Leaving street lights on all night has an unexpected effect on the trees around them, causing them to sacrifice growth for better protection against insects.
Street lighting has many benefits, but there is a price to pay for the loss of darkness. It certainly interferes with our view of the stars and animals are suffering. When scientists from the Chinese Academy of Sciences noticed how little insect damage was occurring to tree leaves in urban areas, lighting was a suspect.
“We observed that, compared to natural ecosystems, tree leaves in most urban ecosystems generally show few signs of insect damage. We were curious as to why,” Dr Shuang Zhang of the Chinese Academy of Sciences said in a statement.
We might expect that if urban insects are not munching on tree leaves as much, the cause would be related to animals, not plants. Perhaps the light or air pollution was directly affecting the insects. Alternatively, nighttime lighting may help predators catch herbivores, leaving less to chew on.
However, Zhang and colleagues also explored the possibility that trees exposed to light at night focused on protecting themselves through the addition of unpleasant chemicals, rather than growth. They tested this by collecting about 5,500 leaves from Japanese shrines (Styphnolobium japonicum) and green ash (Fraxinus pennsylvanica) trees near street lights in Beijing, and some located further away from the lights. All were the same distance from busy roads, so exposure to air pollution did not vary.
Both species produced tougher leaves that made them harder to chew when exposed to 24-hour light. Surprisingly, this made leaves from brighter areas less attractive to herbivorous insects.
Other differences varied by species. Japanese shrines exposed to more light had lower concentrations of nitrogen and phosphorus, both vital nutrients. Green ash trees, on the other hand, had more nitrogen when exposed to supplemental light and less insect-repelling tannin, but their leaves were smaller.
“The underlying mechanism for this pattern 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. Moreover, these leaves may allocate a greater proportion of resources to structural compounds, such as fibers, which may lead to an increase in leaf durability.
The authors speculate that the difference between species may be because green ash trees are less attractive to insects under ordinary conditions and thus may have less pressure to redirect resources toward prevention.
Zhang and colleagues are not ruling out increased predation on insects as another contributing factor, but that would be more difficult to test.
There’s a reason trees don’t grow stronger leaves all the time. Energy invested in building protection comes at the cost of growth and eventually seed production. You may not care if insects struggle to feed, but if it means you have less shade in the summer, the issue may seem more important, especially in a city prone to overheating. The authors also noted: “It is worth noting that leaves with higher resistance tend to decompose at a slower rate, potentially negatively affecting the rate of nutrient cycling.”
Furthermore, Zhang noted, “Reducing herbivory can lead to cascading trophic effects in ecology. Lower levels of herbivory mean lower amounts of herbivorous insects, which can result in lower numbers of predatory insects, insect-eating birds, etc. The decline of insects is a global pattern observed over the past decades. We need to pay more attention to this trend.”
Zhang acknowledges that a study on two species in a single city may not be universally applicable. However, it adds to existing arguments for turning off street lights for part of the night, or having them on motion sensors so they only shine when needed.
The study is published in Frontiers in Plant Science.