Species-specific budburst responses to diurnal temperature cycles during chilling and forcing in temperate deciduous...

Cold temperatures ('chilling') perceived by tree buds progressively release winter dormancy and initiate sensitivity to warmer temperatures ('forcing') required for budburst. Phenology of temperate trees may also respond to daylength, but it remains unclear whether chilling and forcing accumulation is linked to the diurnal cycle. Here, we tested whether chilling accumulation is influenced by light-dark cycle frequency, and whether forcing is more effective during daylight. We harvested twig cuttings from four temperate tree species (Fagus sylvatica, Carpinus betulus, Quercus robur and Acer pseudoplatanus) with contrasting chilling and forcing requirements. In Experiment 1, twigs were harvested in November and exposed to 3°C under three light-dark cycle frequencies: 12h light/12h dark, 6h/6h, and 4h/4h (1, 2 or 3 cycles/day) for one or two months (short vs. long chilling). Twigs were then transferred to two forcing regimes with the same daily mean but different day/night temperatures (25°C/10°C and 20°C/15°C). In Experiment 2, twigs were harvested after substantial chilling and exposed to opposite day/night forcing regimes (25°C/10°C vs. 10°C/25°C). In Experiment 1, shorter light-dark cycles under long chilling delayed budburst in C. betulus, whereas effects on budburst success depended on chilling duration in C. betulus and Q. robur. Interestingly, budburst occurred significantly earlier at 25°C/10°C than at 20°C/15°C in all species, up to 12 days in F. sylvatica, suggesting more effective daytime forcing or non-linear forcing accumulation. Experiment 2 demonstrated that forcing was more effective during daylight, with 10%, 19% and 51% more degree days required for budburst under warm nights in Q. robur, C. betulus and F. sylvatica, respectively, but no difference in A. pseudoplatanus. Our study shows that light-temperature interactions and non-linear forcing responses can shape budburst timing, implying that tree phenological responses to warming may depend not only on its magnitude but also on its timing within the diurnal cycle.