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Climate change and city lights are tricking trees into growing leaves too soon

ARI SHAPIRO, HOST:

As the climate changes, our seasons are changing, too.

THERESA CRIMMINS: We are seeing warmer temperatures earlier in the season, and many plants and animals are responding by undertaking their springtime activity earlier in the year than they used to.

SHAPIRO: Theresa Crimmins is director of the USA National Phenology Network, which tracks these sorts of changes.

CRIMMINS: But we don't have all of the nuance sorted out, like how it all fits together.

MARY LOUISE KELLY, HOST:

Well, here's a little bit of nuance about how trees in the city perceive springtime compared to their rural counterparts.

LIN MENG: I found trees start to grow leaves and turn green six days earlier in cities compared to rural areas.

KELLY: Lin Meng is a postdoctoral fellow at Lawrence Berkeley National Lab. She and her team studied the springtime observations of volunteers nationwide, along with more than a decade of satellite imagery of 85 cities across the U.S.

SHAPIRO: And they found two culprits tricking urban trees into budding early. First, heat - cities are a few degrees hotter than the countryside. And second, all those city lights.

MENG: I further found artificial light in cities acts as an extended daylight and cause earlier spring greening and later autumn leaf coloring.

SHAPIRO: Her work appears in the journal, Science.

KELLY: Meng says she first got interested in this phenomenon in 2015, when she went to see cherry blossoms in a park in Beijing. The blossoms had come in early, and then a snowstorm hit.

MENG: And what I saw on the next day was an almost complete loss of those emerging blossoms - flowers all over the ground. And that moment inspired me to think how warming environment with great fluctuation affect tree greening.

SHAPIRO: There are real-world implications when trees leaf out too early.

MENG: Earlier spring greening, earlier pollen season, higher risk of pollen allergy for humans.

SHAPIRO: And it could confuse pollinators like bees, which are crucial to the ecosystem.

MENG: It may also cause a mismatch with the timing of pollinators. That may affect insect development, survival and reproduction.

KELLY: And as if we didn't have enough supply chain hiccups already...

CRIMMINS: Agriculture can be impacted to quite a great extent, things like cherries and almonds and peaches.

KELLY: Theresa Crimmins again. If a fruit tree grows flowers too soon...

CRIMMINS: Those flower buds - if they're hit by frost, they're killed. And then the tree is not able to put on another bunch of flowers. And so if you don't have flowers, you're not going to have fruit.

SHAPIRO: The study is just one more reminder of the ways our urban footprint echoes through the natural world.

(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.