Complex anthropogenic interaction on vegetation greening in the Chinese Loess Plateau

Highlights

• Human land use management is the main driver of rapid vegetation change.

• Loess Plateau is greening, but urban sprawl enhances local browning.

• Water source concentration area is the key location of vegetation restoration.

Abstract

Vegetation greening steered by land use management in the Chinese Loess Plateau has been widely reported, however studies that quantitatively assessing and explicitly linking the anthropogenic forcing on vegetation greening and browning are scarce. Here in this study, we calculate the increment and rate of change of fractional vegetation cover (FVC) from 1998 to 2018 in the Loess Plateau, and compare the results with changing rainfall, soil types, and Gross Domestic Product (GDP), to detail a systematic assessment of the role of the climate-vegetation-human nexus. We have observed that nearly 80% of the study area has undergone greening, and noticed that rainfall was not the main driver of rapid vegetation change, instead of human land use management such as, irrigation along the Yellow River, snowmelt-runoff irrigation, and irrigation from reservoirs formed by check dams contributed the most for the increased FVC in the Chinese Loess Plateau. Concurrently, rapid vegetation browning is almost fully driven by urban expansion. Our findings show that GDP growth promotes both browning and greening, indicative of sustainable development in the Loess plateau region. These contrasting trends reveal that the relationship between human activities and greening is very complex.

Introduction

The Loess Plateau in China, a global hotspot of land degradation, was covered by dense vegetation in the ancient past (Chen et al., 2001). But, excessive logging about 3000 years ago in the region has resulted in the present nature of the fragile ecological environment of the Loess Plateau (Xu et al., 2004; Fu and Gulinck, 1994; Kaiser, 2004). In order to restore the status of the ecological environment of the Loess Plateau, the Chinese government has been implementing a series of ecological missions including the Grain for Green (GFG) Project since 1999, one of the largest greening plans in the world (Hua et al., 2016). The main goal of the GFG project is to alleviate the continuous soil and water loss, and desertification (Xu, 2001; Chen et al., 2015; Liang et al., 2019). The GFG Project also greatly supports the ambitious plan of the Chinese government to achieve carbon neutrality by 2060 (Fuhrman et al., 2020).

The most intuitive change after the GFG implementation is that more than 2 × 10⁶ ha of farmland has been converted to grassland and forests (Chen et al., 2015; Deng et al., 2014). Arnon et al. (2014) studies observed that even in the most desertic areas with little vegetation coverage before, there have been varying degrees of greening recently. In 2018 alone, the afforestation area of the Loess Plateau reached 2.83 million ha (Han et al., 2020). A large number of studies thus indisputably shown that the Loess Plateau is becoming significantly greener (e.g., Chen et al., 2019).

Complex factors such as climate, irrigation, urban expansion, land-use management, and various conservation programs are the major decisive factors that cause vegetation changes in the Loess Plateau (Fehmi and Kong, 2012; Lu et al., 2015; Feng et al., 2016; Qu et al., 2018; Spracklen et al., 2012; Fang and Pomeroy, 2010; Raut et al., 2010; Liu et al., 2017; Rochedo et al., 2018). For instance, climatic variables including rainfall, temperature, evapotranspiration, and solar radiation can naturally control the vegetation growth, in which rainfall is considered generally a major driving factor leading to greening (Cai et al., 2020; Shao et al., 2019). Fehmi and Kong (2012) reported that vegetation is generally lush in areas with abundant rainfall. On the other hand, the regional long-term rainfall changes are rather a slow process (Ge et al., 2020). Other than the rainfall, land use management and irrigation-based water supply increase the greening prospects. Easy to grow plants such as shrubs and grasses grow plenty in the Loess Plateau supported by artificial irritation, thus contributing to the partial greening of the Loess Plateau (Raut et al., 2010). However, considering the constant requirement of water supply, and the increasing cost caused by irrigation distance, such models can only be adapted to local conditions (Huang et al., 2005). Snowmelt-induced runoff is another practical irrigation method that significantly promotes the distribution and growth of local vegetation (Bandyopadhyay et al., 2019).

After the launch of the national Chinese projects that aim at improving the ecosystem stability, a widespread increase in vegetation cover was reported in both satellite and field inventory data (Sun et al., 2015). The government also promotes greening by providing “subsidies” to farmers, and making the local eco-tourism industry a main economic growth model, thereby improves the income of local farmers (Wang et al., 2019a). However, with the rapid growth of vegetation, groundwater reserves in the Loess Plateau have decreased significantly, which has resulted in the occurrence of more drought in places where rainfall is scarce. This contradiction is particularly significant in areas of intense greening (Han et al., 2020). Furthermore, soil moisture is an important factor for the normal operation of ecosystem functionalities in arid areas (Liu et al., 2020), which determines the urgent issue of sustainability of the greening process (Jin et al., 2007). The driving factors of greening in the Loess Plateau are therefore very complex.

Dynamic changes in vegetation for any region can be easily carried using multi-phase satellite image difference analysis (Murray et al., 2019). Because of their long archived images, Moderate Resolution Imaging Spectroradiometer (MODIS) based vegetation indexes has been popular to evaluate the spatio-temporal changes in vegetation (Lu et al., 2015; Clark et al., 2010). Normalized Difference Vegetation Index (NDVI), the most widely used product of MODIS, is typically used to assess the changes in vegetation cover (Wu et al., 2018). However, due to the interference of complex spectral properties of soil components, the NDVI results are uncertain (Leeuwen et al., 2006). Therefore, a fractional vegetation cover (FVC) solution based on linear spectral unmixing was proposed (Rundquist, 2002). FVC reflects the intuitive measurement of vegetation cover changes (Wu et al., 2014), which provides the possibility for the study of the relationship between vegetation changes and main control factors.

Multi-period FVC based datasets alone however are inconclusive to support the causes of vegetation changes in the Loess Plateau. While a few studies noticed that rainfall is the most important driver for greening processes in the Loess plateau (e.g., Fehmi and Kong, 2012), some other studies explained the variation in soil type reflects the land use management on vegetation to a certain extent (e.g., Carter, 2002). Jiang et al. (2017a) on the other hand noticed that Gross Domestic Product (GDP) can quantify the relationship between human economic activities and greening. Considering contrasting findings, we therefore assessed the vegetation greening and browning in the Chinese Loess plateau by quantifying the relationship between satellite-derived FVC with rainfall, soil type, and GDP data. This allowed us to assess the dynamic differences in greening and browning since the GFG project in different locations to explore three outstanding issues: (i) quantify the areal percentage of Loess Plateau that has turned to green since the implementation of the GFG project? (ii) understand the regional differences in vegetation cover changes and identify the key location for future greening?, and (iii) assess and quantify the human activities that affect greening and browning. Our study further explores a major knowledge gap of driving factors of greening, supports the sustainable development goal 15 of united nations ‘protect, restore and promote sustainable use of terrestrial ecosystems, sustainably manage forests, combat desertification and halt and reverse land degradation and halt biodiversity loss’ (https://sdgs.un.org/goals).