Disturbance Effect of Highway Construction on Vegetation in Hexi Corridor, North-Western China

Abstract

The ecological environment of the Hexi Corridor in China is fragile owing to its unique geographical conditions. Since the 21st century, the construction of highway projects in the Hexi Corridor has increased with the implementation of China’s western development policy. The impact of highway construction on vegetation along roads is uncertain and requires attention. In this study, the normalized difference vegetation index (NDVI) was obtained based on remote sensing data, and the correlations between the vegetation index, climate changes, and highway construction from 2000 to 2019 were analyzed. The results showed that the NDVI of the Hexi Corridor showed a significant increasing trend, and the spatial difference was obvious, which was largely controlled by climatic conditions. Generally, the vegetation index was negatively correlated with temperature, but positively correlated with precipitation, and the correlation between the NDVI and precipitation was greater than that of temperature. The impact of highways on vegetation is a long-term process. In the eastern region of the Hexi Corridor, where the ecological environment is better, highway construction promoted vegetation development. However, in the western region with high desertification, the damage caused by highway construction to vegetation was serious, and the recovery rate of degraded vegetation was slow after the completion of highway construction. Although the vegetation development in the Hexi Corridor was mainly affected by precipitation, in the western region, highway construction was the main factor controlling vegetation disturbance within a certain range on both sides, and its contribution to vegetation disturbance reached 60%. The research results can improve understanding of the disturbance effect of highway construction on vegetation in arid areas.

1. Introduction

The Hexi Corridor is a narrow geographical channel located west of the Yellow River between the Qilian Mountains and Badain Jaran Desert. It is a part of the “Ancient Silk Road”, which is not only a traffic artery connecting the Central Plains of China and the Western Regions, but also an important ecological security barrier. The Hexi Corridor is surrounded by the Gobi and other deserts, and the land in the corridor is poor due to the arid climate and scarce water resources [1]. Since the 21st century, the engineering construction of the Hexi Corridor has increased with the implementation of China’s western development policy. Among them, highway projects are often constructed earlier and widely distributed, and cause significant disturbance to the ecological environment [2,3,4]. Vegetation is the most intuitive and comprehensive indicator of the ecological environment of an area [5,6]. For the Hexi Corridor, which has a fragile ecology and scarce biomass, the role of surface vegetation is particularly prominent, and it is more sensitive to changes in the ecological environment. Spatial and temporal changes and driving factors of vegetation in arid desert areas have been research hotspot in recent years [7,8,9,10]. As a key area of the “Belt and Road” strategy, it is necessary to study the disturbance effect of highway construction on vegetation in the Hexi Corridor to provide a scientific basis for future road planning and evaluation.

Previous studies have shown that vegetation development is affected by natural factors and human activity [11,12,13,14,15]. Natural driving factors are mainly climatic conditions, in which the influences of temperature and precipitation are relatively large [16,17]. At present, the correlation coefficient method is mostly used to perform bivariate analysis of vegetation index, temperature, and precipitation [18,19]. The disturbance of vegetation by human activities is in the exploratory stage, and there is currently no established method or theory. Most existing studies describe this disturbance effect by calculating the residuals of climatic factors [20,21]. The characteristics of vegetation development vary temporally and spatially. Owing to the improvement of high-altitude detection technology and the increase in satellite density, remote sensing data can achieve multiple goals, such as high precision, time, space continuity; therefore, it is often used for the dynamic monitoring of vegetation [22,23]. For example, Sternberg et al. studied grassland degradation in the Mongolian Plateau using field surveys and remote sensing techniques [24]. Pan et al. used the NDVI and land cover data obtained by satellites to investigate the relative contribution of climatic and non-climatic drivers to the variations of grasslands [25].

The results showed that the impacts of non-climatic drivers on grassland variations were more intense. Chang et al. investigated the spatial and temporal variabilities of rain use efficiency (RUE) for grassland ecosystems over Northwestern China using the NDVI and precipitation data [26]. Results indicated that the effects of temperature and annual mean precipitation on RUE were opposite.

The construction and operation of highways have a significant impact on the spatial and temporal changes in vegetation along the route, including direct and indirect disturbances. Direct disturbances may include, but are not limited to, removing the topsoil, destroying native vegetation, and causing devastating damage to the plant community over a long linear range [27]. Indirect disturbances also include multiple aspects such as changes in plant ecological activities, community structure, and the spread of invasive alien species [28]. These disturbances may also affect the stability of the ecosystem and balance of desertification through long-term cumulative effects [29,30]. Xu et al. showed that highway construction causes a decrease in vegetation coverage and the aggravation of desertification, which has a significant impact on the ecological environment along the highway [27]. Klarenberg et al. believed that in addition to deforestation, road paving would also produce “unseen” regional scale effects on forests, owing to the dynamic changes of vegetation [31]. Liu et al. showed that highway construction will result in a decrease in arbor number, photosynthesis rate, and biomass, and the disturbance effect decreases with distance from the road [32]. To improve the ecological environment in areas disturbed by highways, research on vegetation restoration of degraded grasslands has increased in recent years [33,34,35].

Although it has been established that highway engineering has an important disturbance effect on vegetation, much uncertainty remains regarding the quantification of highway contributions, owing to the complexity of factors affecting vegetation development. In addition, highway disturbance areas are generally narrow and long, and there are differences in geography, climate, vegetation types at different times and spaces. It is biased to study the disturbance effect of highways on vegetation only from time, space, or a single influencing factor. In this study, the correlations among vegetation change, highway construction, and climatic conditions in the Hexi Corridor from 2000 to 2019 were analyzed. The main controlling factors in different regions of the Hexi Corridor were determined according to the disturbance contribution of different influencing factors. The vegetation evolution trend in the western region, with a higher degree of desertification, was further discussed using regression analysis. The research results not only provide suggestions for highway planning in arid areas, but also have practical significance for mastering the impact of highway engineering on the ecological environment.

2. Materials and Methods

2.1. Study Area

The Hexi Corridor is located in the northeastern margin of the Qinghai–Tibet Plateau and northwest of Gansu Province. It is a narrow and long accumulation plain, whose distance from east to west is up to 1100 km, and 100–200 km from the south to the north (Figure 1) [36]. This region has a typical continental climate, with scarce precipitation, strong evaporation, and large temperature difference [37]. The precipitation is affected by geographical location, from 50 mm in the northwest in to 550 mm in the southeast. The annual average evaporation is 1000–3500 mm, increasing from southeast to northwest, which is opposite to precipitation.

The Hexi Corridor has a special mountain-oasis-desert ecosystem pattern that can be divided into the Qilian Mountains, corridor plain oases, and arid desert ecological areas [38]. The Qilian Mountains in the south have the most extensive forest ecosystem, which is a lifeline for the development of the Hexi Corridor. The corridor plain oasis in the central region has been transformed from the original natural oasis and desert ecosystem by human beings and is the main agricultural irrigation, runoff consumption, and strong transformation area. Runoff disappears in the arid desert ecological area in the northern region. This area has a fragile ecosystem and faces many problems, such as drought, wind erosion, vegetation degradation, and scarce precipitation. Since the beginning of the 21st century, the average vegetation index in the study area has been low (below 10% year-round). The vegetation cover is fragile and susceptible to external disturbances.

As a key area of the “ Belt and Road “ strategy, the Hexi Corridor has now formed a developed highway transportation network. The construction of highways has significant impacts on local vegetation. The excavation of the site completely destroyed the local vegetation. The transportation and accumulation of building materials greatly hindered the vegetation development, and causing vegetation degradation. According to the field investigation, areas with obvious vegetation disturbances caused by highway construction were mainly distributed in the western region of the Hexi Corridor, concentrated in Jiuquan City, Yumen City, and Dunhuang City. This may be related to the poor local ecological environment. This is because harsh conditions are required for vegetation development in highly desertified environments. Therefore, the feedback from the ecological environment is relatively strong, even under weak engineering disturbances.

2.2. Data

Vegetation disturbance is the result of the combined effects of natural factors and human activity. According to the research content of this study and the actual situation in the Hexi Corridor, only temperature and precipitation were used as natural factors, where road density was used as the human driving factor.

(1) Normalized difference vegetation index (NDVI)

The NDVI is considered to be an effective indicator of vegetation growth, and has been widely used in vegetation change monitoring [39]. In this study, the NDVI was used to quantify vegetation cover, which was obtained from the GEOVIS Earth Open Platform [40]. The raw data were derived from NASA’s Landsat 5/7/8 satellite images from 2000 to 2019. Using the Google Earth Engine platform, all effective Landsat observational data throughout the year were obtained by removing clouds and shadows. The NDVI were calculated using the Formula (1) based on each Landsat effective observation. The maximum NDVI value of each pixel in one year was obtained combined with linear interpolation and S-G smoothing method. The time-series data of the maximum NDVI in the study area are shown in Figure 2. The spatial resolution of the data set is 30 m. The range of the NDVI is between −1 and 1. Generally, a higher NDVI value indicates a larger vegetation coverage. The value of NDVI = 0 represents rock or bare soil, and the negative values usually represent water or snow-related surfaces.

NDVI = (NIR − R)/(NIR + R),

(1)

where NIR and R represent the spectral reflectance of the near-infrared band and red band.

(2) Road density factor

The vector information of the road network in China is updated and published every year. In the study, the starting and ending times of the construction of all national and provincial highways in the Hexi Corridor were queried. Combined with the road vector data provided by the RiverMap server, the annual road network changes from 2000 to 2019 were obtained. In previous studies, the buffer zone was often used to characterize the disturbance range of the road to the vegetation, that is, a certain distance was extended to both sides from the center line of the road as the vegetation disturbance area [41,42]. Beyond the buffer zone, it was considered that the road had no disturbance effect on vegetation. In the Hexi Corridor, there are multiple roads parallel or intersecting, such as national highway G312 and expressway G30, but their construction time is different. Therefore, the intensity of road construction increases with time, but the buffer zone of parallel roads is almost the same, which cannot reflect the change in road construction intensity. The road density was used as an evaluation factor, which can be obtained by the line density tool in ArcGIS. The search radius of the line density was set to 5 km. The closer to the center of the road, the greater the road density, and the road density is 0 beyond the range of 5 km. The calculation results of road density for each year are shown in Figure 3. As can be seen, road density increased rapidly before 2011, then began to slow down, and remained stable in 2017, indicating that the highway construction was basically completed. It should be noted that road density can characterize the strength of engineering construction, but not represent the true disturbance area of highways on vegetation. In addition to the influence of engineering construction, the road disturbance area is also affected by other factors, such as vegetation type, soil, climate, and terrain conditions. The closer to the highway, the more significant the impact of the highway on the ecosystem. However, its final disturbance range is irregular and dynamic, and it is generally difficult to determine the boundary of the fixed disturbance area.

(3) Climatic factors

The annual average temperature and precipitation were used as climatic factors, which were derived from 42 meteorological stations in Gansu Province. According to the latitude and longitude of each meteorological station, the Kriging method of the ArcGIS tool was used for spatial interpolation to obtain the regional temperature and precipitation distribution raster images. Taking 2017 as an example, the annual average temperature and precipitation in the study area are shown in Figure 4. From Figure 4a, the temperature was higher in the western region and Lanzhou City, and relatively low in the central region due to the influence of local high altitude. From Figure 4b, the precipitation in the southern mountainous was significantly higher than that in the northern Gobi Desert, which gradually decreased from southeast to northwest. The climate of Hexi Corridor was mainly affected by the interaction between westerlies and monsoon, and the range and intensity of monsoon influence varied in different years. Figure 5 shows the trends of annual average temperature and precipitation in the Hexi Corridor from 2000 to 2019. The interannual fluctuations of temperature and precipitation were significant, showing a slight upward trend. The most obvious warming areas were mainly distributed in Linze and Gaotai counties in the middle of the corridor, and the area with the largest increase in precipitation was the southern mountainous area. Relevant studies have shown that global warming drives the intensification of the water cycle, which may be the essential reason for the gradual warming and wetting of the climate in Northwest China [43].

2.3. Methods

The factors affecting the vegetation growth in the Hexi Corridor are complex. Therefore, it is not enough to obtain the disturbance effect of highways only by simple comparative analysis of the remote sensing and engineering data. At present, a relatively mature vegetation disturbance analysis method has been formed in the field of ecology [44]. The main methods used in this study were trend analysis, correlation coefficient method and grey correlation analysis. The trend analysis of the NDVI in the Hexi Corridor from 2000 to 2019 was carried out by the unary linear regression method. This method can reduce the influence of accidental factors on vegetation development and accurately reflect the long-term change trend of vegetation. The correlation coefficient method was used to analyze the correlations between each factor and vegetation index in this study. This method was proposed by Pearson and is generally used to study the response of vegetation coverage to climatic factors such as temperature and precipitation. It can reflect the closeness of the relationship between the two variables [45]. The correlation coefficient is based on the deviation between the two variables and their respective average values, and the correlation degree between the two variables is reflected by multiplying the two deviations. The formula for calculating the correlation coefficient r is shown in Equation (2).

$$r=\frac{\sum _{i=1}^n\left(x_i-\overline{x}\right)\left(y_i-\overline{y}\right)}{\sqrt{\sum _{i=1}^n{\left(x_i-\overline{x}\right)}^2}\sqrt{\sum _{i=1}^n{\left(y_i-\overline{y}\right)}^2}},$$

(2)

where n represents the total number of time series, x_i represents the NDVI of year i, and $\overline{x}$ is the average NDVI in all years. y_i represents the road density, temperature, or precipitation, and $\overline{y}$ is the corresponding average value. The value of r is between −1 and 1, and r = 0 indicates that the two variables are not correlated. An r value close to −1 represents a strong negative correlation, and it indicates a strong positive correlation when the r value is close to 1. The critical value of the correlation coefficient r is 0.5139, that is, when the absolute value of r is greater than 0.5139, the correlation between the two variables is significant [46]. It should be noted that the correlation coefficient does not explain the causal relationship. Even if there is a high correlation coefficient between the two variables, it cannot be proved that one of the variables is the cause of the change in the other variable. Therefore, in the correlation analysis, it is necessary to comprehensively consider the possible factors.

To further study the disturbance effect of highway construction on vegetation in the Hexi Corridor, it is also necessary to quantitatively separate the influence of various factors on vegetation on the basis of correlation analysis, which is also the difficulty in ecological research. The main research methods include regression analysis, grey correlation analysis, and principal component analysis, and grey correlation analysis was adopted in this study. This method provides a quantitative description for the development trend of a system. The purpose is to clarify the main relationship between the factors in the system and find out the most influential factors. The quantitative value of the correlation between two systems or two factors is called the correlation degree. In this study, the correlation degree was characterized by the correlation coefficient between vegetation and each factor, and the proportion of a correlation coefficient was called the contribution degree of this factor.

3. Results

3.1. Temporal and Spatial Changes of the NDVI from 2000 to 2019

From Figure 2, spatially, owing to the influence of topography and human transformation, the areas with a higher NDVI were mainly in the Qilian Mountains, urban areas, and near rivers, such as Shiyang River, Heihe River, and Shule River. The NDVI was low in the Beishan Mountains and the northern adjacent desert areas due to the sparse rainfall. In terms of time, Figure 6 shows the interannual variation of the average NDVI in the Hexi Corridor, which generally showed a trend of fluctuating increase from 2000 to 2019, with an annual growth rate of 0.004. The average NDVI was 0.180, and the maximum value was 0.220 in 2018. Due to the large-scale drought in northern China in 2001, the minimum value was 0.136 in 2001. The vegetation development in the Hexi Corridor can be divided into two stages. From 2000 to 2011, the average NDVI was relatively low, but it increased rapidly. During this period, the NDVI of each year was not higher than the average value. Since 2012, the vegetation coverage of the Hexi Corridor was relatively good and in a stable stage.

Due to the large span from east to west in the Hexi Corridor, the climatic conditions in the eastern and western regions are different, resulting in significant differences in the NDVI. Figure 6 also shows the interannual variation of the average NDVI in the east and west regions. The boundary of the east and west regions was set as Jiuquan center. The western region included Jinta, Subei, Yumen, Guazhou, Dunhuang, and Akesai, and the remaining areas belonged to the eastern region. From Figure 6, the growth trend of the NDVI in the east of Hexi Corridor was significantly greater than that in the west, which was directly related to the increase in precipitation in the east. The precipitation in the western region was scarce, and these areas were greatly affected by human activities, which was not conducive to vegetation growth. Therefore, the NDVI growth in the west was very slow.

Highway construction will lead to a decrease in the NDVI along the road. However, because the study area is large, and the disturbance area of highway to vegetation is only within a certain range on both sides, it is difficult to visually show the change in the NDVI along the highway in Figure 2. Therefore, a representative area of about 300 km² with highway crossing was selected, as shown in Figure 7. In Figure 7a,b, only the national road G312 crossed. In Figure 7c, national highway G312 and expressway G30 were parallel. It can be seen that the NDVI was 0 at the location where the highway crossed directly. In addition, the NDVI on both sides of the highway also decreased with the highway construction, especially after the two highways were parallel (Figure 7c). Therefore, highway construction in the Hexi Corridor did have an impact on local vegetation.

3.2. Correlation Analysis between the NDVI and Disturbance Factors

(1) The correlation between the NDVI and road density

The correlation coefficient between the NDVI and road density from 2000 to 2019 was calculated by the correlation coefficient method, as shown in Figure 8. The results showed a significant correlation between the NDVI and road density within a certain distance on both sides of the highway. The correlation coefficient was between −0.935 and 0.905, and the area with significant correlation (|r| > 0.5139) accounted for 28%. It still needs to be explained that because the search radius of road density was 5 km, the range shown in Figure 8 was also 5 km on both sides of the highway, which did not represent the real range of highway disturbance to vegetation. Li et al. showed that highway construction has an inhibitory effect on vegetation growth within 1 km on both sides [47]. Ibisch et al. believed that the disturbance range of 1 km is an underestimated distance [4]. The main concern of this study was the disturbance effect of highway construction on vegetation in different sections of the Hexi Corridor from east to west, so there was no discussion on the disturbance range of the highway.

Spatially, the correlation coefficient showed a positive trend in the east and negative in the west. The correlation coefficient of most areas in the eastern region of Hexi Corridor was greater than zero, such as Lanzhou, Wuwei, and Zhangye. The positive correlation coefficient in Lanzhou reached 0.81. The construction of highways promoted vegetation development in areas with a better ecological environment, especially in the surrounding areas of the large cities. In the western part of Jiuquan, such as Jinta, Subei, Guazhou, and Dunhuang, the correlation coefficients around the highway were mostly negative, and the maximum negative correlation coefficient was −0.89. In addition, the correlation coefficient of Minqin County in the eastern Hexi Corridor was mostly negative. The degree of desertification in these areas was high, and the ecological environment was fragile which was greatly affected by human activities. Highway construction hindered the development of vegetation in these areas.

(2) The correlation between the NDVI and climatic factors

Figure 9 shows the correlation coefficient distribution between the NDVI and climatic factors. As can be seen, there was a correlation between vegetation development and temperature and precipitation in the Hexi Corridor from 2000 to 2019. The correlation coefficients also showed significant spatial differences. From Figure 9a, the areas where the NDVI was positively correlated with annual average temperature were mainly distributed around the urban centers of Zhangye, Jiuquan, and Dunhuang, as well as Minqin County, with the maximum positive correlation coefficient reaching up to 0.85. The increase in temperature in these areas promoted vegetation development. This may be related to urban greening projects, indicating that an increase in temperature within a certain range could promote urban greening construction. The areas with negative correlation coefficient were located in Lanzhou, the northern margin of the Qilian Mountains, and the northwest of the study area, with a minimum negative correlation coefficient of −0.45. The increase in temperature in these areas hindered vegetation development. In areas with high desertification, the increase in temperature was not conducive to vegetation growth.

From Figure 9b, the areas with positive correlation between vegetation development and annual average precipitation were mainly distributed in Shiyang River, Heihe River, and Shule River, Subei County, and the Qilian Mountains, with the maximum positive correlation coefficient reaching 0.87. The increase in precipitation in these areas plays a promoting role in vegetation development. The areas with negative correlation coefficient were located in the surrounding areas of Lanzhou, Jinchang, Zhangye, and Jiuquan, with a minimum negative correlation coefficient of −0.75. Comparing the disturbance effects of the two climatic factors, the temperature and precipitation in the Hexi Corridor had almost opposite effects on vegetation development. The increase in temperature hindered the vegetation growth at the northern margin of the Qilian Mountains, and the increase in precipitation promoted the vegetation growth in the Gobi area, especially in Subei County. According to the calculation, the area with significant correlation between the NDVI and temperature (|r| > 0.5139) accounted for only 6.8% of the Hexi Corridor, and the area with significant correlation with precipitation accounted for 39.8%. Therefore, the effect of precipitation on vegetation in Hexi Corridor was more significant.

3.3. Contribution Analysis of Evaluation Factors on Vegetation Disturbance

The above analysis showed that highway construction, temperature, and precipitation played a cross role in vegetation development, but the main controlling factors of vegetation disturbance cannot be obtained only from the correlation analysis. Therefore, to obtain the disturbance degree of each factor to the vegetation in the Hexi Corridor, their contribution was calculated on the basis of correlation analysis, as shown in Figure 10. The contribution of precipitation was significantly greater than that of temperature and highway, which indicated that the vegetation change in the Hexi Corridor was mainly controlled by precipitation. The contribution of the highway to vegetation disturbance was limited to a certain range on both sides of the highway (Figure 10a). It was mainly controlled by temperature and precipitation after exceeding this range, especially precipitation. It should be noted that even in the highway-affected area, the average contribution of the highway to vegetation disturbance was only 30%, which was smaller than that of temperature and precipitation. It can be concluded that the vegetation disturbance in the Hexi Corridor was largely controlled by climatic conditions.

The contribution of the highway to vegetation disturbance was obviously high in the west and low in the east. Taking Jiuquan City as the boundary, in the western region with a higher degree of desertification, the contribution of the highway to vegetation disturbance was relatively large, reaching up to 60%. This indicated that highway construction in areas with high desertification was still the main controlling factor of vegetation destruction, such as in the Gobi and desert areas, because the vegetation in the ecologically fragile area showed a greater degradation trend than that in good areas under the same highway disturbance conditions.

3.4. Analysis of Vegetation Evolution Trend in the Western Region of the Hexi Corridor

The above research results showed that in the western region of the Hexi corridor with high desertification, highway construction had a strong disturbance effect on vegetation. To further analyze the evolution trend of vegetation under highway disturbance, the regression analysis of the NDVI and road density was performed in the area west of Jiuquan City. In the range of 5 km on both sides of the road, the variation of the average NDVI and maximum road density with year is shown in Figure 11. The road density has increased since 2000, and reached a stable value of 5.50 km/km² in 2011. At the same time, the average NDVI decreased first and then increased, reaching the lowest value of 0.08 in 2011. From the perspective of change rate, the road density increased rapidly from 2000 to 2011, corresponding to the large decrease rate of the average NDVI. From 2011 to 2019, the highway construction was basically completed, and the vegetation index no longer declined, showing an upward trend.

The linear relationship between the average NDVI and road density in the western region of the Hexi Corridor from 2000 to 2011 was fitted, as shown in Figure 12a. For comparative analysis, the average NDVI within 5 km on both sides of the road and the maximum road density in the eastern regions were also linearly fitted, as shown in Figure 12b. From Figure 12a, the negative linear relationship between vegetation index and road density in the western region of Hexi Corridor was significant, during the period of highway construction (R² = 0.91), which can be expressed by Equation (3). From 2000 to 2011, vegetation degraded rapidly with the construction of highways, and the degradation rate increased with the highway construction intensity. When the road density increased by 1 km/km², the average NDVI value decreased by 0.0046. This indicated that the negative feedback of vegetation in such areas to highway construction was timely and significant. From Figure 12b, highway construction caused less disturbance to vegetation in the eastern regions. With the increase in road density, the average NDVI showed a slight growth trend, that is, the vegetation index was positively correlated with road density, which was consistent with the above correlation analysis conclusion. The results of regression analysis showed that the disturbance effect of highways on vegetation was significant in areas with a poor ecological environment.

Since the road density remained stable from 2011 to 2019, the average NDVI and time in this period were fitted, as shown in Figure 13. The average NDVI on both sides of the eastern and western roads in the Hexi Corridor showed a significant positive linear relationship with time, and the western region was more obvious (R² = 0.98), which can be expressed by Equation (4). After 2011, the highway construction was basically completed, and the vegetation around the highway began to recover, but the recovery rate in the western regions was slow with a rate of 0.0035/a. Even if the ecological green belt was built, the recovery rate was very slow. From Figure 13b, the average NDVI growth rate was 0.0078/a in the eastern regions, which was significantly greater than that during the highway construction period, indicating that the vegetation restoration in the ecologically good areas was faster. Therefore, for areas with high desertification and a fragile ecological environment, even if a series of greening projects are implemented, the damage of highway construction to vegetation is far greater than its recovery ability in a short period of time, which should be paid attention to in highway construction.

NDVI_avg = −0.0046 d + 0.103,

(3)

NDVI_avg = 0.0035 t − 6.958,

(4)

where NDVI_avg represents the average value of NDVI, d represents the road density, and t represents the year.

4. Discussion

Climatic factors and human activity have important impacts on the dynamic changes of vegetation in arid and semi-arid areas, and the existing research has not yet reached a consensus. Wang et al. analyzed the effects of climate and human factors on vegetation coverage in arid and semi-arid areas of northern China. The study also proved that precipitation is the main climatic factor controlling the overall distribution pattern of vegetation coverage. However, in a short period, the impact of human factors on vegetation coverage was more serious than climate factors, and the overall impact was negative [48]. In this study, by analyzing the contribution of temperature, precipitation, and highway construction to vegetation disturbance, we concluded that the correlation between vegetation growth and climate factors, especially precipitation, was large in most areas of the Hexi Corridor. However, the vegetation development in some areas was greatly affected by highway construction, which was consistent with the existing research results.

4.1. Variation Characteristics and Main Controlling Factors of the NDVI in the Hexi Corridor

The results showed that the spatial distribution of the NDVI in the Hexi Corridor was characterized by high in the southeast and low in the northwest. From 2000 to 2019, the NDVI showed an overall upward trend, but the spatial difference was large. The NDVI in the eastern region showed a significant upward trend, but the western region changed little. This was mainly related to local climatic conditions, human activities, and regional policies, among which climatic factors were the main reasons for the difference. During the study period, the temperature and precipitation of most meteorological stations showed an upward trend. The correlation between the NDVI and temperature in most areas of Hexi Corridor was not significant, but it had a higher correlation with precipitation. Especially in areas with less human activities, such as the Qilian Mountains, Subei, and Minqin County, precipitation played a dominant role in vegetation development, and the positive correlation between the NDVI and annual average precipitation was very significant. This was also related to the selected annual maximum NDVI. Compared with the annual average NDVI, the annual maximum NDVI is often in summer, and its response to precipitation is stronger. The eastern region was rich in precipitation, and the increase in precipitation during the study period was also more significant. Therefore, the NDVI in the eastern region was high and its increase was obvious, while the western region was the opposite.

In addition, since 1999, the policy of returning farmland to forest has been implemented in Gansu Province, which has also significantly promoted the improvement of the vegetation index in the Hexi Corridor. With the increase in the cumulative area of returning farmland to forest, the area of sloping farmland and wasteland suitable for returning farmland to forest gradually decreased. Therefore, the NDVI of Hexi Corridor increased with time and gradually remained stable. Spatially, the southeast of Gansu Province was the key area of returning farmland to forest. In the Hexi Corridor, the area of returning farmland to forest in Lanzhou, Wuwei, and Zhangye was relatively large. In most areas west of Jiuquan, due to the harsh natural environment, the land type was mainly desert, and the task of returning farmland to forest was less. This was also an important reason for the slow increase in the NDVI in the western region.

4.2. Response of the NDVI to Highway Construction in the Hexi Corridor

Climate changes dominated the overall changes of vegetation in the Hexi Corridor, while the impact of human activities on vegetation cannot be ignored. This study only considered the impact of highway construction. Different from the results of Wang et al. [48], the study area has a large span from east to west, and the impact of human activities on vegetation was not all negative, but related to spatial location and climate environment. Because the environmental background varies spatially in the Hexi Corridor, the disturbance effect of highway on vegetation is also different. The impact of highway construction on vegetation was a long-term process. The highway construction in the Hexi Corridor area was concentrated in 2000–2011, and the vegetation disturbance during the highway construction period was inevitable. However, after 2011, the study area was basically in the stage of highway operation and vegetation restoration. For areas with a good vegetation development environment, vegetation restoration on both sides of the highway was relatively easy. The highway greening project increased the vegetation coverage to the level before the highway disturbance. The cities in the eastern region of Hexi Corridor are relatively concentrated. The construction and operation of highways promoted the rapid development of the urban economy. The roadsides in these areas took the lead in forming ecological green belts. In the long run, this is the inevitable result of the effect of highway engineering on vegetation under the strategy of sustainable development in China, which is also the goal of highway ecological construction. Therefore, the highway construction in the eastern region promoted vegetation development. However, for the western region with high desertification, the vegetation growth conditions were poor, and the highway construction had a significant negative correlation with the vegetation index. In these areas, the damage of highway construction to vegetation was deeper, and the subsequent highway greening project found it difficult to achieve results in a short time. Therefore, highway construction hindered the development of vegetation for a long time.

The improvement of highway transportation network led to the development of other human projects in the Hexi Corridor, such as urbanization, wind power, photovoltaic, water conservancy, and hydropower. Their effects on vegetation are different. For example, the development of urbanization leads to the decrease in vegetation coverage in the suburbs on the one hand and increases the area of urban green space in the city center on the other hand. Photovoltaic and wind power projects need to occupy a large area of land during construction, which inevitably destroy the surrounding ecological environment. Generally, the determination and planning of a highway is a complex process, which needs to consider multiple variables simultaneously. Scientific methods should be adopted in this process. For example, Chiteculo et al. used GIS and AHP to determine the optimal routes, which can reduce the damage to vegetation on the basis of economic security [15].

In addition, China attaches great importance to the development of ecological balance, so a series of ecological restoration projects have been launched. In addition to the above-mentioned policy of returning farmland to forest, there are many policies, such as afforestation projects, grass–livestock balance projects, mine restoration projects, and water-saving agriculture. These policies are key factors in promoting vegetation cover restoration in the Hexi Corridor, which also alleviate the contradiction between human development and vegetation degradation. Therefore, the impact of human activities on vegetation is complex, and the project planning should be considered from a long-term perspective to ensure the balanced development of humans and ecology.

5. Conclusions

The development of vegetation in ecologically fragile areas responds significantly to human engineering activities. In this study, the response of vegetation index to climate and highway construction in the Hexi Corridor from 2000 to 2019 was quantitatively analyzed. Taking the western region of the corridor with a higher degree of desertification as an example, the evolution trend of vegetation under the highway disturbances was further discussed. The results showed a significant correlation between vegetation indices and highway construction. The disturbance of highways to vegetation was a long-term process and was affected by spatial location and climatic factors. In the western region of the Hexi Corridor, the disturbance effect of highway construction on vegetation was strong, whereas in the eastern region, which has a better ecological environment, especially around the city, the disturbance effect was weak. The vegetation indices were also strongly correlated with the temperature and precipitation, and these two factors had almost opposite effects on vegetation development. The increase in temperature hindered vegetation growth in the Gobi area and the northern margin of the Qilian Mountains, while the increase in precipitation promoted vegetation growth in these areas. In general, vegetation development in the Hexi Corridor was controlled by climatic conditions, especially precipitation. However, in the western region with high desertification, the contribution of highway construction to vegetation disturbance on both sides was very large, reaching up to 60%. In this region, vegetation degraded at a relatively rapid rate with the implementation of highway construction. When highway construction was completed, the vegetation around the highway began to recover, but the recovery rate was very low compared to that in the eastern region due to the dry climate conditions.