Light pollution disrupts the seasonal rhythms of plants and trees, lengthening the pollen season

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City lights shining all night deeply disrupt the phenology of urban plants – changing as their buds open in the spring and as their leaves change color and drop in the fall. New research I co-authored shows how bright nighttime lights are lengthening of the growing season in citieswhich can affect everything allergic to local economies.

In our study, my colleagues and I analyzed trees and shrubs at approximately 3,000 sites in US cities to see how they responded under different lighting conditions over a period of five years. Use of plants the natural day-night cycle as a signal of seasonal change with Temperature.

We found that artificial light alone advanced the date when the leaf buds burst in the spring of about nine days on average compared to sites without night lighting. The timing of leaf color change in the fall was more complex, but leaf change was still delayed by an average of almost six days in the lower 48 states. In general, we found that the brighter the light, the greater the difference.

We also projected the future influence of nighttime lights for five US cities – Minneapolis, Chicago, Washington, Atlanta and Houston – based on different future global warming scenarios and up to a 1% annual increase in intensity. nocturnal light. We found that increased nighttime light would likely continue to shift the start of the season earlier, although its influence on the timing of fall color change is more complex.

why is it important

This type of change in the biological clock of plants has important implications for the economic, climate, health and ecological services provided by urban plants.

On the positive side, longer growing seasons could allow urban farms to be active for longer periods of time. The plants could also provide shade to cool neighborhoods earlier in the spring and later in the fall as global temperatures rise.

But changes in the growing season could also increase the plants’ vulnerability to spring frost damage. And this can create a gap with the schedule of other organizations, like pollinatorson which certain urban plants depend.

Urban light intensity varies from city to city and from neighborhood to neighborhood within cities. Yuyu Zhou, CC BY-ND

A longer active season for urban plants also suggests an earlier and longer pollen season, which can exacerbate asthma and other respiratory problems. A study in Maryland found a 17% increase in asthma hospitalizations in years when plants flowered very early.

What is not yet known

How fall color timing will change as nighttime lighting increases and temperatures rise is less clear. Temperature and artificial light together influence fall color in complex ways, and our projections suggest that the delay in coloring date due to global warming may stop by mid-century and possibly reverse due to artificial light. This will require more research.

It also remains to be seen how urban artificial light will evolve in the future.

A study found that urban light at night had increased approximately 1.8% per year around the world from 2012 to 2016. However, many cities and states are trying to reduce light pollutionincluding requiring shields to control where the light goes and switching to LED streetlights, which consume less energy and have less effect on plants than traditional streetlights with longer wavelengths.

The phenology of urban plants can also be influenced by other factors, such as carbon dioxide and soil moisture. Additionally, the faster temperature increase at night compared to daytime could lead to different day-night temperature patterns, which could affect plant phenology in complex ways.

Understanding these interactions between plants and artificial light and temperature will help scientists predict changes in plant processes under a changing climate. Cities already serve as natural laboratories.


Yuyu Zhouassociate professor of environmental sciences, Iowa State University

This article is republished from The conversation under Creative Commons license. Read it original article.

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