Provenance-Specific Chilling and Forcing Requirements Shape Spring Phenology in Three European Temperate Tree Species

Global warming is altering spring phenology in temperate forests, with important consequences for tree survival, growth, and ecological interactions. However, temperature requirements for dormancy release and budburst vary among populations adapted to different climatic conditions, complicating predictions of spring phenology across broad geographic regions. Here, we quantified the chilling and forcing requirements of three deciduous tree species (Fagus sylvatica, Quercus robur, and Tilia cordata) using four provenances per species spanning a latitudinal gradient from Spain to Poland. Saplings were overwintered under either ambient or warmed open-top chambers and were transferred monthly from November to February to a climate chamber under constant forcing conditions. We found that reduced chilling due to earlier transfer substantially delayed budburst, with T. cordata showing the highest chilling requirement, followed by F. sylvatica, whereas Q. robur exhibited the lowest. We detected both co- and counter-gradient patterns of genetic variation in budburst timing. In Q. robur and, to a lower extent, in T. cordata, Polish provenances budburst later than Spanish ones, while German and Swiss populations were intermediate (co-gradient). In contrast, F. sylvatica showed the opposite pattern with the Spanish provenance tending to budburst latest and the Polish one earliest (counter-gradient). These differences likely reflect genetic differentiation in chilling and forcing requirements among provenances, likely driven by variation in frost risk at their sites of origin. Importantly, insufficient chilling significantly reduced budburst success by 25%-85% across species, with the strongest effect in T. cordata, where success fell below 10% in saplings transferred in November or December across all provenances, potentially constraining canopy development and impairing growth and reproduction. These findings underscore the critical role of winter chilling in regulating budburst timing and canopy development, as well as provenance-specific adaptation, suggesting that species adapted to low chilling might be candidates for assisted migration under rapid climate warming.