Streetlamps create an artificial summer for urban trees

Plants have an internal circadian clock that influences their development and physiology. This clock is reset by the rising sun in the morning, but street lights have banished darkness in urban areas for the past couple of centuries. More recently, the yellow glow of sodium lamps has given way to more energy-efficient LEDs. However, these LEDs shine a different kind of light, with a peak in the red and blue parts of the spectrum, perfect for photosynthesis. Ermes Lo Piccolo and colleagues have investigated what effect these lights have had on urban trees. They have found that, for some species, the longer availability of light has even managed to delay their winter dormancy.

The team bought fifteen lime trees (Tilia × phatyphyllos) and fifteen plane trees (Platanus × acerifolia) from a plant nursery and then grew them on at the University of Pisa. The trees were planted with five trees to each light, with two light intensities and a control group.

At night, streetlamp illumination severely affected the net photosynthetic rate. Net photosynthetic rate values were positive for trees of both species in both the low and high illumination groups. Only the young April leaves had a negative photosynthetic rate, which was still higher than the leaves in the control group. The botanists made sure that the leaves were using carbon dioxide for photosynthesis by using gas exchange analysis. They also found that the illuminated trees had a higher chlorophyll content in their leaves. The scientists found that this extended work at night had consequences in the morning.

For some aspects, the effect of streetlamps on plants might be considered to be a continuous light treatment. Many studies on the effects of continuous light treatment on plants found that the extension of the CO₂ assimilation process at night could induce disorders in the plant photosynthetic apparatus… In our experiment, both tree species in the −300 and − 700 groups had lower P_n [net photosynthetic rate] values at sunrise than the trees in the control groups, especially during the summer (fully-expanded leaves) and this trend was more evident in [lime] trees than in [plane] trees under streetlamp-illumination. The P_n value decreased because of biochemical limitations (as suggested by the decrease in the apparent carboxylation efficiency P_n/C_i values) instead of stomatal limitations (not significant or minimal differences between trees in the −300/−700 and control groups).

Lo Piccolo et al. 2023.

Lo Piccolo and colleagues have suggested that streetlamp illumination might negatively affect the light-harvesting capacity of plants during the early hours of the day due to an impairment of the PSII/PSI electron transport rate.

Another problem for the leaves is diet. Leaves produce sugars but also make starches when carbon dioxide assimilation brings more carbon than the leaf can handle through sugar biosynthesis. During the night, the starch is depleted to provide carbon for metabolism and growth until the next morning. Night-time photosynthesis alters the chemical balance between sugars and starches and breaks the circadian cycles.

The switch from summer to winter brings colder weather; in the past, it also used to bring less light. The encroaching sunsets meant that light was an excellent indicator of the season and so worked as a trigger for winter dormancy. What happens when you live with artificial suns?

Lo Piccolo and colleagues found that lime trees still prepared for winter, but the plane trees did not. Instead, they could be active for up to two months later, still growing shoots.

The findings show that there are consistent and species-specific responses to artificial lights. The switch from sodium to LED could change the responses from trees. The change in light could be another challenge for trees, producing a sudden shock for plants already tackling the creep of climate change.

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Lo Piccolo, E., Lauria, G., Guidi, L., Remorini, D., Massai, R. and Landi, M. (2023) “Shedding light on the effects of LED streetlamps on trees in urban areas: Friends or foes?,” The Science of the Total Environment, 865(161200), p. 161200. Available at: https://doi.org/10.1016/j.scitotenv.2022.161200.