"Trees’ spring awakening is becoming less sensitive to altitude differences"

Introduction: Researcher UAntwerp and his international colleagues conduct research on Swiss Alps forest species.

In the Swiss Alps, the time lag between tree leafing at high and low altitudes has shortened dramatically since the 1960s due to climate change. This is the finding of an international study conducted in collaboration with the University of Antwerp, which has just been published in the journal PNAS (Proceedings of the National Academy of Sciences of the United States of America).

Led by biologist Yann Vitasse, three researchers - among whom UAntwerp research fellow Dr Yongshuo H. Fu - analysed more than 20,000 observations recorded by volunteers in Switzerland and collected by MeteoSwiss since 1960. The records in question covered the dates on which leaves or needles appeared on four forest species: beech, spruce, larch and hazel. The study was 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), the University of Antwerp and Beijing Normal University.

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 (University of Neuchâtel and WSL).

The observations recorded by volunteers across the country are enabling the researchers to detect some 'big picture' changes. As data, they are invaluable for anticipating the consequences of global warming more accurately. “The results of this study 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 dates of leaf-out at the top and bottom of a mountain.”

“Back in the early 1960s, the time lag in leafing was roughly five weeks per 1000 m of altitude difference, whereas now it is no more than three weeks”, explains Yongshuo H. Fu, research scientist at the University of Antwerp and Beijing Normal University. The researchers noted that the time lag decrease is especially marked following 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 (without 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 are being exposed to this temperature range at higher altitudes. 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, thus reducing the time lag in leaf emergence at these different altitudes.

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

“Future global warming could make the emergence of spring even more uniform across mountainous areas”, concludes Vitasse. “This could have consequences for the structure and functioning of forest ecosystems, particularly as a result of related changes in plant-animal interactions. But the true impact of such changes remains unclear at the moment.”

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