Spring foliage appearing too early in the Alps?

Spring greening: A larch cone and fresh shoots blossom on a twig of an old larch tree near Chur in eastern Switzerland. Keystone

Because of global warming, trees in the Alps are coming into leaf earlier than they used to – which could have negative consequences for forest ecosystems. 

In the early 1960s, spring came to the mountains about five weeks after arriving in the lowlands – as observed by the appearance of leaves on the trees. Today, that delay is only about three weeks, says the Swiss Federal Research Station for Forest, Snow and LandscapeExternal link in a report published on Monday. 

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Researchers came to this conclusion after assessing more than 20,000 observations recorded by volunteers and collected over the past six decades by MeteoSwissExternal link, the Federal Office for Meteorology and Climatology. The data set pertains to beech, spruce, larch and walnut trees. 

Premature leaf and needle development is even more pronounced at higher altitudes, meaning that those trees sprout foliage much faster after a warm winter. 

The researchers explain the phenomenon in part by how long the trees are exposed to temperatures of 0-8°C (32-46°F) at the end of winter. The trees need this frost-free cold phase so that the buds can awake from their hibernation and develop normally in spring. 

For foliage development, trees also need enough sunlight. Yet leaves that sprout too early encounter shorter days, which can lead to slower bud development. 

According to the study, this in turn has consequences for the structure and functioning of forest ecosystems, especially the interactions between plants and animals. The actual effects of these changes are still largely unknown. 

The study appears in PNAS, the weekly journal published by the National Academy of SciencesExternal link of the United States.