Impacts of recent climate extremes on spring phenology in arid-mountain ecosystems in China

Phenological responses of terrestrial ecosystems to climate extremes are of growing concern due to the increasing frequency and intensity of extreme climatic events associated with climate change which will in turn affect vegetation seasonality more than gradual changes. However, studies are rare in arid mountain regions where plant development is commonly regulated by both temperature and precipitation. To better understand how arid mountain (AM) ecosystems may respond to climate anomalies, we identified recent extreme climatic events (including intense warming, severe drought, and excessive wetness), and analyzed spring onset of vegetation growth in the Qilian Mountains of northwestern China. Phenological sensitivity was assessed from satellite-based data as departures from maps displaying mean onsets of growth for years 1983-2013. Our results revealed remarkable shifts in the start date of the growing season (SOS) under climate extremes, with different responses depending on ecosystem and altitude. Recent warming induced a general advancement of SOS. Higher spring temperatures enhanced the accumulation of heat needed for bud-burst and leaf expansion; elevated winter temperatures reduced the chilling days before bud dormancy release, significantly decreasing the risk of freezing injury in AM plants. Changes in SOS observed in this study suggested that AM plants may have relatively low chilling requirement for dormancy release. Contrary to warming, a drought resulted in a widespread delay in spring phenology, with sensitivity peaking over a shrubland ecosystem at medium elevations. This result demonstrated that subalpine shrubs were most susceptible of the studied ecosystems to hydroclimatic extremes, highlighting the great importance in this biome of concerns. Moreover, during the year when multiple extreme events (e.g. intense warming and heavy rainfall) coincided, their combined influence appeared to arise from synergistic mechanisms that are in urgent need of further research.