Light Pollution Tricks Tree Buds into Bursting Early

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(Credit: Andris Tkacenko/Shutterstock)

When cities light up the night, it confuses the trees.

In places where night-time light pollution is at its worst, trees burst into bloom a week earlier than trees rooted under dark skies, according to a 13-year study from researchers at the University of Exeter in the United Kingdom. Ringing in spring earlier in urban areas could have important ramifications for entire ecosystems, as changes in tree health send ripples throughout local food chains.

Timing is Everything

For trees, timing the budding process perfectly helps them take full advantage of bountiful summer sunlight to store energy, eventually shedding their foliage when the cost of hanging on to their leaves becomes greater than the energy they receive from them. Bursting buds too early risks frost damage, too late, and they miss out on valuable sunlight.

It was already known that changes in temperature, such as those caused by “urban heat islands,” can cause trees to bud at odd times. But the Exeter researchers wanted to determine if increasing levels of artificial, night-time light could also affect trees’ phenological cycles.

They turned to citizen scientists to answer the question, gathering location-tagged data on when trees began to bud between 1999 and 2011 from Nature’s Calendar, a website that allows people to log seasonal events. They then cross-referenced these data with satellite images over the same time period to document night-time light levels across the UK while also accounting for changes caused by temperature changes.

Confused by the Light

Three of the four species researchers included in the study began budding earlier in bright areas compared to trees located underneath dark skies. The most extreme effect was seen in the European ash, which began to grow leaves a week earlier on average when exposed to higher levels of artificial light at night.

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