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Key messageBud break in European beech is generally known to be under strong photoperiodic control, but our data indicate that bud break in the northernmost populations is temperature limited.AbstractEuropean beech (Fagus sylvatica) is among the dominant tree species in Europe, and is expected to increase its distribution northwards due to climate change. Although spring bud break in European beech is known to be under strong photoperiodic control, studies of populations from high elevations suggest a greater role of temperature in colder environments. This indicates that warming could yield advanced bud break for populations at high elevation and at high latitudes. Using live material collected in winter, spring and autumn from wild populations in Norway, representing the northernmost distribution range of European beech, we tested frost tolerance and effects of temperature and day length on bud break in controlled conditions. Furthermore, we observed bud break during spring in field conditions. Bud break in controlled conditions was faster with later sampling dates and higher temperatures, but showed little or no activity under short photoperiod. Field observations revealed earlier bud break in warmer sites, and remarkably, that bud break dates have advanced by more than 10days in less than three decades. Frost tolerance tests showed a gradual decrease in bud intactness from January to March, and a gradual increase from October to December. Our data indicate that bud break dates in the northernmost European beech populations are limited by temperature, similar to observations of populations at high elevation in Central Europe. Further warming can thus be expected to yield earlier bud break at the species' northernmost distribution range, possibly increasing the susceptibility of shoot tips to late spring frost events.