4.1. Relationship between NDVI Residual and Land Degradation
The RESTREND method proposed NDVI residuals as an indicator to quantify human-induced land degradation in water-restricted ecosystems [20
]. The negative trend of residuals is regarded as a signal of human-induced land degradation [21
]. Although humid regions account for 8.26% of the study area, most parts of northern China are hyper-arid, arid, semi-arid and dry sub-humid regions (Figure 1
). Thus, in northern China, precipitation was significantly positively related to NDVI change [2
]. Serious land degradation ultimately resulted in a long-lasting and observable loss of vegetation cover and biomass productivity over time and in space [25
]. Thus, the RESTREND method is applicable to northern China.
However, the reduction in residuals did not have a one-to-one correspondence with land degradation. For instance, some positive residual trends may have indicated harmful land-use development, such as expanding cultivation into dry lands [21
]. Therefore, not all reductions in residuals indicated degradation [22
]. Since the relationship between residual reduction and human-induced land degradation is complex, it is necessary to check carefully to determine the cause of the land degradation trends [21
4.2. Scale Effects in Terms of NDVI Pixel Size
Despite the diversity of NDVI data sources, GIMMS NDVI 3g is the only global vegetation dataset that covers the study period [20
]. Therefore, the spatial resolution in this study was determined primarily by the GIMMS NDVI 3g dataset. However, northern China includes diverse vegetation types distributed along temperature and precipitation gradients [30
]. Furthermore, some desert and desertified ecosystems in northern China may contain sparse vegetation [10
]. In addition, in deserts or heavily degraded areas, it is still possible to have patches of dense vegetation. Consequently, the NDVI that we used in this study may not show any vegetation signal for dense vegetation patches of 2 km × 2 km surrounded by bare sand in its coarse resolution. In other words, the low-resolution NDVI may not be able to accurately reveal the actual distribution characteristics of vegetation and may lead to errors in the results. It should also be noted that although using finer-resolution data may improve the accuracy of the analysis and provide more explanatory power, it may also result in noise overwhelming signals [23
]. Therefore, future studies are needed to explore the ‘‘optical’’ pixel or patch size in the application of the RESTREND analysis [23
4.3. Dynamics of Human-Induced Land Degradation in Northern China
Human activities and climate change are the two dominant factors in land degradation. From the viewpoint of sustainable land use, it is practical to alleviate or govern land degradation via the adjustment of human activities. However, distinguishing the human impacts on land degradation from those of climate change has traditionally been difficult [9
]. RESTREND is a feasible approach to detect human-induced land degradation [20
]. Evans and Geerken verify the applicability of this approach and ascertain its evaluation capability for land degradation [20
]. RESTREND is based on the scientific assumption of the strong correlation between vegetation productivity and precipitation [25
]. Because precipitation is a dominant factor in vegetation growth in arid and semiarid areas, this approach is applicable to research on land degradation in northern China [35
]. The prediction model of NDVImax based on optimal cumulative precipitation is constructed (Table 2
) to ensure the precision of human-induced land degradation detected by the RESTREND approach.
The results showed that human-induced land degradation in northern China in the past three decades presented a reversing trend, but there are still differences in different stages and regions. In space, degraded and restored land are unevenly spread, and the regional difference is significant. The areas with a high degree of degradation are mainly distributed in the northwest inland, and human-induced land degradation in the agro-pastoral ecotone of northern China presents a rapid restoration state, which is consistent with the results of Wang et al. [38
]. Although land degradation has been widely considered one of the most serious environmental issues in China, its distribution and cause is still debated [8
]. For instance, Yang [39
] believes that degraded land under the influence of humans is mainly distributed in Inner Mongolia and in the semiarid agriculture and pasture interlaced zone along the Great Wall. Zhang et al. [40
] emphasized that extremely severe degradation areas are mainly distributed in the arid and semiarid areas in North China and the Northwest.
The land degradation of northern China is affected by both natural and human-induced factors, but the dominant factors of different spatial-temporal scales vary [40
]. It is believed that different results may be due to multiple scale effects in land degradation and the complexity and uncertainty of human activities in northern China [41
]. For example, there are many areas with high land degradation values in the Northwest (Figure 4
), where precipitation increases (Table 2
) and drought mitigates during 1982–1990. This indicates that climate change itself cannot explain the condition of land degradation [40
]. Therefore, human activities, such as deforestation and overgrazing, are most likely the dominant factors in land degradation development in these areas and in the western parts of the agro-pastoral ecotone of northern China [43
]. In the northeast after the 1990s, precipitation decreases, potential evapotranspiration increases and drought intensifies the effects [42
] (Figure 2
). In contrast to the rapid land degradation reported by Ma et al. [43
], we found that the land degradation of the area caused by humans reverses significantly (Figure 5
). This signifies that human activities have a decisive effect on the reversal of land degradation, even if climate factors are adverse to the restoration of land degradation.
Clearly, research on the difference in the spatial-temporal scale will result in discrepancies in land degradation detection results. Many research results show that climate change is the dominant factor in land degradation formation and development in northern China, while human activities impact contemporary land degradation [10
]. Land degradation is a long process of gradual change. Therefore, the evaluation of results on the long-time scale can weaken the uncertainty of the results evaluated on the shorter time scale [40
]. In summary, it is considered, based on the results of the past three decades, that human-induced land degradation in northern China persists, but the development trend has been reduced since the 1990s. Although the trends of climate change in the different periods and areas are different, human activities are progressing toward the acceleration of land degradation reversal.
4.4. Regional Differentiation of Human Factors in Land Degradation of Northern China
Our results show that desertification and the relative role of natural and human factors showed obvious spatial heterogeneity, similar to Wei et al. [9
]. Therefore, we selected hot spots of land degradation and restoration in northern China (Figure 7
and Figure 8
) to further discuss the regional differentiation of the driving force of land degradation and to identify the following three types.
In the first type, climate change is beneficial to the reversal of land degradation, but the land degradation situation becomes worse, and human activities dominate land degradation. For instance, the Tarim River Basin is affected by increased precipitation and basin flow in the 1980s, and this climate factor is beneficial to the reversal of land degradation [45
]. However, our results showed that human-induced land degradation in the area is serious. Our result is consistent with that of Tao et al. [46
], Xu et al. [45
] and Jiang et al. [47
], who also think that human activities such as large-scale land reclamation and excessive water consumption, not climate change, are responsible for the drying and land degradation of the downstream river channels of the Tarim River.
In the second type, human activities play a key role in the development and reversal of land degradation. Our results show that human-induced land degradation of the Alxa Plateau presented a development trend and then gradually reversed after 2000. The climate in this area is more stable and has little effect on land degradation [48
]. It reported that land degradation developed under the influence of increased population and livestock in the 1990s [49
]. However, an obvious reversal trend since 2000 under the influence of active human activities, such as the reduction of the intensity of agricultural activities and the execution of effective ecological rehabilitation projects, was observed [49
]. Therefore, we argue that the development and reversal of land degradation in the Alxa Plateau may be dominated by human activities. It is also pointed out in research by Wang et al. [48
] that land degradation of the Alxa Plateau that occurred before 2000 is the consequence of a rapidly increasing population and that the reversal of land degradation that occurred after 2000 is the result of integrated water management implementation.
In the third type, the climate factor contributes to the development of land degradation, but it reverses under the influence of human activities. For instance, the precipitation in the western side of the Northeast China Plain decreased significantly in the 1990s, and the climate factor clearly induced an increase in land degradation [50
]. However, we found that land degradation caused by humans reversed significantly, which is similar to the conclusion obtained by Qiu et al. [50
], who point out that human factors, such as land protection policy and returning farmland to forests or grassland, dominate the reversal of land degradation in the western side of the Northeast China Plain, instead of detrimental climate change.
Human-induced land degradation in northern China developed rapidly in the 1980s, locally reversed in the 1990s and continued restoration since 2000 (Figure 4
, Figure 5
and Figure 6
). Its change trend was not the same as the precipitation change in the same period (Figure 2
). Analysis of the hot spots of human-induced land degradation indicates that the effect of climate and human factors on land degradation in different areas was complex. However, active human activities, e.g., the Three-North Shelterbelt Project (1978–present), Beijing and Tianjin Sandstorm Source Treatment Project (2001–2010), Returning Farmlands to Forest Project (2003–present), and Returning Grazing Land to Grassland Project (2003–present), are obviously the important driving forces of the reversal of land degradation in northern China [51
]. For key fortification objects, the effect of human activities on the mitigation of land degradation is particularly significant [52
]. For instance, the land degradation of areas such as Mu Us and Horqin Sandy Lands reversed significantly, fitting the reversal trend of land degradation under the influence of policies such as grazing prohibition [16
]. In summary, we think that the dominant factor of land degradation and restoration of hot spots is human activities rather than climate change. Climate change may be the long-term driving force of land degradation, but the effect of human activities on the development and reversal of land degradation in the short term is more significant.
Human activities are the most active and primary factor in land use, and artificial disturbance and governance can aggravate or effectively mitigate regional land degradation. Of course, the main driving factor of land degradation is regional disparity [2
]. Therefore, land use policies should be formulated for land degradation in northern China according to circumstances [17
]. Meanwhile, it is necessary to comprehensively consider the unique combination of regional constraints and resources and to take advantage of neglected resources, improving the livelihoods of local residents and promoting environmental conservation [54