The modest influence of internal factors on autumn phenology compared to climate factors

Autumn phenology plays a vital role in regulating the length of growing season and carbon uptake in terrestrial ecosystems. Under global climate change, delayed autumn phenology has been widely reported, but as to the potential drivers and mechanisms, it is are still unclear, especially the impact magnitude and direction of internal factors. In this study, based on satellite-derived the end of growing season (EOS) and photosynthetic data over the Northern Hemisphere (>30° N) from 1982 to 2014, we comprehensively investigated the impacts of developmental and environmental factors on EOS and compared their relative effects across different climate zones and vegetation types. We found the magnitude of EOS shift was highly heterogeneous across climate zones and vegetation types. It delayed more in humid regions than in drought regions. The response of EOS to temperature and precipitation in warm-dry areas was opposite to that in other areas. Spring phenology had an important legacy effect on EOS. But no direct sink limitation effect of growing season photosynthesis on autumn phenology was detected in any climate zone or vegetation type, as reported in some other field observations. While internal factors played a certain role in modulating EOS, their total impact was lower than the influence of temperature and precipitation in most regions and vegetation types. Our findings emphasize the complexity of factors influencing EOS, and call for elaborate investigation on the driving mechanisms of autumn phenology at different scales and under different climate backgrounds.