Abstract
The Mongolian Plateau (MP) plays an important role in the global carbon cycle and has significant impacts on the ecological security of northeastern Asia. Affected by land degradation and desertification, the vegetation cover on the MP experiences major changes under the influence of human activity and climate change. In contrast to previous holistic studies on the MP, this research focusses on the features of vegetation cover and divides the MP into three subregions (desert, grassland, and forest) to analyze vegetation dynamics and identify the driving factors behind vegetation change in this region. The residual analysis method is used and its “errors” are discussed. The results show the following. 1) The area of vegetation restoration is larger than the area experiencing vegetation degradation, and, overall, vegetation is greening and vegetation degradation is tending to reverse on the MP. 2) The ranking of vegetation change intensity is forest > grassland > desert subregions. 3) Climate change is the principal control on vegetation restoration across the whole MP, including the grassland and forest regions. Human activity similarly affects both vegetation restoration and degradation, but the effect of human activity is greater than that of climate change in the desert region. This research confirms that vegetation restoration activity is effective in the desert subregion of the MP.
Keywords : vegetation dynamics, climatic factors, RESTREND method, Mongolian Plateau, Driving factors

1 INTRODUCTION

Land degradation can cause global environmental change via its impact on the terrestrial ecosystem, hydrologic cycle, energy exchange, and other processes, and represents a major global environmental problem that encompasses regional ecological security and food security. The relationships that link fragile eco-environments, land degradation, and poverty can restrict regional socio-economic development (Gao and Mao, 2003; Tong and Long, 2003). The Committee for Development Policy (CDP) has developed a series of Sustainable Development Goals (SDGs) and SDG 15 (“Life on Land”) aims to combat desertification, restore degraded land and soil, and strive to achieve a land-degradation-neutral world by 2030 (SDGs, 2015).
Land degradation is a complex phenomenon that is associated with a reduction in land productivity, especially in arid and semi-arid areas (Kassas, 1995). Although land degradation occurs across arable land, grassland, forests, and other areas with production capability (Liu, 1995), the essence of land degradation is desertification (UNCCD, 1994). Yang (1997) proposed that land degradation can be seen as a reduction in the biological or economic production capacity of land caused by excessive human activity, and that the primary problem that drives land degradation is desertification. Zhu (1994) defined land degradation as the decline of land productivity and the evolution of other landscapes towards desertification, caused by unmitigated economic activity in a fragile ecological environment. Zhou and Pu (1996) suggested that land degradation includes desertification, which is a part of land degradation. However, these different interpretations of what constitutes land degradation all include the central tenet that land degradation causes a decrease in the land productivity. Researchers have reached a consensus that land degradation has a serious impact on global economic and social development, which will be difficult to recover from over a short period of time (Okin et al., 2001). In addition, without any control measures, land degradation will accelerate, and the cost of later rehabilitation will increase exponentially (Glantz and Orlovsky, 1983).
Land degradation is driven by a combination of human activity and climate change. Global warming has decreased the growth of vegetation in some regions (Wieder et al., 2015), and reduced vegetation productivity to some extent (Asseng et al., 2015). Temperature, precipitation, wind speed, and other climatic factors influence regional soil, vegetation, and other surface characteristics, leading to climate-related vegetation degradation (Guo et al., 2012). Compared with climatic factors, it is difficult to quantify the direct impact of human activity on vegetation change. Rapid population growth, industry development, animal husbandry, and ecological restoration projects all have some impact on vegetation change. All anthropogenic activity, such as deforestation, mineral exploitation, and improper land use, as well as emissions of industrial and domestic waste, and greenhouse gas emission, break the balance of natural ecosystems and result in serious ecological and environmental problems (Fleskens and Stringer, 2014; Liu Q. et al., 2014).
The Mongolian Plateau (MP) is located in the hinterland of Eurasia, which spans arid, semi-arid, and semi humid climate regions. This unique geographical location makes the eco-environment of this region especially vulnerable to interference from external factors. The MP is often used to study land degradation because the region is sensitive to both human activity and climate change, and also faces the problems of desertification and vegetation degradation (Zhang et al., 2008). The MP plays an important role in the global carbon cycle, and its ecosystem status and changes have an important impact on the environment of the whole of northeastern Asia (Zhang, et al., 2009; Zhang, et al., 2003). Changes to the eco-environment of the MP will affect carbon sinks in the surrounding regions, as well as further afield, through dust transmission. In recent decades, in the context of global warming and excessive reclamation, overgrazing, poor protection, soil degradation, and desertification of the MP have increased, resulting in severe pressure on the ecological environment (Liu, et al., 2016). Consequently, financial resources and policy measures have been implemented in an attempt to protect and improve the ecological environment of the MP, and these countermeasures have been relatively effective. However, soil degradation and desertification in the region remain very serious problems, and directly threaten the ecological security of China and the whole of East Asia (Mi et al., 2015; Fujiwara et al., 2007). Using the MP as our research focus to study vegetation change has important and urgent practical significance for regional vegetation restoration and ecological security in Southeast Asia.
There are regional differences in the extent and impact of the various factors that influence vegetation change. Smaller scale causes of vegetation degradation on the MP have been somewhat overlooked due to the focus on larger regional effects in previous research. Therefore, it is necessary to develop a clearer picture of the factors that drive vegetation dynamics in the different types of ecosystems on the plateau. In this study, we aim to determine the nature of vegetation change and identify its key drivers across the whole plateau and in each of its subregions (i.e., the desert, grassland, and forest subregions).

2 MATERIALS AND METHODS