Exploring the control of diurnal cycles on chilling and forcing accumulation in tree bud dormancy release

Cold temperatures (known as ‘chilling’) are perceived by tree buds in winter and are responsible for dormancy release after species-specific exposure times, which marks the start of the buds' sensitivity to warmer temperatures (known as ‘forcing’). Temperate trees are also sensitive to changing daylength, but it remains unresolved whether the accumulation of chilling and forcing is related to the diurnal cycle. This study explores whether trees "count" chilling based on night/day cycles rather than purely through temperature accumulation or exposition, and whether forcing temperatures are more effective during daylight.

We harvested twigs from four temperate tree species with contrasting chilling and forcing requirements for dormancy release in late November 2024 , i.e. before they would experience significant periods of cold. Twig cuttings were then placed into transparent boxes filled with water and kept in climate chambers at 2°C/4°C (night/day) under three diurnal cycles: 12h/12h, 6h/6h (2 cycles per day), and 4h/4h (3 cycles per day) for one month (short chilling) or two months (long chilling). After these six treatments, all cuttings were transferred to forcing conditions with 12h daylight under two temperature regimes: 10°C/25°C and 15°C/20°C, i.e. with the same mean temperature but warmer or colder temperature during daytime. The timing and success of bud break were visually monitored twice a week.The experiment is ongoing. We hypothesize that chilling accumulation is influenced by diurnal cycles, with faster dormancy release for twigs exposed to shorter diurnal cycles. Additionally, we anticipate that daytime temperatures play a more significant role in forcing accumulation, leading to faster budburst in the 10°C/25°C treatment compared to the 15°C/20°C, especially for species known to be photoperiodic sensitive such as European beech. 

Our study will provide insights into how trees perceive and respond to temperature in relation to daylight, which is crucial for understanding and predicting phenological responses accurately in the context of climate change.