Minnesota's fall colors season is changing, right before our eyes

Trees seen from Summit Avenue, St. Paul.
Warmer temperatures are making for a later, brighter fall color season. But it's not all good news.
Bridget Bennett for MPR News

In Minnesota, there's one small bright spot amid the joyless landscape of climate change.

Fall colors will become more vibrant as the climate warms, and the leaf-peeping season might last longer some years.

However, extreme variations in weather could make for some years where trees just skip the colorful part and turn brown.

Here's what to expect for Minnesota's fall colors future, as described by University of Minnesota forest ecologist Lee Frelich.

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Changing tree species, brighter colors

Aspen and tamarack are dying, and maples and oaks are invading northern Minnesota.

"Fall colors will be brighter than they used to be because maple is going to have more oranges and reds compared to the birch and aspen, which was all yellow," Frelich said.

Eventually, parts of the north will lose many of their trees and resemble a grassland. The Boundary Waters, for example, will become a grassland with scattered oak trees.

Trees will turn later in the season

Frelich traveled to Ely, Minn., over the weekend and noticed something odd.

"Most of the trees are still green there, which is amazing because they used to have fall color at Labor Day," he said.

Temperature determines when orange and yellow will appear in leaves. Red colors come from anthocyanins, which are new pigments made after chlorophyll goes away.

Trees produce the green chlorophyll consistently during the summer. But as summer cools into fall, the chlorophyll degrades and reveals the leaves' carotenes — the stuff making yellow and orange pigments.

Trees need several nights of temperatures in the 40s before they stop making chlorophyll, the green pigment, and reveal their fall colors, Frelich said.

The later-than-usual green in Ely is symptomatic of what climate data show. Nighttime temperatures are increasing even faster than days, which are warming about a half-degree per decade.

Plus, as temperatures grow warmer overall, it's likely some fall colors seasons will last longer than normal.

The bad stuff

Not all climatic shifts are good for leaves.

Droughts, made worse by climate change, can choke off fall colors. A vibrant autumn requires plenty of rain toward the end of summer.

Frelich also remembers a time when rapid changes in weather made the trees skip their colorful stage. It was a period of hot weather followed by cold then hot again.

"The trees were just confused," Frelich said. "They start to go into fall then came back to summer, started to go into fall and just said, 'We give up we're just gonna turn brown.'

"I'm a little worried that with a warmer climate, that might happen more often in the future."

This article is based off an interview by MPR News chief meteorologist Paul Huttner for Climate Cast. Listen to past episodes of the show here.