Some like it ‘cold’

It is well-known that trees in temperate climates use cold temperatures as a measure to predict the arrival of spring: only when a certain amount of cold (referred to as 'chilling') has been experienced do these trees begin to develop their buds under warmer (spring) conditions. This can be seen as a safety mechanism to avoid leaf-out after a warm period in the middle of winter when the risk of damaging frost events is still high. A new study by Frederik Baumgarten and colleagues from the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) quantified the temperature range of effective chilling for six common deciduous tree species. Using 1’170 twig cuttings placed in different climate chambers, the researchers found that temperatures up to 10°C contribute to release dormancy, which is higher than previously thought. However, some species, such as maple and lime, benefited most from freezing temperatures (e.g. -2°C), an important finding that can improve the prediction of spring phenology. To ensure that winter is over, temperate and boreal trees not only "count" the cold days, but some even weight the especially cold ones!