Late to bed, late to rise—Warmer autumn temperatures delay spring phenology by delaying dormancy

Spring phenology of temperate forest trees has advanced substantially over
the last decades due to climate warming, but this advancement is slowing
down despite continuous temperature rise. The decline in spring
advancement is often attributed to winter warming, which could reduce
chilling and thus delay dormancy release. However, mechanistic evidence of
a phenological response to warmer winter temperatures is missing. We aimed
to understand the contrasting effects of warming on plants leaf phenology
and to disentangle temperature effects during different seasons. With a
series of monthly experimental warming by ca. 2.4 °C from late summer
until spring, we quantified phenological responses of forest tree to
warming for each month separately, using seedlings of four common European
tree species. To reveal the underlying mechanism, we tracked the
development of dormancy depth under ambient conditions as well as directly
after each experimental warming. In addition, we quantified the
temperature response of leaf senescence. As expected, warmer spring
temperatures led to earlier leaf-out. The advancing effect of warming
started already in January and increased towards the time of flushing,
reaching 2.5 days/°C. Most interestingly, however, warming in October had
the opposite effect and delayed spring phenology by 2.4 days/°C on
average; despite six months between the warming and the flushing. The
switch between the delaying and advancing effect occurred already in
December. We conclude that not warmer winters but rather the shortening of
winter, i.e. warming in autumn, is a major reason for the decline in
spring phenology.