Chilling accumulation and photoperiod regulate rest break and bud burst in five subtropical tree species

The environmental regulation of spring phenology in boreal and temperate trees is generally well-understood, but little is known about the regulation in subtropical trees. It has been shown recently that similarly to the more northern trees, subtropical trees also exhibit rest (endodormancy) and chilling requirement of rest break (chilling requirement of endodormancy release), but the effects of photoperiod remain largely unexplored. Here we did an experimental study of the effects of chilling accumulation and photoperiod on the occurrence (bud burst percentage, BB%) and timing (days to bud burst, DBB) of bud burst in five subtropical tree species growing commonly in subtropical China. In all of the five species examined, both chilling accumulation and photoperiod showed a significant effect on DBB, and several significant effects were found for BB%. The responses to chilling accumulation and photoperiod we found are thought to be adaptive to the conditions of relatively short and warm subtropical winters: first, an independent effect of photoperiod would reduce the risk of frost damage caused by a premature bud burst in the case of false springs, which are especially common in subtropical conditions. Second, an interaction of photoperiod with chilling accumulation would facilitate a timely bud burst in spring after an exceptionally warm winter with reduced chilling accumulation. On the basis of our findings, we put forward a conceptual model for the various effects of chilling accumulation and photoperiod on rest break and bud burst in subtropical trees. The model facilitates future efforts towards developing process-based spring phenology models for subtropical tree species. Our limited but novel results show that 1) the modelling needs to address the effects of photoperiod; 2) because of the large differences found in the responses among the five species examined, the model development needs to be based on species-specific experimental data.