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Plants are sprouting from the ground earlier and earlier once the snow has disappeared. This is now happening on average six days earlier than 25 years ago, according to a newly published study by SLF researcher Michael Zehnder and colleagues.

The findings are in the journal Global Change Biology.

The reason for the change is the significant rise in temperatures.

"On average, as a result of climate change, the ambient temperature is almost two degrees Celsius warmer after the disappearance of snow cover than it was 25 years ago," explains Zehnder. This rapid warming has accelerated the onset of plant growth, thus shortening the time from the end of snowmelt until the meadows green up again.

This has implications not only for the plant world, but also for the economy and society, as it means that the mountain spring, so beloved by day trippers, is getting underway sooner.

"Alpine farming might also start earlier in the future," says Zehnder.

In addition, biodiversity in the Alps will change. This is because not all plants start growing immediately after the snow melts. Some follow their internal growth clock and only sprout when the days are long enough after the snowmelt, while others need warmth above all else, explains Zehnder.

The biologist found that in areas close to the treeline at around 2,000 meters above sea level, vegetation needed more warm days to start growing in years when there was early snowmelt. At higher altitudes too, there are alpine plants that depend on the length of the days and delay sprouting even when it is warm enough for this to happen. However, the study shows that plant communities at these altitudes require approximately the same number of warm days to start growing, regardless of when the snow melts.

In the future, therefore, it is likely that higher mountain areas in particular will green up earlier than is currently the case as the snow melts earlier and temperatures rise. This will also affect the composition of plant communities. Species that primarily respond to warm days could displace those with a strict dependence on day length.

"Climate change is reshaping mountain ecosystems," says Zehnder.

For the study, he used data from 40 Intercantonal Measurement and Information System (IMIS) weather or measuring stations. Each of these has an ultrasonic sensor that measures the snow depth ("snow height") in winter. In fact, they also make measurements in summer—and are thus recording plants growing earlier and earlier in the year.

"This way we receive data without having to be in the field ourselves," explains Zehnder. A computer model, trained on a wealth of data by machine learning (ML), detects whether there is snow under the sensor or whether plants are already growing. This is how Zehnder identifies at what point in spring the snow in the mountains disappears and plants start growing.

Zehnder analyzed data from the period running all the way from 1998 to 2023. In order to record the vegetation at the stations, the biologist also had to go to the stations himself. He did this on foot, as most of them are in the middle of the mountains, at an altitude of between 1,700 and 2,700 meters above sea level. This is because researchers cannot tell from the IMIS station data which plants are growing under the sensors.

"It's important, though, to understand how different plant communities will have different responses to even earlier snowmelt in the future and who the winners and losers will be," says Zehnder.