Leaves on the most common trees lining Beijing’s streets appear to be growing very well.
Shuang Zhang, a biologist at the Chinese Academy of Sciences in Beijing, was curious about why. The leaves showed less evidence of being eaten by insects (herbivory) than Zhang expected, a feature he initially attributed to the use of insecticides. He quickly rejected that hypothesis, however, because insecticides are sprayed only a few times a year and wouldn’t have such a profound effect.
Zhang and his colleagues then turned their attention to another product of urbanization: artificial light. “We find artificial light at night can increase the toughness [of leaves],” he said, “and reduce the level of herbivory.”
New research from Zhang and his colleagues in Frontiers in Plant Science examines how nighttime light exposure changes the leaves on which some insects rely.
The analysis compared leaves from city trees that were exposed to artificial light at night (ALAN) with those that were not. Scientists chose 30 sites along major Beijing roads that have lights on at night. They measured the light at the time of sampling using a photometer; at each site they selected three trees exposed to high levels of light and three exposed to low levels. They then took 30 leaves from each of those trees and began their assessments.
They measured leaf size and quantified the amount of each leaf that had been eaten by insects. Using a tool more commonly meant for evaluating fruit and vegetable hardness, the researchers measured the force needed to penetrate a leaf, giving them “leaf toughness” data.
The two species, Styphnolobium japonicum (L.) Schott (pagoda tree) and Fraxinus pennsylvanica (green ash), both showed an increase in leaf toughness and a decrease in herbivory among leaves directly facing streetlights.
On other measures, however, F. pennsylvanica proved to be more vulnerable to ALAN. Artificial light at night led to a lower carbon-to-nitrogen ratio, decreased tannin content, and smaller leaf size. Nitrogen content increased. S. japonicum (L.) Schott showed reduced leaf phosphorus and nitrogen content.
“We don’t know the exact reasons” for these differences, Zhang said, noting that this study was not designed to tease that out. But, he added, “leaf toughness is one of the most important defense traits in plants.”
The paper hints at one possible explanation: “In higher ALAN environments, S. japonicum (L.) Schott might allocate more resources to defense, leading to a decrease in nutrient content, such as leaf phosphorus and nitrogen content. In contrast, F. pennsylvanica, which has less tasty leaves, might preferentially allocate resources for growth rather than defense.”
Ecosystem Effects
The new research contributes to a growing scholarly 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 the palatability of leaves is measured by the carbon-to-nitrogen ratio, and if artificial light changes that ratio and makes the leaves tougher, the relationship between a given insect and a tree it has fed upon could be disrupted. “If the insects that are there can no longer eat those two planted species, they still need to eat,” she said. “So are they going to now preferentially consume the [nonplanted] tree that’s not benefiting from the artificial light?”
When a city is illuminated all night long, “we’re potentially affecting the entire ecosystem,” Sher continued. If the environment changes and one link in the ecosystem chain is affected, everything else that encounters that link, however indirectly, may also
“Ecosystems can be very resilient and shift 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 those changes “are going to facilitate the untimely extinction of a species.”
Zhang and colleagues suggested that future work might look at how ALAN affects tree reproduction and traits such as seed size. High-intensity artificial light at night may be detrimental to the energy flow between urban plants and the things that eat them, they suggested, a situation that could pose “a potential threat to the maintenance of [urban] biodiversity.”
Zhang said he would like to see several hundred tree species enrolled in future studies to probe why different species respond differently to light pollution. Then, he said, “we could construct the relationship between leaf traits and their responses to artificial light at night.”
This article originally appeared in Eos Magazine.
