The time of new leaf formation of the horse chestnut tree in Geneva has been registered since 1808, and the blooming of cherry trees at the rural station in Liestal since 1894. These time series are valuable because they date much further back than the observations at the stations of the national phenological measurement network.
Observations at the stations of the phenological measurement network only started in the 1950s. The two series of measurements for the horse chestnut tree in Geneva and the blooming of the cherry tree in Liestal, the longest-running in Switzerland, date back considerably further and deliver valuable information about the reaction of the vegetation to the climate in years gone by.
These series of phenological observations are part of the systematic climate observation program GCOS (Global Climate Observing System). This program, which Switzerland joined in 2002, systematically observes 34 different climate variables (GCOS-CH). This climate monitoring is implemented in Switzerland through the activities of numerous national institutions coordinated by the Swiss GAW/GCOS Office at MeteoSwiss.
The timing of budbreak of the horse chestnut tree in Geneva varies greatly. In 1816, the first buds opened on 23 April; the earliest this event was observed on 29 December 2002. A clear trend toward earlier event dates has been observed since 1900. Global climate change as well as structural modifications in the surrounding area and greater warming in the city are all factors at play here. In the last few years this trend has been reversed. The reasons for this are not yet known. It may be due to the tree itself, to changes in the environment or to a changed reaction to the temperature. The official tree died in summer 2015 and since then the observations have been made on a new tree.
One wild cherry tree at the edge of the forest is monitored in Liestal-Weideli. Starting from 1990, a trend toward earlier event dates has been determined in the rural areas near Liestal. The variation width of the observations is considerably smaller than for the budbreak of the horse chestnut tree in Geneva.