Allergies last longer? Blame the light pollution

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 nighttime lights lengthen the growing season in cities, which can affect everything from allergies to local economies.

In our study, my colleagues and I analyzed trees and shrubs at about 3,000 sites in US cities to see how they responded to different light conditions over a five-year period. Plants use the natural day-night cycle as a signal of seasonal change with temperature.

We found that artificial light alone advanced the date when leaf buds erupt in spring by about nine days on average compared to sites without nighttime 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 D.C.; Atlanta; and Houston, based on different future climate warming scenarios and up to 1% annual increase in nighttime light intensity. 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 shift in the biological clock of plants has important implications for the economic, climatic, 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. 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 plant susceptibility to spring frost damage. And that can create a time lag with other organisms, such as pollinators, on which some urban plants depend.

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 the timing of fall color 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.

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