Spatial and temporal variations in the end date of the vegetation growing season throughout the Qinghai–Tibetan Plateau from 1982 to 2011

Abstract

The spatial and temporal variations in the end date of the vegetation growing season (EGS) and their relationships with climate factors across the Qinghai–Tibetan Plateau yet have not been well researched. In this study, we used the rate of the change in the curvature of the S-curve function which integrated a logistic function and an asymmetric Gaussian function and showed a better performance for fitting the LAI (leaf area index) data to extract the EGS from a long-term time series of AVHRR (advanced very high resolution radiometer) LAI data. The spatial distribution pattern of the EGS averaged from 1982 to 2011 presented a gradual decrease from the southeast to northwest plateau. The various vegetation types showed different phenological EGS timing. The EGS occurred earlier with increasing altitude (slope=−3daykm−1, p<0.001). Throughout the entire Qinghai–Tibetan Plateau, the monthly air temperature and precipitation were positively correlated with the EGS, whereas the monthly sunshine duration showed a negative correlation. At the regional scale, a less pronounced increasing EGS trend (shifting about 1 day over 24 years, p=0.084) was observed during the entire study period. By analyzing the trend turning points, we found that the EGS occurred later during 1982–1994 (slope=0.155dayyr−1, p=0.045) and 1999–2011 (slope=0.096dayyr−1, p=0.3), but occurred earlier during 1994–1999 (slope=−0.373dayyr−1, p=0.049). During 1982–2011, the annual changes of EGS negatively correlated with precipitation (p<0.1) in June, but positively with precipitation (p<0.1) in August. As the same time, the annual changes of EGS positively correlated with sunshine duration (p<0.1) in June, yet negatively with sunshine duration (p<0.1) in August. During 1982–1994, the annual changes of EGS positively correlated with air temperature (p<0.01) and negatively with precipitation (p<0.1) in June. During 1994–1999, the annual changes of EGS only negatively correlated with air temperature (p<0.05) in August. During 1999–2011, the annual changes of EGS only negatively correlated with sunshine duration (p<0.1) in August.