Florence study of 283 plane trees reveals an unexpected effect of street lighting

What if streetlights kept trees from recognizing winter? In Florence, a study of 283 plane trees reveals a surprising phenomenon: some keep their leaves well after others. A finding that questions the impact of our illuminated cities on nature.

In Florence, researchers find that some plane trees refuse to shed their leaves

In Florence, plane trees sometimes seem oblivious to the onset of winter. In darker areas, trees gradually shed their foliage. Under streetlights, some remain surprisingly green. This seasonal mismatch goes almost unnoticed by passersby. It has, however, intrigued Luciano Massetti, a researcher at the National Research Council (CNR) of Italy.

Between 2014 and 2017, his team studied 283 plane trees subjected to different lighting conditions. The results, published in 2018 in Urban Forestry & Urban Greening, show a significant difference. The trees exposed to light retain more green leaves from mid-December to late January. The phenomenon became more pronounced during the cold winter of 2016-2017.

Cascine Park reveals a striking contrast with the city’s lit streets

Cascine Park offers a particularly revealing contrast. This vast Florentine green space remains relatively free from urban lighting. The researchers observed a striking phenomenon there. The period without green leaves began at least 20 days earlier than in the other studied sectors. Yet all these trees endure the same regional climate.

This difference could be explained in part by the photoperiod. The term refers to the daily duration of light exposure. Trees use this natural cue to anticipate seasonal changes. When nights lengthen, their activity gradually slows down. They then prepare for their winter rest. But the presence of a streetlamp can disrupt this biological clock.

The observation, however, is not entirely new. As early as 1936, scientists observed similar behavior. Branches near a lamp sometimes kept their leaves longer. The Florentine study provides field data to this botanical curiosity. One question remained: could light alone trigger such a disruption?

In Pisa, an LED experiment reveals a two-month delay, but not in all trees

To delve deeper into this riddle, scientists from the University of Pisa conducted an experiment. They selected 30 young trees: 15 plane trees and 15 large-leaved lindens. Some were exposed to LEDs during the night. The others served as controls. The results were published in 2023 in Science of the Total Environment.

The finding is striking. Under nocturnal illumination, plane trees show a dormancy onset delayed by about two months. The lindens do not exhibit a similar shift. However, this figure warrants an important caveat. It comes from a controlled experiment, with lamps close to the foliage. It does not automatically apply to all urban plane trees.

Behind these still-green trees, nighttime light could be concealing other disruptions

Scientists have also identified a subtler effect. Among trees exposed to LEDs, mature leaves absorb less CO₂ at dawn. This reaction suggests a form of physiological fatigue after a night of illumination. It concerns both species studied. Yet only the plane tree shows a marked delay in dormancy.

These observations raise several questions. Leaves that stay active too long could become more vulnerable to frost. They might also disrupt the tree’s energy reserves. For now, these consequences remain scientific hypotheses. The experiments do not prove that nocturnal lighting reduces the longevity of plane trees. Further research will be needed.

For local authorities, the stakes go beyond mere electricity consumption. A street lighting better suited to living beings could also preserve the natural rhythm of trees. This perspective invites a different view of our illuminated streets. On a winter walk, one detail now deserves attention. Why do some foliage stay green while their neighbors are already bare?