Granger Causality Analysis of Grass Response to Climatic Changes Over Tibetan Plateau

Abstract

Due to the complex natural environment, vegetation on the Tibetan Plateau (TP) has a sensitive response to climatic changes. Thus, it is of great importance to explore the cause effect of climatic shifts on vegetation. Based on the long-term satellite NDVI dataset during 1982–2012, we analysed the causal relationship of vegetation greenness with temperature and precipitation by using the Granger causality test at monthly and seasonal temporal scale for each pixel. The results show that (1) the proportion of pixels with stationary time series for NDVI vs. temperature and NDVI vs. precipitation is greater at month scale than at seasonal scale, which is 99% and 98% at monthly scale, 64% and 71% at seasonal scale, respectively. (2) At month scale, the lagging time period of the average temperature and total precipitation on NDVI is around 12–13 months at monthly scale and show similar temporal profile across various vegetation types. At seasonal scale, the lagging time period is mainly occurred in 3, 4 and 6 quarter and very different in desert steppe, steppe and meadow. (3) For 98% area of the TP, average temperature change is found to granger cause of NDVI. While for 89% of the TP, except for the south-eastern TP, NDVI is supposed to granger cause of average temperature change at month scale. At seasonal scale, average temperature change is granger cause of changes in NDVI approximately accounting for 92% area of TP, where the central part of TP is excluded. However, in the eastern and western TP (about 50% of TP), NDVI is interpreted as granger cause of average temperature changes. (4) In the north-eastern and north western parts (about 98% of TP), precipitation is the granger cause of NDVI changes. While for 94% of the plateau, except for a few areas in the southeastern TP, NDVI is supposed to the granger cause of precipitation change at month scale. The precipitation change is considered as granger cause of NDVI in the south-eastern part of the TP (approximate 61% TP) at seasonal scale. In the central and eastern (about 48% of the plateau), NDVI is granger cause of precipitation change. Overall, climate factors have an interactive relationship with vegetation changes. Climatic factors and vegetation greenness can compose a Grainger causality relationship to each other, but climatic factors have stronger Grainger cause effect on vegetation than vegetation’s Grainger effect on climatic factors. There is more Granger cause effect region at month scale than seasonal scale over the TP.