3.4.1. Driving Forces of NDVI Changes in the Yangtze River Basin
Anthropogenic and climate factors are two key driving factors affecting regional vegetation cover. Among them, the direct influence of climate change is mainly manifested by the changes in temperature and precipitation. In terms of human activities, it is affected by the comprehensive effects of ecological construction, urban expansion, agricultural production, and land use.
Figure 8 shows the influence of anthropogenic and climate factors on NDVI in the Yangtze River Basin.
The interaction of those two factors can vary considerably for different regions. In high-altitude areas with comparatively low human activities, climate change mainly influenced the vegetation cover. In contrast, in low-altitude areas with generally high population densities, vegetation cover becomes more dependent on human activities, such as hydro-power development and soil erosion control. The results show that in 22.91% of the Yangtze River Basin, vegetation coverage is not significantly affected by climate change. In 66.40% of the basin, climate change promoted vegetation growth, of which 33.06% had moderate or high promotion effects. These regions are distributed in the Wuyi Mountains in the Poyang Lake Basin, the Nanling Mountains in the Dongting Lake Basin, the Wuling Mountains in the Upper Mainstream Area, the Hengduan Mountains in the Sichuan Basin, and the Jinsha River Basin. In 10.69% of the basin, climate change had an inhibitory effect on vegetation growth, of which 2.30% had a moderate or high inhibitory effect. The areas were located mainly in the Yangtze River Delta, the eastern of the Hanshui River Basin, and the densely populated areas, such as Chengdu, Chongqing, Wuhan, and Changsha.
The contribution rate of anthropogenic activities to vegetation growth in the Yangtze River Basin was 83.45% (
Table 6). Compared to climate change, areas where human activities had moderate or high promotion effects accounted for a larger proportion (76.74%). Areas where human activities promoted vegetation growth were located in the central Yangtze River Basin, including the upstream area, the Jialing River Basin, Wujiang River Basin, Han River Basin, and Dongting Lake Basin. These regions comprise key regions for water and soil conservation projects, conversion of farmland to forests, and natural forest protection projects in the Yangtze River Basin. The proportion of the area where anthropogenic activities inhibit the growth of vegetation is 13.5%, which is mainly distributed in the upstream Jinsha River basin, Taihu Lake Basin, Southern Qinghai, Eastern Tibet, and many large cities (e.g., urban agglomeration in the midstream of Yangtze River, Yangtze River Delta region, and Chengdu–Chongqing region). These areas have high human activities, such as tourism development, urban construction, mining, and grazing.
In
Figure 9, areas where vegetation growth had been caused by both anthropogenic and climate factors accounted for 73.82% of the Yangtze River Basin. Areas where vegetation growth had been influenced only by climate change comprised 1.55% of the basin and were mainly situated in the upstream of Jinsha River Basin in the eastern region of the Qinghai–Tibet Plateau. Areas where vegetation increase had been affected only by anthropogenic factors accounted for 10.43% of the region and can be found in the Jianghan Plain in the east of the Hanshui River sub-basin and the surrounding areas in the north of the Dongting Lake Basin. In these areas, traditional farming techniques are commonly employed, and human planting activities are quite significant.
In addition, about 8.58% of the NDVI decrease in the Yangtze River Basin was caused by the double influence of anthropogenic and climate factors. These areas are mainly concentrated in the Yangtze River Delta region, the urban agglomeration in the midstream of the Yangtze River, the Hengduan Mountains in the upstream of Jinsha River, and the Western Sichuan Plateau in Lu-Tuo River Basin. Areas with reduced vegetation caused only by climate factors accounted for 0.73% and had a relatively scattered distribution. Areas with reduced vegetation due to anthropogenic factors comprised 4.90% and were distributed in the alpine meadows of the Qinghai–Tibet Plateau upstream of the Yangtze River Basin. The results suggest that the double impact of anthropogenic and climate factors have been major drivers of vegetation change in the Yangtze River Basin in the past 20 years.
3.4.2. The Relative Contribution of Human Activities and Climate Change to NDVI Change in the Yangtze River Basin
The contributions of anthropogenic and climate factors to NDVI have to separated and analyzed individually (
Figure 10). Areas where climate change positively contributed to NDVI change accounted for 84.61% (
Table 7). Among them, areas with contribution rates of 0–20% and 20–40% comprised 44.31% and 23.86%, respectively. These areas are mainly in high altitudes where precipitation is abundant and vegetation cover change is highly influenced by temperature, such as the Hengduan Mountains, Wuling Mountains, Yunnan–Guizhou Plateau, and Nanling-Wuyi Mountains. Areas with more than 80% contribution rates made up only 3.16%, and were mainly in the Three-River Headwater Region of Qinghai–Tibet Plateau and Hengduan Mountains. Areas where climate change contributed negatively to vegetation cover change accounted for 15.40% and were mainly in the Guoluo area in the southeast of Qinghai–Tibet Plateau, the northern Dongting Lake sub-basin, and the eastern Han River sub-basin.
As shown in
Figure 10b, the contributions of anthropogenic activities to vegetation cover change in the Yangtze River Basin have been mainly positive, affecting 92.77% of the region. Those with contribution rates of 60~80% and greater than 80% made up 23.86% and 54.74% of the basin, respectively. Areas where human activities had contribution rates above 80% are the Dongting Lake Basin, Han River Basin, Poyang Lake Basin, northern Jialing River Basin, and the Yunnan–Guizhou Plateau in the Jinsha River Basin.
In 7.24% of the study area, anthropogenic activities had negative contribution rates to vegetation cover change. The contributions of that to vegetation cover increase were greater than those from climate change. Using the actual and the mean change trends of NDVI affected by climate change and anthropogenic activities, the contributions of rates were estimated to be 20.71% and 79.29%, respectively. The results suggest that the anthropogenic aspect is the main factor affecting vegetation cover change in the Yangtze River Basin.
At the sub-basin level, the contribution rates of climate change to vegetation cover change ranged between 0% and 55.48%, and only in upstream Jinsha River did climate change contribute more than 50%. The contribution rates of human activities ranged between 44.52% and 95.88% (
Figure 11). In 11 sub-basins, human activities contributed more than 50%, which means that in these regions, the effect of anthropogenic factors on vegetation cover change areas was greater than that of climate change. Among them, the contribution rate of human activities in the Han River Basin, Taihu Lake Basin, and the middle reaches of the main stream area exceeded 90%. The contribution of climatic factors to the change of vegetation cover in this region was relatively low, and the change of vegetation cover was mainly affected by human activities.
To better spatially characterize the effect of anthropogenic factors on vegetation change, the spatial configuration of the effects of anthropogenic activities was determined using the Getis-Ord G* statistics. The result suggests that the contributions of anthropogenic factors can be characterized as being “hot in the east and cold in the west” in the Yangtze River Basin (
Figure 12). The cold spots and hot spots were found to be closely related to human activities and climate. The hot spot regions accounted for 55.59% of the total area and it mainly concentrated in the Poyang Lake Basin, Han River Basin, Dongting Lake Basin, mainstream area of midstream, Wujiang River Basin, Jialing River Basin, and the Yunnan–Guizhou Plateau in downstream Jinsha River. These regions have a variety of vegetation types and high vegetation cover and are the main areas for environmental management projects (e.g., conversion of farmlands into forests). The cold spots accounted for 25.48% and are mainly distributed in the Jinsha River Basin in the west of the Yangtze River Basin. Cold spot zones are mostly in high altitude areas, which are mainly impacted by climate factors and limited anthropogenic factors.
Based on the change in land use, the impact of human activities on vegetation cover change was discussed by a lot of scholars. With the intensification of human activities, the significant change in land use has profoundly changed the vegetation coverage. As shown in
Table 8, the area of land type change in the Yangtze River Basin from 2000 to 2018 was about 143,978.67 km
2, accounting for about 3.64% of the area of the Yangtze River Basin. The change in land use mainly includes construction land, cultivated land, and forest land, and it is a process of conversion from cultivated land to construction land and forest land. Among them, the largest area was transferred from cultivated land, with a total of 48,778.15 km
2, which was mainly converted into forest land (40.89%) and construction land (33.67%). Woodland was the largest area, with a total of 46,498.05 km
2, mainly from grassland (53.98%) and arable land (42.90%).
We know that land use change has obvious positive and negative effects on vegetation change. On the one hand, the urban development and construction in the basin has destroyed the vegetation coverage. On the other hand, ecological construction projects have improved the vegetation coverage. As can be seen from
Figure 13, the transfer of land use types in the regions with negative residual change are mainly from cultivated land to construction land and from unused land to grassland, with an area of 6953.4 and 3971.25 km
2 respectively. In the region with positive residual change, 19,792.73 km
2 of farmland was converted to forest land, 17,549.23 km
2 of forest land was converted to farmland, 13,609.27 km
2 of forest land was converted to grassland, and 24,136.73 km
2 of grassland was converted to forest land. In addition, human urban development and construction activities have exacerbated vegetation degradation, mainly in urban agglomerations of major river basins (
Figure 14). Arable land and forest land are the main sources of construction land, accounting for 73.65% and 17.89% of the inflow area of construction land, respectively. Large-scale urbanization resulted in the conversion of agricultural land into construction land, which destroyed the surface vegetation around the city and significantly reduced the level of regional vegetation coverage. Therefore, ecological engineering, agricultural development and urban construction are the leading human factors of vegetation change in the Yangtze River Basin.