Chilled to be forced: the best dose to wake up buds from winter dormancy.

Over the last decades, spring leaf-out of temperate and boreal trees has substantially advanced in response to global warming, affecting terrestrial biogeochemical fluxes and Earth's climate system. However, it remains unclear whether leaf-out will continue to advance with further warming because species' effective chilling temperatures as well as the amount of chilling-time required to break dormancy are still largely unknown for most forest tree species. Here, we assessed the progress of winter dormancy and quantified the efficiency of different chilling temperatures in six dominant temperate European tree species by exposing 1'170 twig cuttings to a range of temperatures from -2° to 10°C for 1, 3, 6, and 12 weeks. We found that freezing temperatures were most effective for half of the species or as effective as chilling temperatures up to 10°C, i.e. leading to minimum thermal time to and maximum success of budburst. Interestingly, chilling duration had a much larger effect on dormancy release than absolute chilling temperature. Our experimental results challenge the common assumption that optimal chilling temperatures range around 4-6°C, instead revealing strong sensitivity to a large range of temperatures. These findings could be valuable for improving phenological models and predicting future spring phenology in a warming world.