Background/Question/Methods The advancement of spring phenology in response to warmer temperatures during the last decades is one of the most prominent biological indicators of climate change. However, this spring advancement is slowing down. One often discussed reason is the lack of chilling during the nongrowing season. Another reason might be a delayed induction of endodormancy in tree buds due to warmer autumn temperatures, leading to a shift of the dormant phase. We experimentally assessed the effect of a strong winter warming manipulation by ca. 4.5 °C on spring phenology at two common temperate trees species, early flushing Betula pubescence and late flushing Fagus sylvatica. Further we estimated the induction and progression of bud dormancy by repeated measurement of dormancy depth in seedlings. For dormancy depth assessment we quantifying the days needed for buds to burst under favorable conditions under both temperature regimes.
Results/Conclusions Strong winter warming caused a comparatively low advancement of spring leaf unfolding, and surprisingly at the earlier stage of bud burst there was no significant advancement at all. As a reason we found a clear and substantial delay of endodormancy induction under warming in autumn This delay remained unexpectedly long and only shortly before flushing it was counterbalanced by an increased rate of dormancy release under warmer temperatures. A detailed discussion of the dormancy depth progression will be given. Both species showed a lower percentage of bud burst after warming, indicating incompletely fulfillment of chilling requirements due to shortening of the nongrowing season. We conclude that under continued climate warming the advancement of spring leaf phenology is very likely to cease. The temperature in autumn has a particular relevance here, which has so far been overseen and offers new perspectives for phenological modeling.
