## 3 Results
### Spatiotemporal Characteristics of NDVI
#### 3.1.2 NDVI Spatiotemporal Change  
The vegetation coverage in the coastal region of the Guangdong Province gradually increased. From 2000 to 2020, the average NDVI increased from 0.65 to 0.73, but showed regional differences, with a decrease in the NDVI in the central region and increases in the NDVI in the eastern and western regions (Figure 3). The proportion of areas with higher coverage and above categories decreased from 89.48% to 85.37%. The area with higher coverage continued to rapidly decrease, with the highest proportion decreasing from 47.61% to 16.7%. The area with extremely high coverage continued to increase rapidly, from the lowest proportion of 0.01% to 30.69% (Figure 3).  
There are spatiotemporal differences in the transfer of different vegetation coverage levels and, over time, the transfer became increasingly frequent (Figure 4). The transfer of vegetation coverage in the central region was mainly between the moderate and below categories, while the transfer of vegetation coverage in the eastern and western regions was mainly between higher and above categories.  
The largest type of transfer area from 2000 to 2005 was the transfer of higher coverage to high coverage (23,791 km\({}^{2}\)), which exceeded the unchanged area of high coverage. Other types of transfer areas with larger transfer areas included medium coverage to low coverage (1081 km\({}^{2}\)), medium coverage to high coverage (1321 km\({}^{2}\)), high coverage to medium coverage (2842 km\({}^{2}\)), high coverage to higher coverage (1421 km\({}^{2}\)), and high coverage to extremely high coverage (2654 km\({}^{2}\)).  
Figure 3: NDVI spatiotemporal distribution from 2000 to 2020. (**a**) NDVI distribution and area proportion in 2020; (**b**) 2005; (**c**) 2010; (**d**) 2015; (**e**) 2020. The pie chart represents the area ratio, the same below.  
(2046 km\({}^{2}\)), higher cover transfer to medium cover (1541 km\({}^{2}\)), higher cover transfer to high cover (5424 km\({}^{2}\)), high cover transfer to higher cover (4459 km\({}^{2}\)), higher cover transfer to extremely high cover (2828 km\({}^{2}\)), and extremely high cover transfer to high cover (2059 km\({}^{2}\)).  
From 2010 to 2015, the types of vegetation coverage transfer with larger areas included medium cover transfer to lower cover (1531 km\({}^{2}\)), medium cover transfer to higher cover (1215 km\({}^{2}\)), higher cover transfer to medium cover (3110 km\({}^{2}\)), higher cover transfer to high cover (5043 km\({}^{2}\)), high cover transfer to higher cover (5374 km\({}^{2}\)), and higher cover transfer to extremely high cover (19978 km\({}^{2}\)).  
From 2015 to 2020, the types of vegetation coverage transfer with larger areas included medium coverage transfer to higher coverage (1291 km\({}^{2}\)), higher coverage transfer to medium coverage (1789 km\({}^{2}\)), higher coverage transfer to high coverage (4677 km\({}^{2}\)), high coverage transfer to higher coverage (2910 km\({}^{2}\)), higher coverage transfer to extremely high coverage (9020 km\({}^{2}\)), and extremely high coverage transfer to high coverage (3874 km\({}^{2}\)).  
The dynamic degree of NDVI changes shows that higher coverage is always negative, indicating a continuous decrease in area, but the decrease rate has been relatively slow since 2005 and the dynamic degree of medium and extremely high coverage is always positive, indicating a continuous increase in area (Table 1). The single dynamic degree of NDVI changes shows that among the changes from 2000 to 2005, the dynamic degree of extremely high coverage is particularly high (5840), indicating that the area expansion rate of this level is the fastest at this stage. The rate of decrease in low coverage areas was relatively large from 2005 to 2010 (with a dynamic degree of \(-10\)). In the changes from 2010 to 2015, the dynamic degree of extremely high coverage was the highest (113.138)the Guangdong-Hong Kong-Macao Greater Bay Area and is scattered across the east and southwest (Figure 5).