Contrasting drought impacts on the start of phenological growing season in Northern China during 1982-2015

Drought is a widely occurring extreme climatic event that may have various effects on vegetation phenology and activity. The change of the start of the phenological growing season (SOS) is one key mechanism for ecosystem responses to droughts yet remain unknown at large scale. This study used abnormal changes in the ratio of reference evapotranspiration (ETO) and precipitation (P) to detect the pre-season (defined as the 3 months before the growing season) monthly drought during 1982-2015. After that, the pre-season drought impacts on SOS were analysed. Drought appeared frequently (>3.6 times/10 years) in the northern and southwest China, as well as the coastal area of south China during the last 34 years. The droughts generally accompany a higher temperature and a stronger radiation, but the drought effects on the SOS differ between humid/sub-humid and semi-arid zones in northern China. Specifically, in the temperate humid/sub-humid zones, SOS under drought was 1.08-4.86 days earlier than during normal (no drought) years, and the response was greater to the droughts that appeared in the first two of the three pre-season months. In addition, in the cold, mid-, and warm temperate zones, the advance response was more concentrated in the SOS about DOY130-140, DOY125-150, and DOY80-160, respectively. However, in the north semi-arid zone, the SOS under drought was 0.66-3.45 days later than during normal years, and the response was greater to the droughts that appeared in the last two of the 3 months. These delay responses also concentrated in the SOS about DOY125-150. The contrasting drought impacts on phenology suggest that we should move beyond the vegetation activity under droughts such as the growth, productivity, and mortality, and pay more attention to the timing of vegetation activity to better quantify the carbon budget at Northern Hemisphere.