Wohlleben, who authored The Hidden Life of Trees and manages a beech forest in Hummel, Germany, stressed that like humans, trees also need to sleep at night to grow. Also like humans, trees need darkness to sleep properly, and lights disrupt their circadian rhythm.
"Trees that live near street lights will die earlier because it's like burning a light in your bedroom," Wohlleben said in the report.
Wohlleben's findings are in line with another study, published in the Journal of Ecology, that found that plants are sensitive to light, which informs them of what time of day or season it is -- information that is crucial in helping plants decide what processes to execute. Artificial light interrupted these decision-making processes, the study found.
The tree expert said that urban trees are particularly challenged, not only because they are deprived of sleep, but also because they have to grow without a support network of fellow trees. Also, roots of urban trees are compromised, as they grow under pavement, which stays warm unlike forest soil, which cools down at night.
"Urban trees are the street kids of the forest," Wohlleben said.
He stressed that trees are more human-like than people think, with their health dependent on certain factors that also affect people. For instance, trees grow better when they are with their mother tree, Wohlleben wrote in a Telegraph.co.uk article. The massive mother tree keeps younger trees from growing too quickly by forming thick canopies that allow only a certain amount of sunlight to reach the leaves of younger trees. A slow growth, according to the forester, is crucial for trees to reach an old age.
Also, having a network of other trees for communication and support is important for the health of trees, which send signals through the soil, sharing food and warning each other of imminent danger. Wohlleben shared that after stumbling upon an ancient tree stump that had been felled hundreds of years prior, he discovered that the remains were still living, concluding that surrounding trees had been pumping the stump with sugar to keep it alive.
He also shared that some trees give off a warning gas to inform neighboring trees that there are nearby animals that will eat their leaves, which causes trees pain. Once they receive the warning signals, the other trees are able to protect themselves by releasing toxic substances into their leaves, deterring the animals from eating them.
This supportive, interdependent behavior of trees is one way to keep themselves alive.
"If every tree were looking out only for itself, then quite a few would never reach old age. Regular fatalities would result in many large gaps in the tree canopy, which would make it easier for storms to get inside the forest and uproot more trees. The heat of summer would reach the forest floor and dry it out," Wohlleben wrote. "Every tree, therefore, is valuable to the community and worth keeping around for as long as possible."
This community interdependence is what makes it even more challenging for trees to grow in an urban setting.
Still, improving the health of urban trees has a ripple effect on the health of a city. According to the Planting Healthy Air report by The Nature Conservancy, trees are a cost-effective measure against heatwaves and improve the quality of the air. By creating a canopy of leaves for shade and releasing water vapor, trees are able to keep the air cool. At the same time, they are able to clean the air by screening out fine particulate matter from biomass and fossil fuels that come from highways and industrial areas.
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