*4.2. Characteristics and Reasons for Change in NDVI during the Operation Period*

Within the operation period, the overall NDVI in the core and contrast areas showed an upward trend, where the growth rate in the contrast area was greater than that in the core area; however the increase was small. Due to the implementation of environmental protection policy [44], the vegetation being slowly restored by regeneration, afforestation, and returning farmland to forest. The NDVI along the GH highway rose to a high value and the change is relatively stable in 2000. However, after 2000, the NDVI showed a downward trend. During the nearly 30 years of the operation period, human activities along the GH highway have substantially enhanced, as is reflected by an increase in artificial surfaces along the highway and the construction and widening of other roads along the highway. In addition, the NDVI showed an obvious downward trend in 2008 and 2009, related to the severe snow disaster in southwest China in the winter of 2008. A certain range of vegetation died due to freezing, which affected the maximum NDVI in the following year. The same significant decrease also occurred from 2010 to 2012, which was related to the destruction of vegetation on the original surface due to the construction of the Shanghai-Kunming highway in this section in 2010, and the severe drought in Guizhou in 2012, which resulted in a decline in and reduced growth of NDVI from 2010 to 2012. Furthermore, the NDVI decreased significantly in 2019–2020. Meteorological bureau data (http://gz.cma.gov.cn/ access on 1 October 2022) indicated that a spring drought occurred in the spring in Central Guizhou during 2019–2020. This spring drought caused the vegetation to be short of water during the growth period, leading to inhibition of vegetative growth throughout the year, thus significantly reducing the NDVI value over this period.

The construction and operation of GH highway has increased the intensity of human activities in towns along the line, these activities Include change of cropland area along the GH highway will also affect the vegetation [45], causing the fluctuation of NDVI along the GH highway to be higher than that in other regions. The construction and operation of the GH highway mainly affected the area within 2 km, where a large number of villages, towns, cities, and trunk roads are concentrated. These areas are characterized by strong human activities. In the area far from the GH highway, the vegetation is less affected by the highway, and the heterogeneity of the karst surface becomes the main factor affecting the NDVI. Although the impact of human engineering construction on the environment is not unique to karst, the heterogeneity, vulnerability, and sensitivity of the karst surface are strong [46]. Therefore, the impact of human activities tends to cause greater damage in karst areas. In recent decades, the frequency and intensity of extreme climate events have increased, and the impact of high temperatures and drought on the productivity of the midlatitude ecosystem in the Northern Hemisphere has become greater and greater, leading to a more sensitively responsive of NDVI [47]. Over the past three decades, the reduction of precipitation and the increase in temperature have led to increased evaporation. The soil moisture in many areas has decreased [48], while the sensitivity of vegetation to soil moisture has generally increased [49]. In particular, soil moisture restricts karst ecological restoration [50]. The study area is a typical karst area, with thin soil layer, weak soil water holding capacity, many underground rivers, strong surface water infiltration, and easy soil water loss. Under the trend of long-term temperature rise and precipitation decrease, soil moisture evaporation will inevitably be intensified, leading to soil moisture reduction, which will further affect the growth and development of vegetation along the line. The enhancement of human activities along the GH highway will also lead to intensification of soil erosion, especially with expansion of cities along the highway. The construction of other highways will increase the sensitivity of vegetation in artificial surface areas to

drought, resulting in water shortages during the peak growth season, ultimately leading to declining annual biomass.
