Time-lag effects of climatic change and drought on vegetation dynamics in an alpine river basin of the Tibet Plateau, China

Vegetation is the most important component of terrestrial ecosystem, reflecting the quality of ecological environment. In this study, non-parametric Mann-Kendall test and Sen's slope were applied to analyze the spatiotemporal variabilities of vegetation coverage, precipitation, temperature and drought during the period 1981-2015 in the Yarlung Zangbo River basin (YZRB), based on GIMMS NDVI3g, CHIRPSv2.0, China Meteorological Forcing Dataset (CMFD) and CRU TS 4.03 (scPDSI) datasets. The Pearson Correlation Analysis and geostatistical methods were adopted to investigate the correlations between climatic variables and vegetation coverage as well as the time-lag effects for different vegetation types. The results showed that the spatial distribution of vegetation coverage is generally characterized as increasing from upper to lower reaches. The northwestern region of lower reaches showed a greening trend in vegetation, while degradation of vegetation was detected in southeastern region. The vegetation in the upper and middle reaches was significantly positively correlated with precipitation and drought, while a significantly negative correlation was detected between vegetation and temperature in the middle reaches and southeast region of the lower reaches, as well as vegetation and precipitation in the lower reaches. One-month lag effect of precipitation and temperature on vegetation appeared in the upper reaches, while no lag for precipitation and one-month lag for temperature were detected in the middle reaches, and both precipitation and temperature showed no lag effect on vegetation in southeast region of the lower reaches. The vegetation types in the YZRB were mainly consisted of meadow, alpine vegetation, shrubs and grassland, in which the growth of shrubs was relatively better than other vegetation types. All the four vegetation types showed a slightly greening trend, which were positively correlated with precipitation and drought, while showed a negative correlation with temperature. The findings help to better understand the mechanism of vegetation dynamics and provide references for ecosystem monitoring and assessment in the YZRB.