Cities with anti-light-pollution laws are protecting tree seasonal cycles
Geist B, Meng L, Longcore T, Wang Z, Hölker F
Urban Ecology
If you've noticed a street tree near a bright parking lot or streetlight holding its leaves weeks longer than its neighbors in the dark, this is why: cutting nighttime light exposure through city policy measurably restores normal timing to when trees leaf out and change color.
Streetlights and city glow don't just wash out the stars, they also confuse trees about what season it is, causing them to leaf out earlier or drop leaves later than they should. Researchers compared satellite images of nighttime light in four US cities from 2022 to 2024 and found that New York City and Washington DC, both of which passed laws to reduce artificial light at night, saw their light pollution drop, especially in the blue light band. Trees in those cities also showed smaller disruptions to their spring and fall timing, while Newark and Philadelphia, which have no such policies, saw light pollution keep climbing.
Key Findings
ALAN declined in NYC and Washington DC after mitigation policies were enacted, with the largest reduction in the blue light band
ALAN increased over the same period in Newark and Philadelphia, cities without mitigation policies
Reduced shifts in spring leaf-out and autumn senescence timing in policy cities tracked with the observed decline in ALAN exposure
chevron_right Technical Summary
City light pollution rules actually work: satellite data show that New York City and Washington DC cut their nighttime light levels after passing ALAN-mitigation policies, and their street trees showed smaller shifts in spring leaf-out and fall color change compared to Newark and Philadelphia, where light pollution kept rising.
Abstract Preview
Original paper
Mitigating artificial light at night: effectiveness of policies and implications for plant phenology.
Light pollution, an increasingly recognized environmental hazard, disrupts urban ecosystems, including plant phenology, such as leaf-out and senescence. In recent years, municipalities have enacted...
open_in_new Read full abstractAbstract copyright held by the original publisher.
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