As the planet has warmed, a commonly observed phenomenon has been that the leaves of deciduous trees have been emerging earlier every spring. However, this effect has become less pronounced since the 1980s, according to a release from Technical University of Munich.
Scientists believe the effect may have become less obvious due to warmer winters, which influence the depth of chilling the trees need to become dormant. Less chilling seems to have meant a reduction in the speed at which leaves emerge, though they do still appear earlier than before, scientists say.
An international team of researchers working with an extensive European phenological database was able to determine that tree leaves emerged about four days earlier per rise in 1 degree Celsius for each year from 1980 to 1994, but only 2.3 days earlier per degree rise from 1999 to 2013. The long-term effects of the decrease in sensitivity are not yet known.