Vegetation phenology change and response to temperature in the Qilian Mountains from 1982 to 2014

The Qilian Mountains in the northeast of Qinghai Tibet Plateau are endowed with diverse vegetation types and significant climate change．Understanding vegetation phenology change in Qilian Mountains is of great significance for ecology and development of animal husbandry industry．GIMMS NDVI data and satellite-derived method were used to extract phenology indicators of Qilian from 1982 to 2014: start of the growing season (SOS), end of the growing season (EOS) and length of the growing season (LGS)．The phenological characteristics and their responses to temperature were then examined in different vegetation type regions using Theil-Sen slope, Mann-Kendall test and partial least-square method (PLS)．It was found that the vegetation types in Qilian Mountains from the southeast to the northwest transitioned gradually from shrubs to meadows and grasslands and then to alpine vegetation; SOS was delayed, EOS advanced, LGS shortened gradually．Meadow and shrub were found to be the vegetation types with the most drastic phenology change．From 1982 to 2014, SOS of meadow and shrub was significantly advanced (0.19 days per decade, 0.25 d·(10 a)⁻¹), EOS delayed (0.20, 0.21 d·(10 a)⁻¹), LGS extended (0.39, 0.46 d·(10 a)⁻¹), all significantly．For the whole Qilian Mountains and major vegetation types, the SOS had a turning point from 1997 to 2000, turning points of EOS and LGS both occurred in 2003．After these turning points, phenology trends became insignificant．Temperature at the end of autumn and beginning of winter was found to have a positive effect on SOS in meadows and shrubs, temperature in spring before the phenology date had a significant negative effect on SOS．Temperature in summer and autumn had significant positive effect on EOS．The effect of maximum temperature on phenology was found to be significantly stronger than minimum temperature; in the individual months in spring and summer, effects of maximum and minimum temperature on phenology showed opposite directions．Changes in vegetation phenology will help to remind local herdsmen to adjust their forage planting and grazing．