Plant phenology

What is plant phenology?

Phenology deals with the regularly recurring growth and development processes of living organisms throughout the year. In plant phenology, the start dates (corresponding to a specific day of the year) of so-called phenophases—such as leaf unfolding, flowering, fruit maturing, leaf coloring, and leaf drop of various plant species—are observed.
One example of phenological observations in plants is time of general flowering, when 50% of the blossoms of the dandelion are open. Another example is when 50% of the leaves of the black locust have unfolded or the start of flowering in the european red elder, when the first flowers on three inflorescences have opened.

How is plant phenology monitored?
MeteoSwiss has been operating a phenological observation network since 1951, which now comprises around 160 stations throughout Switzerland. At these stations, 26 different plant species are observed to describe vegetation development. The observations are still carried out manually by observers today.
Switzerland also has two particularly long phenological observation series: the new leaf formation of the horse chestnut has been recorded in Geneva since 1808, and the flowering time of the cherry tree in Liestal since 1894.

Influences on plant phenology
Vegetation development is influenced by various factors such as climate, soil composition, soil flora and fauna, topography and altitude, current weather conditions, day length, and the genetics of the plant. Since vegetation development in spring and summer is strongly dependent on temperature, plants are particularly well suited as indicators of climate change. Long observation series are extremely valuable for drawing conclusions about past climate conditions.
As many of these influencing factors correlate strongly with altitude, vegetation development is significantly influenced by altitude. Roughly speaking, a change in altitude of 100 meters corresponds to a shift in vegetation development in spring and summer of around three days.

Development of plant phenology in Switzerland
In addition to the observations of individual plant species, MeteoSwiss publishes an annual average value for phenological development in spring in Switzerland: the so-called spring index. This index takes into account the flowering and leaf unfolding data of nine different plant species at around 80 locations with long data series. The spring index reveals the strong annual fluctuations and illustrates the general trend towards earlier flowering and leaf unfolding. While the onset of spring vegetation growth hardly changed until 1988, a clear shift to an earlier onset has been observed since 1989
Depending on the plant species, spring development can depend not only on temperature but also on day length, as is the case with beech, for example. Since length of the day has not changed, the timing of leaf unfolding of the beech has changed only slightly since 1951, occurring about five days earlier now. Other species, such as hazel, react very strongly to temperature changes. Compared to 1952, the flowering of hazel at certain stations below 800 meters above sea level is now a full 18 days earlier. Dandelions bloom 14 days earlier, and cherry trees 11 days earlier.
These observations show that vegetation is developing much earlier in spring and summer today than it did a few decades ago. This shift is due to rising temperatures caused by climate change.

Map interpretation
The map shows the development of the start dates of general flowering—when 50% of the flowers are open—for the dandelion and the field daisy. A bar serves as a timeline showing the mean values of the phenophase start dates for the periods 1951-1970 and 2004-2023. This makes it possible to see how the phenophase (general flowering) start date has changed across the two time periods for a particular monitoring station.
When interpreting the map, it's important to note that phenological observations come with inherent uncertainties. Estimating whether a phenophase has been reached is somewhat subjective, despite detailed instructions. In long data series, observer changes and changes in the plants being observed may also play a role. Despite these potential inaccuracies, the observations provide valuable information for understanding plant development processes and the impact of climate change.