Spatiotemporal vegetation cover variations associated with climate change and ecological restoration in the Loess Plateau

Abstract

Vegetation is an important component of terrestrial ecosystems, and it plays an important role in preventing desertification and conserving soil and water in arid and semi-arid regions. This study examined the spatial distribution and temporal variations of vegetation cover in the Loess Plateau over the last three decades using a time series of the normalized difference vegetation index (NDVI) derived from the long-term data record (LTDR) and the moderate resolution imaging spectroradiometer (MODIS) between 1981 and 2010, along with daily climate datasets. Furthermore, we assessed the relationships between the vegetation, climate variables (i.e., temperature and precipitation), and human activities, particularly the “Grain to Green” restoration program. The greenness of the vegetation cover in the Loess Plateau exhibited a significant increase from the northwest to the southeast. Generally, the NDVI exhibited an increase of 0.0015 year−1 (P<0.01) from 1981 to 2010 over the entire plateau, and the rate was particularly high over the last decade. The NDVI increases with a rate of more than 0.01 year−1 during 1991–2000 were distributed throughout the Ziwuling and Huanglongshan Forests in Yan’an, Shaanxi Province, and in the Lvliang and Taihang Mountains, Shanxi Province. However, the distinct feature of the NDVI, namely, its increase of more than 0.01 year−1 in 2001–2010, was associated with key areas of ecological restoration in Yulin and Yan’an in Shaanxi Province. The vegetation cover was largely improved within the last decade due to the “Grain to Green” program. However, revegetation still has great potential in the Huangfuchuan, Kuyehe, Wudinghe, and Yanhe watersheds in the future. In response to global warming, the phenology timing in the Loess Plateau was 3.9 days (1991–2000) and 6.6 days (2001–2010) earlier in spring compared with 1981–1990, and the end of the fall growing season was delayed 3.6 days and 9.6 days, respectively, over the past three decades. The significant and positive partial correlations (R>0.5, P<0.05) between the temperature and the NDVI occurred in the central and southeastern parts of the plateau; this finding indicates that higher temperatures promote the growth of vegetation in less water-stressed areas. However, a negative impact of vegetation degradation due to climate warming was found in the northwestern Loess Plateau. The permafrost slowly degraded in the northwestern Loess Plateau, causing a decrease in the water content of the topsoil, eventually leading to the adverse effects on vegetation.