Abstract:

The Three-Rivers Headwater Region (TRHR) refers to the source area of the Yangtze River, Yellow River and Lancang River, an important water resource. Vegetation activities in this region are closely related to water and ecological security in China. As a critical component of terrestrial ecosystems, vegetation has become one of the important issues in global climate change. Numerous case studies have been conducted on vegetation dynamic trends over relatively short study periods however, few long-term studies from the 1980s to recent years have been completed because the normalized difference vegetation index (NDVI) datasets limited. Additionally there are few quantitative analyses of the consistency of vegetation trends over time. In this study, the latest GIMMS NDVI3g datasets from 1982 to 2012 were used to identify any spatiotemporal patterns in vegetation changes over the growing season and seasonal NDVI at regional, biome and pixel scales in the TRHR. To explore the change process linear regression over progressively longer periods from the initial year of the study was applied to detect vegetation growth changes in TRHR. At the regional scale, although a positive trend of growing season and seasonal NDVI were found during past three decades, there were two distinct periods with opposite trends in spring NDVI, a significant increase and a significant decrease before and after 1998, respectively. As the study period increases, the rates of NDVI increase in the spring and summer growing season markedly reduced, but in autumn the NDVI increased. However, significant NDVI increases were found in most seasons. The season and period with significant trend in NDVI differed among vegetation types. Significant increases in NDVI were observed in spring during the first few periods for coniferous forest and shrub and in spring and summer during the first few periods for desert and meadow areas. The difference in persistence of NDVI trends caused variation in the contribution of seasonal vegetation activity to plant growth. During the first few periods, the largest NDVI increase and the largest contribution were generally found in spring and in summer, respectively. During the last few periods, both the largest NDVI increase and contribution were found in autumn. It can be inferred that increases in NDVI during the growing season was mainly due to both the earlier start of vegetation growth and accelerated vegetation activity during previous periods and from both the accelerated vegetation growth and a delay in the end of the growing season during latter periods.