Asymmetric effects of daytime and nighttime warming on spring phenology in the temperate grasslands of China

Understanding the spring phenology of temperate grasslands and its response to climate change are crucial for diagnosing the responses of ecosystem to regional climate change and projecting regional ecosystem carbon exchange. Using NDVI data from 1982 to 2015, this study investigated the changes of the start date of the vegetation growing season (SOS) for the temperate grasslands of China, and explored the possible effects of average monthly Tmax, Tmin and total precipitation on the SOS across different grassland vegetation types. We improved on prior studies of climate change in China's temperate zone by focusing only on grasslands unchanged over this extended study period. The results showed that the SOS significantly advanced at a rate of 1.84 days/decade from 1982 to 2015, controlled mainly by spring precipitation and spring and winter temperatures, with differing degrees of influence among vegetation types. On average, an increase of 10 mm in spring precipitation would advance SOS by 1.7 days across the temperate grasslands of China. Although warmer temperatures generally led to advanced SOS, this study revealed for the first time that the seasonal effects of daytime and nighttime warming on the SOS of temperate grasslands in China were asymmetric, with Tmax more influential in winter and Tmin more influential in spring. An increase of 1 °C in winter Tmax and spring Tmin would advance SOS by 0.42 and 1.34 days, respectively, compared with effects of 0.24 or 0.64 days for 1 °C increases in winter Tmin or spring Tmax. Given the global asymmetry in daytime and nighttime warming, this study highlights the asymmetric effects of daytime and nighttime warming on the SOS of temperate grasslands in China, and suggests that the impacts of seasonal Tmax and Tmin should be considered separately in the SOS modules of terrestrial ecosystem models for temperate grasslands.