Leaving street lights on all night has an unexpected effect on the trees around them, making them sacrifice their growth for better protection against insects.
Street lights have many advantages, but there is a price to pay for the loss of darkness. It certainly interferes with our view of the stars, and the animals suffer. When scientists from the Chinese Academy of Sciences noticed how insects were damaging the leaves of trees in urban areas, one of them was suspected.
“We have noticed that, compared to the natural environment, the leaves of trees in many urban environments often show no sign of insect damage. We were curious to know why,” said Dr Shuang Zhang of the Chinese Academy of Sciences in a statement.
We would expect that if urban insects do not buy leaves from trees as much, the cause will affect the animals, not the plants. Perhaps the light, or air pollution, directly affected these insects. In addition, night light can help carnivores, leaving few to eat.
However, Zhang and colleagues explored the possibility that trees exposed to light all night might focus on protecting themselves by producing unpleasant chemicals, rather than grow up. They tested this by collecting about 5,500 leaves from a Japanese pagoda (Styphnolobium japonicum) and green ash (Fraxinus pennsylvanica) trees near street lights in Beijing, and some are placed further away from the lights. They were all far from busy roads, so exposure to air pollution was not different.
Both species produced tougher leaves that were harder to chew when exposed to 24 hours of light. Unsurprisingly, this resulted in leaves from exposed areas well do not attract plant-eating insects.
Other differences vary from species to species. Japanese pagodas exposed to more light had lower concentrations of nitrogen and phosphorus, both of which are important nutrients. On the other hand, green ash trees had more nitrogen when exposed to more light, and less insect tannins, but their leaves were smaller.
“The mechanism underlying this mechanism is not yet fully understood,” Zhang said. It is possible that trees exposed to artificial light at night can increase their photosynthesis time. In addition, these leaves can provide a large proportion of materials to structural compounds, such as fibers, which can lead to an increase in the stiffness of the leaves. ”
The authors speculate that the difference between species may be because green ash trees are less attractive to insects under normal conditions, and may have less pressure to move tools to prevent them.
Zhang and colleagues have not ruled out insect infestation as another cause, but it would be more difficult to test.
There’s a reason trees don’t keep their leaves hard all the time. Energy invested in self-defense comes at the cost of growth, and ultimately seed production. You may not care if the insects find it difficult to feed, but if it means you get less shade in the summer, the problem can seem very important, especially in a city with extreme heat. The authors also noted: “It is noteworthy that leaves with a high density tend to decompose slowly, and can negatively affect the speed of nutrient cycling.”
Furthermore, Zhang noted: “Decreasing herbivory can lead to very negative effects on the environment. Lower levels of herbivores mean fewer herbivores, and that can lead to there will be more insectivores, insectivorous birds, etc. The decline of insects is a trend seen worldwide in recent decades.”
Zhang believes that the study on two species in one city may not be applicable everywhere. However, it adds to the existing arguments for turning off street lights for part of the night, or having motion sensors to turn them on when needed.
The study was published in Frontiers in Plant Science.
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