It is of great practical value to apply R/S analysis to study the consistency of the vegetation cover and further to predict the future changing trend of vegetation cover in eastern China. Hurst exponent can be used to indicate the consistency of the NDVI time series. In this study, firstly, R/S analysis was applied to generate the Hurst exponent for every pixel against the background of the 16-year NDVI time series with the aid of MATLAB programming. Then overlay analysis was used to overlap the results of the NDVI changing trend and Hurst exponent of NDVI time series. Thus the spatial pattern of the future changing trend of NDVI was displayed both on annual and seasonal scales (Figure 5-11 and Figure 5-12). Furthermore, zone analysis was applied to generate the statistical results of the future changing trend of NDVI for eastern China and the ten administrative units (Figure 5-10).
Hurst exponent ranges from 0 to 1. When Hurst exponent is greater than 0.5, indicating that NDVI will remain the current changing trend in the future, and when Hurst exponent is lower than 0.5, indicating that NDVI will shift the current changing trend in the future. It is worth noticing that, on annual scale, the mean Hurst exponent of the study area is 0.51, suggesting a weak consistency of NDVI time series for eastern China. Moreover, the areas expected to maintain the current changing trends (57%) are larger than the areas expected to shift the current changing trends (43%) for eastern China.
Figure 5-11(a) displays the spatial characteristics of the future changing trend of the annual NDVI. The future changing trend of the annual NDVI shows distinctive spatial heterogeneity over the study area. Figure 5-10(a) and Figure 5-11(a) show that the consistent improvement area accounts for the largest proportion (25%) of the study area, which spreads from Shandong to Liaoning. The improvement area accounts for 12% of the study area. The consistent degradation area accounts for 21% of the study area, which is concentrated in the north of Jiangsu and scattered over the study area. The degradation area accounts for 24% of the study area, which is primarily distributed in Liaoning and the north of Beijing and Hebei. Areas expected to experience unchanged in the future accounting for 11% of the study area, and areas with uncertain future
changing trend accounting for only 7% of the study area. Above results suggest that the areas with certain vegetation degradation will be larger than the areas with certain vegetation improvement in eastern China in the future.
From the perspective of the future changing trend of the annual NDVI for each administrative unit, the proportion of the areas expected to degrade in the future in nine of the ten administrative units (except for Shandong) is larger than the proportion of the areas expected to improve in the future. Particularly in Beijing and Liaoning, the degraded area expected to reach half of the corresponding area in the future.
5.1.4.2 The spatial pattern of the future seasonal NDVI changing trend
In terms of the future changing trend of the seasonal NDVI in eastern China, accounting for 53% in spring, 57% in summer, and 60% in autumn of the study area predicted to reverse the current changing trends in the future, and accounting for 47% in spring, 43% in summer, and 40% in autumn of the study area predicted to maintain the current changing trends in the future. Above results generated two statements that (1) the areas expected to remain the current changing trend are slightly greater than the areas expected to switch the current changing trend in all seasons; and (2) the areas expected to switch the current changing trend in the future are largest in autumn, followed by summer and then spring.
As displayed in Figure 5-11(b), in spring, the consistent improvement area is located in the middle of Liaoning and the junction area of Hebei and Shandong, accounting for 15% of the study area. The improvement area is sparsely scattered from the south of Jiangsu to Guangdong, accounting for 11% of the study area. The degradation area accounts for the largest proportion (23%), which is mainly distributed in the north of Hebei and Beijing and the east and west of Liaoning. The consistent degradation area accounts for 13% of the study area.
Figure 5-12(a) shows in summer the consistent improvement area accounts for the smallest proportion (11%), which is sparsely scattered from Shandong to Liaoning. The improvement area in Jiangsu is larger than in other regions of the study area. The degradation area and the consistent degradation area account for 32% of the study area,
which is primarily located in Tianjin, the junction area of Shandong and Hebei, the north of Beijing and Hebei, and the east of Liaoning.
Figure 5-12(b) displays that in autumn the improvement area and consistent improvement area account for 24% of the study area, which is primarily located in the junction area of Fujian and Zhejiang and sparsely scattered over the study area. The degradation area takes the largest proportion (28%), which is predominantly located in the middle and north of Shandong, north of Beijing and Hebei, and west of Liaoning. The consistent degradation area accounts for only 8% of the study area.
Regarding the future changing trend of the seasonal NDVI for each administrative unit, the proportion of the areas expected to degrade in the future in eight of the ten administrative units in spring (except for Shandong and Zhejiang), in six of the ten administrative units in summer (except for Shanghai, Jiangsu, Zhejiang, and Fujian), and in seven of the ten administrative units in autumn (except for Shanghai, Zhejiang, and Fujian) is larger than the proportion of the areas expected to improve in the future. It is worth noticing that areas expected to degrade in the future in Beijing reaching 52% in spring, 54% in summer, and 66% in autumn, indicating that the current improving trend of vegetation cover in Beijing may reverse and a server vegetation degradation may occur in the future.
Above results suggest that the areas with certain vegetation degradation will be larger than the areas with certain vegetation improvement for eastern China both on annual and seasonal scales in the future. This result is in line with the results generated by Li et al. (2019). Li et al. (2019) demonstrated that areas with an increasing trend and an anti- persistence characteristic account for the largest proportion both on annual and seasonal scales in the 400 mm annual precipitation fluctuation zone, China, indicating a server vegetation degradation in the future in this region.
However, our result is not in line with the results generated by Tong et al. (2018). Tong et al. (2018) evidenced that the areas with vegetation improvement are larger than the areas with vegetation degradation in the Mongolian Plateau in the future, particularly in Inner Mongolia. Inner Mongolia located in the north of China, the vegetation cover has significantly improved in recent years due to large-scale reforestation and afforestation
programs implementation and practical ecological management in this region (Tong et al., 2018, Zhang et al., 2016, Duan et al., 2011). However, eastern China is located in the eastern coastal area of China, a region has highly developed in the last three decades. Although the vegetation cover has restored in this region in recent decades, the tolerance and resilience of terrestrial ecosystem are limited. Therefore, the continued socio-economic development, urban expansion, and population growth may result in vegetation degradation in eastern China in the future. Above explanation may answer the question of why the areas with vegetation degradation predicted to be larger than the areas with vegetation improvement in eastern China in the future.
Along with rapid urbanization and industrialization occurring in eastern China, environmental concerns have become increasingly serious in recent decades in this region. Large-scale reforestation and afforestation programs have exerted great impacts on regional vegetation restoration and rehabilitation, in the north of the study area in particular, but the terrestrial ecosystem in more regions is becoming more vulnerable and unstable. To prevent the environmental degradation in eastern China effectively, the large-scale reforestation and afforestation programs should be continuously implemented, and, meanwhile, it is urgent to put new vegetation-protection programs into action to protect existing vegetation cover and alleviate the environmental deterioration, particularly in the south of eastern China.
The transformation of land use functions (e.g., forest land, grassland, shrubland, and farmland to built-up land) and enhanced human activities contributed to vegetation degradation directly and inevitably. Therefore, appropriate ecological-conservation policies should be formulated by authorities and put into practice to regulate inappropriate land use change. These policies can not only protect existing vegetated areas but provide a sustainability way to further enhance vegetation resource inventory, particularly in the areas with vegetation degradation currently and the areas expected to degrade in the future.
Figure 5-10. The statistical results of the future changing trends of annual and seasonal NDVI for eastern China and the ten administrative units