A Wilder View: Trees in cities turning green earlier

Posted at 5:43 PM, Dec 08, 2021

MISSOULA — Trees don't get to look at the calendar to see when spring arrives, but they seem to know when spring is here better than we do.

This edition of A Wilder View takes a look at why trees in cities are turning green earlier than expected. The annual shift from winter to spring is a breathtaking event to watch as leaves become green and a lush environment reveals itself.

While it may be winter right now, trees in cities are turning green earlier than expected and one of the reasons why is because of artificial light. City lights brighten the night skies, billboard signs are lit up on roadways, and car headlights all contribute to shifting the regular day to night cycle that plants and trees rely on.

In order to stay alive during harsh winters, trees hit the pause button on their growth. Since temperatures can vary dramatically throughout the winter the length of daylight is the signal trees look for to safely start growing again and turn green.

On average cities are typically 1.8° to 5.4°F warmer than rural areas. This is known as the urban heat island effect. These changes in city environments may affect seasonal changes even more than climate warming and this alteration can affect the allergy and mosquito season, water cycles, and also effect pollinators.

Cities can act as laboratories to look at these responses to the environment before they spread more broadly.

Scientists use satellite imagery to see when plants turn green. By comparing spring green-ups in the 85 largest US cities scientists found on average, that trees start to turn green six days earlier in cities compared to rural areas. But artificial light shifts trees to start greening up on average nine days earlier.

The Missoula outdoor lighting ordinance that went into effect on Nov. 2, 2016, may lessen the impact our light has on leaves.