From New York to Paris to Beijing, urban trees are enjoying an extra-long growing season, a new study finds.
The reason, researchers say, is that trees take their cues from shifts in temperature and sunlight, sprouting leaves when days grow warmer and longer in the spring, and shedding them when the sun retreats in the fall. In cities, concrete buildings and asphalt roads retain heat, while streetlights shine all night, muddling the normal seasonal cues.
The new research, based on satellite data from 428 cities across the northern hemisphere, found that the growing season in cities now starts 12.6 days earlier and ends 11.2 days later, on average, than in surrounding rural areas. The findings were published in Nature Cities.
Authors say that light appears to play a bigger role than temperature in extending the growing season. That’s because while cities are slightly warmer than their rural environs, they are exponentially better lit.
Over the last decade, cities have seen nighttime light pollution grow by close to 10 percent per year. The authors expect the impact of artificial light to become even more pronounced as cities increasingly swap sodium streetlights for brighter LEDs.
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