The mass elevation effect of the Tibetan Plateau and its implications for Alpine treelines

The immense and towering Tibetan Plateau (TP) acts as a heating source and shapes the climate of not only the Eurasian continent but also the entire world. The mass elevation effect of the TP was first observed in the 1950s; however, due to the scarcity of meteorological observation stations and limited climatic data, little information on the mass elevation effect of the plateau and its implications for the position of Alpine treelines in the southeastern part of the TP is quantitatively known. This paper compares monthly mean air temperature differences at elevations of 4000, 4500, 5000, 5500 and 6000m between the main plateau, the Qilian Mts. in the northeastern corner of the plateau and the Sichuan Basin to the east of the plateau to quantify the mass elevation effect of the plateau. The TP air temperature data are retrieved from Terra moderate-resolution imaging spectroradiometer (MODIS) land surface temperature (LST), and the free-air temperatures over the westernmost Sichuan Basin are estimated using the measured lapse rate from Mt. Emei, which is located in the western portion of the Sichuan Basin. The results demonstrate the following important characteristics. (1) Owing to the mass elevation effect, air temperatures gradually increase from the eastern edge to the interior main TP. The monthly mean air temperature in the interior main plateau is approximately 2-7 degrees C higher than in the surrounding mountains and adjacent lowland areas. At an elevation of 4500m (corresponding to the mean altitude of the TP), the monthly mean temperature differences between the plateau and the Sichuan Basin range from 3.58 degrees C (April) to 6.63 degrees C (June); the monthly temperature differences between the plateau and the Qilian Mts. range from 1.6 degrees C (July) to 7.7 degrees C (March). (2) The mass elevation effect of the plateau pushes the 10 degrees C isotherm upward in the warmest month and is indicative of a warmth index of 15 degrees C month up to elevations of 4600-4700 m, which enables the treeline altitude in the interior TP 500-1000m higher than along the eastern edge. Therefore, mass elevation effect contributes to the occurrence of the highest treeline in the Northern Hemisphere, which is present on the southeastern TP.