The spring index shows the difference (in days) in spring vegetation growth onset compared to the long-term average for the period 1991 to 2020. The phenological spring phases are summarized in the annually determined index. Since temperature is one of the central factors influencing vegetation growth onset, the spring index is an effective tool for measuring the effects of climate change on vegetation.
The spring index is determined by means of the first ten phenological spring phases of the year and updated at the end of each May. It draws on the observations documented in the corresponding year at the nearly 80 stations in the phenological measurement network, using datasets of adequate length.
The spring vegetation in 2025 developed seven days earlier than the long-term average for the period 1991–2020. This was very early, ranking seventh since 1954. The average temperature from January to April was 1.7 °C higher than normal. Hazel bushes began to flower on 25 January in Ticino and at the end of January on the northern side of the Alps. Overall, hazel bushes flowered 9 days earlier this year than the 30-year average from 1991 to 2020. Fruit tree blossoming began in the last days of March, 9 to 10 days earlier than usual. At the same time, dandelions and cuckoo flowers bloomed 11–12 days earlier than average. The unfolding of horse chestnut and hazel leaves and the emergence of larch needles in April was 5–6 days earlier than average, while beech trees sprouted 4 days earlier than average in mid-April.
The following ten phenological phases are used to characterise the phenological spring as a whole; they occur between January and May:
The deviation from the average occurrence date is determined by means of a main component analysis. This method is practical for structuring, simplifying and visualising complex datasets. In addition, it can be used to filter spatial and chronological dependencies. The result of the first main component is then converted back into number of days’ deviation from the average.