The leaves of the most common trees along the streets of Beijing seem to be growing very well.

Shuang Zhang, a biologist at the Chinese Academy of Sciences in Beijing, wanted to know why. The leaves showed fewer signs of insect herbivory than expected, a fact he initially attributed to the use of insecticides. However, he quickly dismissed this hypothesis because insecticides are only sprayed a few times a year and therefore do not have such a profound effect.

Zhang and his colleagues then turned their attention to another product of urbanization: artificial light. “We found that artificial light at night can increase the resilience (of leaves),” he said, “and reduce the level of herbivory.”

New research results from Zhang and his colleagues in Frontiers of plant science investigates how nighttime light exposure changes the leaves on which some insects depend.

The analysis compared leaves from urban trees exposed to artificial light at night (ALAN) with those that were not. The scientists chose 30 locations along Beijing’s main streets that are lit at night. They measured the light at the time of sampling using a photometer; at each location, they chose three trees that were exposed to high light and three that were exposed to low light. They then took 30 leaves from each of these trees and began the study.

They measured leaf size and calculated how much of each leaf had been eaten by insects. Using a tool normally used to determine the hardness of fruits and vegetables, the researchers measured the force needed to penetrate a leaf, obtaining data on “leaf toughness.”

The two species, Styphnolobium sylvestris (L.) Schott (cord tree) and Fraxinus pennsylvanica (Green Ash) both showed an increase in leaf toughness and a decrease in herbivory in leaves directly opposite streetlights.

However, other measures F. pennsylvanica were more susceptible to ALAN. Artificial light at night resulted in a lower carbon-nitrogen ratio, lower tannin content and smaller leaf size. Nitrogen content increased. S. japonicum (L.) Schott showed reduced phosphorus and nitrogen content in the leaves.

“We don’t know the exact reasons for these differences,” Zhang said, noting that this study was not designed to find out. But, he added, “leaf hardness is one of the most important defensive traits of plants.”

The paper gives a possible explanation: “In environments with higher ALAN S. japonicum (L.) Schott could allocate more resources to defense, which would lead to a decrease in nutrient levels, such as phosphorus and nitrogen content of the leaves. In contrast, F. pennsylvanicawhose leaves are less palatable might allocate resources to growth rather than defense.”

Impact on the ecosystem

The new research adds to growing scientific interest in the effects of artificial light on plants, said conservation biologist Anna Sher of the University of Denver, who was not involved in the study.

She explained that leaf palatability is measured by the carbon-nitrogen ratio. If artificial light changes that ratio and makes leaves tougher, the relationship between a particular insect and a tree it was feeding on could be disrupted. “If the existing insects can no longer eat those two planted species, they still have to eat,” she said. “So will they now preferentially eat the (unplanted) tree that doesn’t benefit from the artificial light?”

If a city is lit all night, “we are potentially affecting the entire ecosystem,” Sher continued. If the environment changes and one link in the ecosystem chain is affected, everything else that comes into contact with that link, even indirectly, can also be affected.

“Ecosystems can be very resilient and adapt to changing conditions, but it’s important for us to know what those changes are,” Sher said. Ultimately, she concluded, scientists can ask whether these changes “will favor the premature extinction of a species.”

Zhang and his colleagues suggested that future work could examine how ALAN affects tree reproduction and traits such as seed size. High-intensity artificial light at night can affect the flow of energy between urban plants and their predators, the researchers said, a situation that could pose “a potential threat to the conservation of (urban) biodiversity.”

Zhang said he would like to include several hundred tree species in future studies to find out why different species respond differently to light pollution. Then, he said, “we could establish the relationship between leaf traits and their responses to artificial light at night.”

This article originally appeared in Eos Magazine.