Provenance-specific chilling and forcing requirements regulate spring phenology of three European temperate tree species

Global warming alters spring phenology in temperate forests, with significant implications for tree vitality, growth, and ecological interactions. However, temperature requirements for dormancy release and budburst differ among populations adapted to different climatic condition, complicating predictions of spring phenology across broad geographic regions.

Here, we quantified 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 exposed to either ambient or warmed (+5 °C) open-top chambers and subsequently transferred at monthly intervals from November to February to a 20°C forcing chamber.

We found that reduced chilling (due to earlier transfer into warming conditions) substantially delayed budburst, with T. cordata showing the highest chilling requirement, followed by F. sylvatica, whereas Q. robur exhibited the lowest. Interestingly, 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. In contrast, F. sylvatica showed the reverse pattern with the Spanish provenance tending to budburst latest and the Polish one earliest. These differences likely reflect provenance-specific frost risks and resulting genetic differentiation in chilling and forcing requirements. Remarkably, insufficient chilling significantly reduced budburst success by 25–85 % across species. The effect was most pronounced in T. cordata, where success dropped below 10 % in saplings transferred in November or December, regardless of provenance. These findings underscore the critical role of winter chilling in regulating budburst and maintaining tree vitality, as well as provenance-specific adaptation, suggesting that species adapted to low winter chilling might be candidates for assisted migration under rapid climate change.