Opposite effects of winter day and night temperature changes on early phenophases

Changes in day (maximum temperature, T_MAX ) and night temperature (minimum temperature, T_MIN ) in the preseason (e.g., winter and spring) may have opposite effects on early phenophases (e.g., leafing and flowering) due to changing requirements of chilling accumulations (CAC) and heating accumulations (HAC), which could cause advance, delay or no change in early phenophases. However, their relative effects on phenology are largely unexplored, especially on the Tibetan Plateau. Here, observations were performed using a warming and cooling experiment in situ through reciprocal transplantation (2008-2010) on the Tibetan Plateau. We found that winter minimum temperature (T_MIN ) warming significantly delayed mean early phenophases by 8.60 d/°C, but winter maximum temperature (T_MAX ) warming advanced them by 12.06 d/°C across six common species. Thus, winter mean temperature warming resulted in a net advance of 3.46 d/°C in early phenophases. In contrast, winter T_MIN cooling, on average, significantly advanced early phenophases by 5.12 d/°C, but winter T_MAX cooling delayed them by 7.40 d/°C across six common species, resulting in a net delay of 2.28 d/°C for winter mean temperature cooling. The opposing effects of T_MAX and T_MIN warming on the early phenophases may be mainly caused by decreased CAC due to T_MIN warming (5.29 times greater than T_MAX ) and increased HAC due to T_MAX warming (3.25 times greater than T_MIN ), and similar processes apply to T_MAX and T_MIN cooling. Therefore, our study provides another insight into why some plant phenophases remain unchanged or delayed under climate change.