Many recent studies indicated that the regional climate of northwest China had changed from a warm-dry to a warm-wet pattern. Therefore, it is important to assess vegetation cover change in this area. Our objective is to evaluate the normalized difference vegetation index (NDVI) record in eastern, middle and western parts of the Qilian Mountains and assess past long-term vegetation cover change with tree-ring chronologies.Temporal and spatial changes of NDVI, and its relationship with tree-ring width were analyzed using 8 km × 8 km resolution multi-temporal NOAA/AVHRR-NDVI data from 1986–2003 and five ring-width chronologies in the Qilian Mountains. Then, we used liner regression to reconstruct annual NDVI varieties for 1843–2003.Vegetation cover in the Qilian Mountains decreased from east to west, and the main growing season lasted from June to August. In addition, during the period 1986–2003, an increasing NDVI-trend was evident in three regions in the Qilian Mountains, indicating that vegetation activity has been rising recently. In the eastern, central and western parts of the Qilian Mountains, values of NDVI during the growing season have increased by 3.28%, 4.82% and 7.75% per year, respectively. We also found a strong relationship between NDVI and the first principal component (PC1) of the five tree-ring width chronologies (r = 0.74, p < 0.01). Growing season NDVI was reconstructed based on correlation analysis. Six periods characterized by high and low values were reflected in the NDVI curve, and the poorest period of plant growth was 1923–1932. Moreover, the amplitude of the fluctuation was larger during 1989–2003, accompanied by an increasing trend since 1991.
A, Variations of reconstructed, growing season (June–August) NDVI values during 1843–2003 in the Qilian Mountains by using tree ring width RES chronologies PC1. B, Averaged NDVI from June to September during 1843–2001 in Delingha. C, Palmer drought severy index (PDSI) during 1891–2003 in the Qilian Mountains. (Average of NDVI (line)
and 10 years low pass filtering (smoothed curve))
(Picture/Chinese Journal of Plant Ecology) |
Introduction
