Spatial-temporal variations in the thermal growing degree-days and season under climate warming in China during 1960-2011

Vegetation growth and phenology are largely regulated by base temperature (T-b) and thermal accumulation. Hence, the growing degree-days (GDD) and growing season (GS) calculated based on T-b have primary effects on terrestrial ecosystems, and could be changed by the significant warming during the last century. By choosing 0, 5, and 10 degrees C, three key T-b for vegetation growth, the GDD and GS in China during 1960-2011 were developed based on 536 meteorological stations with homogenized daily mean temperatures. Results show that both the GDD and GS showed positive sensitivity to the annual mean temperature. The start of the growing season (SOS) has advanced by 4.86-6.71days, and the end of the growing season (EOS) has been delayed by 4.32-6.19days, lengthening the GS by 10.76-11.02days in China as a whole during 1960-2011, depending on the T-b chosen. Consistently, the GDD has totally increased 218.92-339.40 degrees Cdays during the 52years, with trends more pronounced in those based on a lower T-b. The GDD increase was significant (Mann-Kendall test, p<0.01) over China except for the north of Southwest China, while the significant GS extension only scattered over China. Whereas the extensions of GS(0) and GS(5) were dominated by the advance in SOS, the GS(10) extension was closely linked to the delay in EOS. Regionally, the GS extension in the eastern monsoon zone and northwest arid/semi-arid zone was driven by the advance in SOS and delay in EOS, respectively. Moreover, each variation has a substantial acceleration mostly in 1987 or 1996, and a speed reduction or even a trend reversal in the early 2000s. Changes in the thermal growing degree-days and season are expected to have great implications for biological phenology, agricultural production, and terrestrial carbon cycle in the future.