In the Swiss Alps, the time lag between leafing of trees at high and low altitudes has shortened dramatically since the 1960s due to climate warming. This was the finding of a study conducted by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and the University of Neuchâtel in collaboration with the Swiss Federal Institute of Technology Lausanne (EPFL) and the universities of Antwerp and Beijing. The study was published in the journal PNAS (Proceedings of the National Academy of Sciences).

Led by biologist Yann Vitasse, three researchers analysed more than 20,000 observations recorded in Switzerland since 1960 by volunteers and collected by MeteoSwiss. The records in question covered the dates on which leaves or needles appeared on four forest species: beech, spruce, larch and hazel.

Five decades of citizen science bear fruit

"These forest species are among the most widespread in Switzerland, especially in mountainous areas. They provide a representative overview of the staggered start to spring along altitudinal gradients," says Yann Vitasse. "The observations recorded by volunteers across the country are enabling the researchers to detect some 'big picture' changes. As data, they are invaluable for better anticipating the consequences of global warming."

The results of this study also highlight one such consequence: a general tendency for leaves to start emerging earlier. This earlier leafing has been taking place faster at high altitudes, thereby reducing the time lag between the date of leaf-out at the top and bottom of a mountain. Back in the early 1960s, the time lag in leafing was roughly 5 weeks per 1,000 m of altitude difference, whereas now it is no more than 3 weeks. The researchers noted that the time lag decreases in particular after a warm winter.

Cooling needed before spring

One partial explanation of the phenomenon may lie in changes involving exposure to temperatures ranging between roughly 0 and 8°C. This cooling phase (but no frost) is necessary for buds to emerge from their winter rest. They can then 'react' to warm spring temperatures, allowing the leaves to develop normally.

Global warming means that more trees at higher altitudes are being exposed to this temperature range. Conversely, such periods are becoming less frequent at low altitudes. As a result, buds may be able to emerge from dormancy increasingly early high up on mountains and increasingly late lower down, thereby reducing the time lag in leaf emergence at these different altitudes.

This phenomenon is compounded by another consequence of an earlier spring awakening: day length. At low altitudes, leaves that emerge too early are exposed to a shorter day length, which may effectively slow down bud development.

"Future global warming could further uniformise the emergence of spring in mountainous areas," concludes Yann Vitasse. "This could have consequences for the structure and functioning of forest ecosystems, particularly as a result of resulting changes in plant-animal interactions. But the true impact of such change is still largely unclear today."