Leaf-out in northern ecotypes of wide-ranging trees requires less spring warming, enhancing the risk of spring frost damage at cold range limits

Aim: Trees need to avoid frost damage to their young leaves by leafing out after the occurrence of the last frost, yet they also need to start photosynthesis early in the season to achieve sufficient growth. This trade-off leads to the hypothesis that ‘safety margins’ against spring frost should become shorter, the longer the winter duration, perhaps reaching an asymptotic limit where frost damage would occur in most years. Physiologically, shorter safety margins in high-latitude ecotypes might be achieved by lower degree-day requirements for leaf-out, compared to low-latitude ecotypes. Location: Europe. Time period: 1902–2009. Major taxa studied: Temperate trees. Methods: Using herbarium collections of Acer platanoides, Carpinus betulus, Fagus sylvatica and Prunus spinosa made over 108 years at 40° to 60° N latitude, we related historic leaf-out dates to winter and spring temperatures (chilling and degree-days), winter duration, and date of last frost occurrence in the relevant years and locations. Results: In all species, frost safety margins decreased towards high-latitude regions with long winters, with each day increase in winter duration reducing frost safety margins by 0.48 days in Fagus and 0.32–0.21 days in Prunus, Acer and Carpinus. These latitudinal differences correlate with northern ecotypes’ shorter degree-day requirements for leaf-out. Main conclusions: The decline in spring frost safety margins in regions with long winters supports the new hypothesis that species may reach their geographic range limit where they ‘bump up’ against experiencing regular frost injury to their young leaves. Larger datasets are necessary to further corroborate our hypothesis and future efforts should thus be directed toward increasing the latitudinal range of existing phenological databases.