Effects of winter chilling and photoperiod on leaf-out and flowering in a subtropical evergreen broadleaved forest in China

Accurate forecasting of spring plant phenology is critical for understanding structural and functional changes in plant communities and their ecosystems in light of global warming. To date, the relative importance of winter chilling and photoperiod for leaf-out and flowering phenology remains unknown in subtropical and tropical forests. Here, we established a climate chamber experiment to test the effects of photoperiod and chilling on the timing of leaf-out and flowering for 37 subtropical woody species using dormant twigs. We found that most cuttings reached leaf-out, but only a few cuttings reached flowering. Longer chilling (including both moderate and strong chilling treatments) may improve leaf-out success and advance leaf-out timing, but it showed little effect on flowering. Moderate chilling was enough for the initiation of leaf-out. Photoperiod showed little effect on both leaf-out and flowering. To our knowledge, our study is the first to explore how different winter chilling conditions may lead to tree phenology progressing in various subtropical species in a new order and to show that leaf-out order is largely unpredictable when chilling requirements are not met. Our experiment indicates that dormant twigs of subtropical woody species can be successfully applied to leaf-out rather than flowering. Our results suggest that lack of chilling not only led to a considerable delay in leaf-out for subtropical species, but it also caused substantial changes in the order of species leaf-out. Thus, warmer winter temperatures might impact subtropical forest ecosystems more than formerly believed.