The advancement of spring phenology in response to warmer temperatures during the last decades is one of the most prominent biological indicators of climate chance. However, this spring advancement is slowing down, possibly due to a lack of enough chilling temperatures during winter. Deciduous trees are in a (endo-) dormant state in winter, which prevents them from premature bud burst during the first warm days of the season, when the risk of late frost is still high. Cool temperatures in winter are necessary to release this dormancy, so that the trees become sensitive to warm temperatures and finally open their buds.
We investigated the effect of autumn and winter warming on the development of the dormancy state and on spring leaf phenology in seedlings of four common deciduous tree species (Fagus sylvatica, Quercus robur, Betula pendula, Alnus glutinosa). We applied a set of monthly warming manipulations of 2°C over 4 weeks, respectively, from September until April. We further tested the effect of winter temperatures at a range from -6 to 12 °C on chilling and dormancy release.
Results/Conclusions
Our results show that the effect of warming on spring phenology strongly depends on the time of the year at which it occurs. While warming in spring caused an overall earlier leaf out, we found a surprisingly high delaying effect of autumn warming in all species. For every 1°C warming in October, trees flushed in average 2.4 days later in the following spring. On the other hand, directly after autumn warming, dormancy depth was lower in those plants that had experienced warmer temperature. Thus, we conclude that warmer temperatures in autumn delay dormancy induction, hereby delaying the whole process of dormancy development, which eventually lead to a later spring phenology in spring. In contrast, the influence of warming in winter appears to be relatively low. Our study highlights the importance of autumn temperature for plant phenology and the growing season length.