Relationship between extreme climate and vegetation in arid and semi-arid mountains in China: A case study of the Qilian Mountains

Understanding how extreme climates affect vegetation is of great importance for the protection of ecosystems. However, most of previous studies focused on the impact of average climate change on vegetation, and few on the effects of extreme climate on vegetation change. Such studies are particularly scare in arid and semiarid mountainous areas where vegetation is relatively homogenous and ecosystems are more vulnerable to climate change. In this study, we analyzed the vegetation response to extreme climate change in the Qilian Mountains, northeastern Tibetan Plateau, employing 11 extreme climate indices and the normalized difference vegetation index (NDVI), and further explored possible future trends in NDVI and extreme climate indices using detrended fluctuation analysis (DFA). The results indicated that NDVI in the Qilian Mountains showed an overall increasing trend from 1982 to 2015, with a rate of 0.004 per decade; 38.11% of the region showed a significant increasing trend and only 3.34% showed a significant decreasing trend. The vegetation areas with a significant increasing trend were dominated by grassland, cropland, and forest. The extreme climate indices accounted for approximately 29.8% of the change in NDVI, with the extreme precipitation intensity index (Rx1day) contributing the largest amount (17.1%), followed by the extreme temperature intensity index (TNx; 12.7%). Compared with extreme temperatures, the interannual variation in extreme precipitation has a greater impact on vegetation growth in the Qilian Mountains because moisture is the main climatic factor that limits vegetation growth in this region. There was a significant time-lagged effect of climate factors on vegetation growth in general, with a lag up to three months. The beginning of vegetation growing season was controlled by temperature-related extreme indices, but not directly by precipitation-related indices, and the cumulative effect of extreme precipitation and the time lag of NDVI's response appeared in the later part of the season. The DFA results showed that the extreme temperature intensity, extreme precipitation intensity, and the NDVI are expected to continuously increase in the future.