City lights that stay on all night are severely disrupting the phenology of urban plants—when their buds open in spring and when their leaves change color and drop in fall. New research I co-authored shows how nightlight hours are prolonging 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 nearly 3,000 sites in US cities to see how they responded under different light conditions over a five-year period. Plants use the natural day-night cycle as an indication of temperature as well as seasonal changes.
We found that artificial light alone extended the date when leaf buds broke in spring by an average of about nine days, compared to sites without nighttime illumination. The timing of autumn color change in leaves was more complex, but leaf change was delayed by an average of about six days in the lower 48 states. In general, we found that the brighter the light, the greater the difference.
We estimated the future impact of nighttime lighting for five US cities—Minneapolis, Chicago, Washington, Atlanta and Houston—based on different scenarios for future global warming and a 1 percent increase in nighttime light intensity. of annual increase. We found that increasing night light would likely continue to change early in the season, although its effect on the fall color change timing was more complex.
why it matters
Such changes in plant biological clocks have important implications for the economic, climate, health and ecological services provided by urban plants.
On the positive side, longer growing seasons can allow urban farms to remain active for longer. Plants can provide shade to cooler neighborhoods earlier in the spring and later as global temperatures rise.
But a change in the growing season can increase the plant’s susceptibility to damage from spring frosts. And it can lead to a mismatch with the timing of other organisms, such as pollinators, that some urban plants rely on.
A longer active season for urban plants also suggests an earlier and longer pollen season, which can exacerbate asthma and other breathing problems. A study in Maryland found that hospitalizations for asthma increased by 17 percent in years when plants bloomed too early.
what is not yet known
How the timing of the fall color will change as the night light increases and the rise in temperature becomes less obvious. Temperature and artificial light together affect fall color in a complex way, and our estimates suggested that a delay in color date due to climate warming could be halted mid-century and possibly reversed by artificial light. This will require more research.
How urban artificial lighting will change in the future also remains to be seen.
One study found that urban illumination at night has increased by about 1.8 percent per year worldwide from 2012-2016. However, many cities and states are trying to reduce light pollution by requiring shielding to control where light goes and is transferring to LED street lights, which use less energy. and have less effect on plants than traditional street lights with longer wavelength.
The phenology of urban plants can also be influenced by other factors such as carbon dioxide and soil moisture. Additionally, the rapid increase in temperature at night compared to the time of day can lead to different patterns of day-night temperature, which can 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 are already functioning as natural laboratories.
Yuyu Zhou, associate professor of environmental science, Iowa State University
This article is republished from The Conversation under a Creative Commons license. Read the original article.