Leaving street lights on all night has an unexpected effect on the trees around them, forcing them to sacrifice growth for better defense against insects.
Street lighting has many benefits, but the loss of darkness comes at a cost. It disrupts our view of the stars, and animals suffer. When scientists at the Chinese Academy of Sciences found that insects caused little damage to tree leaves in urban areas, lighting was one of the suspects.
“We noticed that, compared with natural ecosystems, tree leaves in most urban ecosystems generally showed few signs of insect damage. We were curious to know why,” Dr. Shuang Zhang of the Chinese Academy of Sciences said in a statement.
You might think that if urban insects aren’t eating as much tree leaves, the cause would be related to animals, not plants. Perhaps light or air pollution directly affects insects. Alternatively, nighttime lighting could help predators catch herbivores, leaving fewer to munch on.
However, Zhang and his colleagues also investigated the possibility that trees exposed to light at night focus on their defense by activating unsavory chemicals, rather than on their growth. They tested this hypothesis by collecting nearly 5,500 leaves from Japanese pagoda trees (Styphnolobium japonicum) and the green ash (Fraxinus pennsylvanica) trees near street lights in Beijing, and some placed further away from the lights. All were the same distance from busy roads, so exposure to air pollution did not vary.
Both species produce tougher leaves, making them harder to chew when exposed to 24-hour light. Unsurprisingly, leaves from brighter areas are therefore less attractive to herbivorous insects.
Other differences varied by species. Japanese pagodas exposed to more light had lower concentrations of nitrogen and phosphorus, two essential nutrients. Green ash trees, on the other hand, had more nitrogen when exposed to more light, and less tannin, an insect deterrent, but their leaves were smaller.
“The mechanism underlying this phenomenon is not yet fully understood,” Zhang said. “It is possible that trees exposed to artificial light at night can extend the duration of their photosynthesis. In addition, these leaves may allocate a greater proportion of resources to structural compounds, such as fibers, which could lead to increased leaf strength.”
The authors hypothesize that the difference between species may be due to the fact that green ash trees are less attractive to insects under ordinary conditions and therefore may be under less pressure to redirect resources toward deterrence.
Zhang and his colleagues do not rule out that increased predation on insects is another contributing factor, but that would be more difficult to test.
There’s a reason why trees don’t always harden their leaves. The energy invested in building defenses comes at the expense of growth and, ultimately, seed production. You may not care that insects are struggling to feed, but if it means you have less shade in the summer, the problem may seem more relevant, especially in a city prone to overheating. The authors also observed: “It’s worth noting that tougher leaves tend to decompose more slowly, which can negatively affect the rate of nutrient cycling.”
Furthermore, Zhang noted, “Decreased herbivory may lead to cascading trophic effects in the ecology. Lower levels of herbivory imply lower abundance of herbivorous insects, which in turn could lead to lower abundance of predatory insects, insectivorous birds, etc. Insect decline is a global trend observed in recent decades. We should pay more attention to this trend.”
Zhang acknowledges that a study of two species in a single city may not be universally applicable. Still, it supports existing arguments for turning off streetlights for part of the night or mounting them on motion sensors so they only come on when needed.
The study is published in Frontiers in Plant Science.
