**4. Discussion**

### *4.1. Effects of Climatic Factors on Vegetation Growth*

Vegetation growth was found to be strongly correlated with climatic factors, and their interaction was the main focus, regarding global climate change [25–27]. A normalized difference vegetation index (NDVI) could be used to monitor and estimate vegetation activities over different spatiotemporal scales, without damaging or altering vegetation [28]. Considering the differences in the responses of vegetation to climate variability under various eco-environmental conditions [29], in the Hexi Corridor, the response of the NDVI to climate change exhibited a large disparity spatially, especially for different vegetation types. In this study, grassland and farmland showed a more obvious response than other vegetation types to changes in precipitation, consistent with the results of other studies in this area [30,31]. A negative impact of temperature on vegetation growth mainly occurred in relation to bare land and desert, as well as marginal artificial grassland, attributable to the increasing evaporation associated with warming [32].

### *4.2. Effects of Human Activates on Vegetation Growth*

The spatiotemporal change of land use in the Hexi Corridor was found to be closely related to local population growth and economic development. The area of artificial oases was shown to be positively correlated with population number in [33]. According to the data of the Gansu Statistical Yearbook, the figures for the permanent population and settlement changed from 4.66 million with a settlement of 5641 in 2000, to 4.91 million with settlement of 6864 in 2020; while the GDP changed from 25.68 billion to 217.91 billion in the oases of the Hexi Corridor. Meanwhile, the night light index was also increased from 12.62 to 49.5 (Figure 7). However, human factors, such as population changes and increased cultivation of land, appear to have been stronger driving forces, which were directly responsible for the changes in desertification [10]. For example, areas of forest and grassland surrounding farmland were converted to farmland, to meet increasing demand, resulting in serious destruction of the ecological environment, such as soil loosening and increased wind erosion. One mitigation measure adopted to improve the ecological environment was the step-by-step return of farmland to forest, whilst maintaining forest– grassland conservation [34]. The change in forest area was affected by both human and natural factors, of which logging for new housing and overexploitation of groundwater resources were the main factors responsible for the 0.01% reduction in forest area [35]. Meanwhile, the 0.1% decrease in grassland area was primarily ascribed to overgrazing, owing to the annual increase of livestock supporting capacity and economic development

needs (according to the Statistical Yearbook of Gansu Province, the livestock numbers in the Hexi Corridor increased from 9.29 million in 2000 to 14.65 million in 2020) [36]. This caused a series of environmental problems and destroyed regional biodiversity. Overgrazing has extensively degraded Chinese grasslands. A reduction in the stocking rate, of 30–50% below the district averages, is required to increase the profitability of livestock production and protect vital ecosystem services. Additionally, short-time exclusions from grazing in a degraded desert have great potential to restore vegetation and soil properties [37].

**Figure 7.** Night light index and residential areas of the Hexi Corridor in (**a**) 2000 and (**b**) 2020.

Further analysis of the spatial distribution of NDVI change associated with the different vegetation types indicated that vegetation (or crop) growth increased in grassland and farmland areas, was relatively stable in forest areas, but decreased slightly in bare land and desert areas. In other regions, vegetation growth exhibited no obvious changes. Under the background of global warming, the vegetation growth in the Hexi Corridor has increased, while the area of farmland, especially that involved in crop growth, has increased since 2007, because the high consumption of water for agricultural production in the middle and lower areas of the river basins has been effectively constrained by governmental control of water resources. Correspondingly, the downstream bare land and desert vegetation presented a slight increase during the study period.

Human activities have had a positive impact on vegetation productivity in the Hexi Corridor, mainly in the oasis areas, which primarily reflects artificial irrigation, fertilization, and other management measures [38]. Irrigation water productivity and food production should be improved through promoting water-saving irrigation technology, maintaining the current use of fertilization, agricultural films and agricultural pesticides, and improving the use efficiencies of agronomic inputs, instead of increasing their amount [39]. On the basis of vegetation change, PANDA night light data, and land use type, the relationship between the changes in oasis vegetation and human activities has been discussed. The results showed that the dynamic changes of oases in the Hexi Corridor were influenced mainly by agricultural activities, supplemented by the effects of natural factors, which is in accord with other similar regions [40–43].

### **5. Conclusions**

Changes in land use and vegetation are the direct outcome of interactions between human activities and the natural environment. The spatial pattern of land use change represents the intensity and mode of the human–land relationships at different region scales. On the basis of the analysis of land use types in the Hexi Corridor, this study found that the area of certain land use types (i.e., bare land or desert, grassland, forest, and snow or glaciers) decreased from 2000 to 2020, while the areas of farmland, residential land, and water bodies increased.

Generally, vegetation growth improved continuously during 2000–2020; however, there were obvious regional differences, i.e., the highest (lowest) values of growth were found in the southeast (northwest) of the region. In the area of marked increasing NDVI, farm land and planted grass land were 55.65% and 33.79%, respectively. In the area of marked increasing NDVI, farm land and construction land were 73.05% and 13.84%, respectively. Vegetation changes at different elevations fluctuated more in the areas with frequent human activities.

Changes in vegetation were more affected by precipitation than by temperature; 41.36% of oasis change had a marked positive correlated with precipitation, while 5.38% of oasis change was negatively correlated with temperature. Based on the analysis of the statistics of the Gansu Statistical Yearbook, settlement numbers, and night light index, it was determined that human activities have gradually come to dominate the NDVI changes in farmland, residential land, and artificial grassland areas.

In China, a number of policies were implemented to improve the ecological environment in oases, whose effect was evaluated by means of remote sensing. The findings of this study highlight the great importance of enhancing the protection and management of existing areas of grassland and forest around bare land or desert areas, by means of both rational use of water resources, and strengthening awareness of ecological protection.

**Author Contributions:** W.D. and Y.J. conceived the idea and designed the research framework. Y.J. and J.W. discussed the content of the analysis part. J.C., Z.X. and W.S. carried out data collection and preprocessing. Y.J., W.S., C.W. and J.W. undertook data analysis and manuscript preparation. W.D., W.S., L.M. and X.C. contributed to manuscript refinement. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was jointly supported by the following research projects: supported by National Natural Science Foundation of China (42071018, 42101139); State Key Laboratory of Cryosphere Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy Sciences (Grant Number: SKLCS-ZZ-2022); Innovation and Development Project of China Meteorological Administration (CXFZ2022J039); Natural Science Foundation of Gansu Province (20JR10RA453); State Key Laboratory of Frozen Soil Engineering (State Key Laboratory of Frozen Soil Engineering, SKLFSE202006), and Opening Foundation of Key Laboratory of Desert and Desertification (KLDD-2020-010).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

