Naturalist's Corner - Why Leaves Change Color

Clonal aspen stand, CO

Returning home from an extended October hiking trip in the Southwest last year, I was struck by the riot of colors that greeted me in our Driftless area. While I was impressed by the beautiful displays of gold and yellow of the aspen stands (all clones of single root systems) out west, I was worried I'd be too late to see the diverse fall colors that the Driftless area forests display.

Not to worry.

I was delighted to find that colors were just beginning to peak. Rather than large swaths of trees in the same color, we have a veritable party of different colors going vertically up the bluffs as you drive along the Mississippi River.

And it got me wondering...what sparks that change?  

It's not just one thing. The weather, the length of days and the individual tree's pigments all combine to create the color palette and timing. 

Each tree species has unique colors that depend on pigments already present in the leaf. For trees with yellow, orange and brown fall pigments (carotenoids), those colors are already in the leaves along with  the green chlorophyll during spring and summer. The chlorophyll masks these pigments during the summer. As the leaf prepares for winter, chlorophyll production slows and stops. That's when the bright yellows and oranges appear.

For trees that display more red and purple (anthocyanins), that pigment is produced in fall when excess sugar gathers in leaf cells.

This is usually preceded in August with a gradual lightening - or to my eyes, slight greying - of the deeper green colors of early summer as chlorophyll slows.

The shorter length of the day and less sunlight triggers leaf cells to start building up in the stem end, trapping the sugars and eventually sealing off the stem (when that happens, the leaf falls).

Finally the weather - sunny days and cool nights help create the most colorful leaves.  If there has been abundant rain in spring and summer to produce plenty of sugar for the trees, the colors will tend to be more brilliant. Drought and drier summers and fall, produce less sugar and can dull the colors. 

Driftless area states have "Fall Color-Finding" sites that keep you updated on the peak and past-peak times of autumn leaf changes:

• Minnesota
• Wisconsin
• Iowa

You can read more details about this process, at the US Forest Service webpage  and an interesting article from the November 1, 2024 edition of the Washington Post exploring the annual life cycle of a leaf and the effect our changing climate has on deciduous trees.

This short video produced on October 13, 2020 by Minnesota Master Naturalists for their Trail Tuesday series, goes through the process.

Check out my recommendations for favorite fall color hikes in the Driftless.

See you on the trails!