Light Pollution Is Waking Trees Up Early for Spring

June 30, 2016

The bright lights of the big city may trick trees into budding up to a week early, new research finds.

Light pollution in the United Kingdom was linked to early “budburst,” or the date that green leaves just begin to emerge from a budding tree. Later-budding trees such as the European ash (Fraxinus excelsior) were most affected by light pollution, researchers report today (June 28) in the journal Proceedings of the Royal Society B.

“It was an amazing result, really,” study researcher Richard ffrench-Constant, an etymologist at the University of Exeter, told Live Science. [Photos: See Light Pollution Around the World]

Domino effect

Ffrench-Constant came to be interested in trees by way of insects. He studies the winter moth caterpillar (Operophtera brumata), which relies on early spring leaves for sustenance. (More mature leaves produce bitter tannin compounds to ward off caterpillars and other herbivores.) Thus, the timing of budburst is crucial for winter moths.

“The moths are continuously struggling to time the hatch of the eggs exactly at the time that buds on the tree burst, and then the caterpillars will have the juiciest, freshest, greenest leaves,” ffrench-Constant said. The caterpillars’ success or failure reverberates up the food chain, he added: Bird species time the hatching of their eggs to the caterpillar hatch so that their offspring will have ample food.

“There’s a lot riding on the hatch of these caterpillars in the forest ecosystem,” ffrench-Constant said.

Trees use temperature cues to determine when to burst into bud, but experimental work has also shown that many rely on light cues, too. In particular, pigments in the trees respond to red light with growth and far-red light with inactivity. Red light wavelengths are prevalent in sunlight, while far-red light wavelengths are more dominant in shade or nearly dark locations.

Citizen science

Light pollution is already known to directly alter behavior in animals like bats and sea turtles. Ffrench-Constant and his colleagues wondered if streetlights and other city illumination might affect trees, too. They used citizen observations from the U.K. phenology network of nearly 42,000 individual trees of four species: the European sycamore (Acer pseudoplatanus), the European beech (Fagus sylvatica), the pedunculate oak (Quercus robur) and the European ash (F. excelsior).

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