early successional plants, which many herbivores like to graze, may be encouraged by the roadside [
30]. Early-stage plants with faster growth rates may green up earlier in the spring than grasses in the surrounding area, making fodder available to herbivores [
31]. However, this pattern was not observed in the desert ecosystem.
There was a distinctive difference in species composition among the vegetation along the two types of roads; the vegetation along the paved road is mainly composed of shrubs while herbaceous species make up the vegetation at all distances from the dirt roads. The re-establishment of species composition is known to take more time than other values such as biomass and canopy cover [
17,
32]. Within 3–4 years, the vegetation along the paved road, which was completed without any reclamation efforts, has not shown signs of natural revegetation in terms of species composition and richness nor in canopy cover and biomass.
Herbaceous species are especially reduced in richness, cover, and biomass at all distances from the paved road. These herbaceous species comprise most of the rare herbivorous grazers’ diet [
33]. As an exception, a few browsers, such as the wild camels, can feed on shrubs; nevertheless, high traffic and noise pollution generated from the roads do not allow the wild animals to graze freely near the paved road.
Natural revegetation in dry areas is often started by the invasion of pioneering annual plants [
34,
35]. The low abundance of annuals in the vegetation along the paved road could be a result of either a reduced annual rainfall of a given year or damage to soil seed banks during disturbances. A strong temporal dependence on the amount of annual rainfall results in high annual growth variability for a given year. Wet years could overwhelm the effects of disturbance on the annuals as their biomass and cover increase by multiple orders of magnitude [
36]. Even though the research year of 2015 was a wetter than average year, the “rainfall” of that year did not affect the vegetation. Therefore, damage to soil seed banks could be expected in the vegetation along the paved roads. The vegetation along the paved roads was a dirt-road (gravel) corridor before the road’s development. The passage of large trucks hauling mining products, and the paved road’s construction are both disturbances that can severely damage annuals’ seeds [
37].
In desert ecosystems, the distribution of perennial herbaceous plants creates spatial heterogeneity [
38,
39,
40]. Perennials create a microhabitat with a mild microclimate by trapping litter, reducing evaporation, and increasing soil moisture and nutrients [
27,
41,
42,
43,
44]. The nutrient-rich soils under perennials’ canopy facilitate soil organisms and other annuals [
45,
46]. Moreover, seed banks are abundant in the soils, and they are essential for plant recruitment [
47]. The lack of perennial plants in the vegetation along the paved road potentially results in the natural restoration of the vegetation.
The vegetation along the dirt road corridors could be considered naturally revegetating because herbaceous species dominate in the vegetation. The dirt roads used by local herders in Mongolia’s rangeland are low traffic at most times. Mongolian herders are nomadic, and traditionally, they move seasonally four times in a year. The dirt roads are used for these seasonal movements. In the desert, the distance between movements is 40–50 km on average, longer than herders’ movement in other parts of Mongolia [
48]. Since 1990, vehicle ownership among Mongolian herders has increased from nearly 0 to 100 percent for their seasonal movement, leading to a spatial extension of dirt-road corridors. Legally, land ownership in Mongolia is “communal”, suited for traditional nomadic pastoralism, but this system now allows people to freely create new road tracks and corridors and extend them. Due to the inability of constructing roads linking every herder’s seasonal camp, the formation of new dirt-road corridors is likely to increase. The environmental costs will be higher for the development of paved roads than that of the formation of dirt-road corridors. Precise and cautious monitoring and management plans for the dirt-road corridors are essential over a short period.
The results of OT mining reclamation activities have not been disclosed publicly. Although the development of paved roads avoided further deterioration of desert ecosystems, the effects of mining product transportation by dirt roads on desert vegetation between 2000 and 2010 were not taken into account by the mining or the government. In similar desert habitats, there is no attempt to revegetate after road abandonment. Therefore, both OT mining and the state must conduct a project to reclaim abandoned roads in order to expand the capacity of the rangeland area, thereby promoting rare wildlife habitat and the livelihood of local herders.
Having paved roads could be of significant socio-economic value in developing countries [
49] such as Mongolia, but the environmental costs of the paved road should be cautiously considered prior to road planning. Furthermore, new legislation may be required to prevent the local population from creating or extending the dirt-road corridors in the desert.