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Gardeners who have sprouted seeds indoors know the power of artificial light to coax plants into action. It turns out humans across the United States—and potentially the world—are tweaking entire forests in much the same way.

As cities and suburbs sprawl, turning night into day with streetlights, headlights and myriad other lights, scientists have discovered that we are inadvertently meddling with hallmarks of the seasons.

New evidence from satellites reveals that “urban nighttime light has significant impact” on when leaf buds open in the spring and leaves turn color in the fall in urban deciduous trees across the United States, says Yuyu Zhou, an Iowa State University scientist involved in the new research.

In the past, lab experiments have shown that artificial light exposure can prompt plants to flower or grow faster.  Studies in cities have noted similar changes in urban trees, including later loss of leaves and earlier budding. But much of this research has looked at local or regional levels. And it’s hard to untangle how much these changes stem from higher temperatures tied to what’s known as the urban heat-island, and how much can be blamed on lights.

Over the past dozen years, however, space agencies launched two satellites capable of detailed monitoring of nighttime light—the Suomi National Polar-orbiting Partnership and the NOAA-20. Zhou, along with scientists at a handful of U.S. universities, NASA and the federal Oak Ridge National Laboratory, tapped this new night vision to track links between artificial light and tree behavior on a much bigger scale.

The researchers took satellite images tracking nighttime light pollution across the continental United States. They matched them with on-the-ground observations from 2011 to 2016 of two key markers of seasonal change in trees—when leaf buds opened on trees in the spring and when leaves started to change colors in the fall. That data came from the USA National Phenology Network, a system of professional scientists and volunteers who report the timing of plant behavior across the country. To control for the effects of temperature, the scientists also incorporated detailed weather data.

They found that in much of the country, trees near urban light are effectively being fooled into thinking spring is starting earlier and fall is starting later. On average, the buds on trees started opening almost 9 days sooner in places exposed to artificial nighttime light than comparable places that were darker. Later in the year, trees in brighter night spots waited an average of 6 more days before starting show their fall leaf colors, the scientists reported earlier this year in PNAS Nexus.

Depending on the situation, this light could amplify or counteract the season-changing effects of global warming. In the springtime, this effect happened regardless of the air temperature. So the light pollution effects could come on top of a shift to earlier budding triggered by warmer springs due to climate change. A model combining the effects of temperature and expanding light pollution showed that in 2100, in 5 U.S. cities spread across different climate zones, from Minnesota in the north to Houston in the south, buds would open 17 to 21 days earlier than in 2015.

The fall is more complicated. There, the data showed that light pollution prompted trees to change leaf color later in cooler areas, but had the opposite effect in places hotter than 71°F. The researchers aren’t sure why this happened. But they noted that plants closer to the equator have been shown to be less sensitive to the length of day, which might partly explain why trees in warmer spots showed less reaction to light pollution. That means that as the planet warms further, the heat-driven push for leaves to stay green later might be countered in warmer parts of the country by light pollution.

Overall, these changes in trees could have both costs and benefits. If leaves emerge sooner, they could be more vulnerable to damage from late-winter cold spells. It might also put the trees out of sync with other organisms, such as insects, whose lives are timed to coincide with changes in plants. On the flip side, a longer growing season could mean more of the benefits that come from leafy trees, the researchers say: shade, absorption of rainwater and carbon dioxide, and filtering air pollutants.

While the scientists weren’t studying the biological processes inside plants driving the effects of light pollution, one possibility is that it could be messing with the internal, light-sensitive clock that plants use to gauge what time of day it is and where in the year they are located. Yes, trees have a circadian rhythm, just like people.

One added twist is that lighting technology could influence how trees respond in the future. Trees are most sensitive to light in the red part of the spectrum. Fluorescent lights emit almost no red, while the high-pressure sodium-vapor lights commonly used in street lights have more long-wavelength red light than do widely-used LED lights. Whether these changes in lights could reverberate with that maple tree in the front yard “is still unknown,” wrote the scientists.

Meng, et. al. “Artificial light at night: an underappreciated effect on phenology of deciduous woody plants.” PNAS Nexus. April 18, 2022.

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Topics: Anthropocene Biodiversity Cities Decarbonization Food & Agriculture Health Climate Parables

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