Artificial city

Study shows how artificial light affects seasonal rhythms of plants in US cities • News Service • Iowa State University

Researchers at Iowa State University have found that urban nighttime light alters plants’ natural circadian rhythms.

AMES, Iowa – A new study by researchers at Iowa State University shows how artificial light has affected the natural seasonal processes of plants in urban areas across the United States.

The study, published in the peer-reviewed journal academic journal PNAS Nexusdemonstrates how urbanization affects the natural world, leading to noticeable changes for humans, said Yuyu Zhou, associate professor of geological and atmospheric sciences at Iowa State and corresponding author of the study. For example, artificial light levels during nighttime hours in urban settings alter the natural circadian rhythms of plants, lengthening the pollen season for many plants in these regions. That means city dwellers who suffer from allergies may have to deal with sneezing and itchy eyes for longer periods of the year, Zhou said.

“From this study, we found that urban night light had a significant impact on the phenology of urban plants,” Zhou said. “We found that artificial light significantly advanced spring phenology and delayed fall phenology in the United States.”

Zhou analyzed how urban environments affect plants in several ways in a correspondence article published in Natural climate change in April. His previous studies have examined how heat trapped in cities, known as the heat island effect, alters the seasonal cycles of plants. Researchers also analyzed how changes in carbon dioxide concentrations and water and nutrient availability alter plant processes in urban environments.

For the latest study, Zhou and his colleagues evaluated NASA satellite data of artificial light at night, or ALAN, in US cities from 2012 to 2016. They then compared this data to seasonal changes in plants observed over approximately 3,000 urban sites. They concluded that ALAN advanced the date of leaf bud break in the spring by almost nine days and delayed leaf coloration by about six days in the fall. The overall effect is a longer active season for many plants, Zhou said.

This news could have important implications for allergy sufferers, and it also has utility for urban agriculture, a growing practice in which crops and other plants are grown and distributed in urban settings.

Studying the interaction of urbanization and plant phenology also offers insight into the impacts of climate change, Zhou said. The study indicated that the combination of increasing ALAN with rising temperatures intensifies the early onset of spring for many urban plants. The ways in which ALAN and temperature interact to influence plant cycles in the fall are more complex, he said.

But the study clearly shows that unraveling the interactions between ALAN and temperature will better prepare scientists to predict and respond to changes in plant processes in years to come.

“Urban environments can serve as natural laboratories to study plant responses to climate change,” he said. “Urban research can be used as a lens to give us clues about how the carbon and water cycles of the Earth system will evolve under a changing climate.”