With global warming, flowering and leafing of trees is occurring earlier and earlier in all temperate regions of the world. Humans have been observing these natural events for centuries, heralding the arrival of warm weather after sometimes harsh winters.
A team of international researchers has recently presented the five world's longest series, some of them comprising several centuries of observations. The researchers draw attention to the rapidity with which the start of vegetation advances over the years, which reflects the acceleration of global warming observed from the 1950s onwards and which intensified again in the 1980s.
Remarkably, two of the five time series originate from Switzerland: the Grand Conseil of the Republic and Canton of Geneva has observed the first bud break of a horse chestnut tree (Marronnier de la Treille) every year since 1808, and the Landwirtschaftliches Zentrum Ebenrain and MeteoSwiss have observed the flowering of a wild cherry tree in the Canton of Basel-Landschaft since 1894. Other series include the cherry blossom in Kyoto, Japan, a culturally significant event whose onset has been recorded since 812 A.D., the emergence of oak leaves recorded by the Marsham family in southeastern Great Britain from 1736 to 1958 and then relayed by observations of a citizen since 1950, and finally the reconstruction of the blossom dates of several shrubs in China since 1834 using observations and records from archives since 1834.
Across these five series, leaf-out and flowering during the period 1985-2020 started six (in China) to 30 days (in Switzerland) earlier than before 1950. Particularly impressive: the 2021 cherry blossom in Kyoto, Japan was the earliest ever observed in the last 1200 years.
The time series of flowering and bud opening in spring are not only interesting indicators for science, but are also phenomena easily observable by the general public, including children. They are concrete and tangible indicators of accelerating climate change. The authors argue that these series could therefore be a very good tool to communicate about the impact of climate change on living organisms and to make politicians and future generations aware of the urgency of current climate change and its impact on the living world.
Dr Yann Vitasse
Swiss Federal Institute for Forest, Snow and Landscape Research (Eidg. Forschungsanstalt WSL)