Posted 12:25 p.m. Friday, Oct. 16, 2020

Red maple leaf with a hint of yellow and orange. Concentrations of diverse molecules in a leaf can make it appear multiple colors.

Chemist explains the basic science behind the beauty 

When the countryside fills with blazing oranges, deep reds and bright yellows, it is a welcome sign of fallBut why are fall leaves so colorful? In one word: chemistry. University of Wisconsin-La Crosse Chemistry Professor Heather Schenck answers some common science questions related to fall foliage. 

What causes fall leaves to change color? 

Chlorophylls are a class of molecules with similar structures. They are generally differentiated by a letter after the family name of chlorophyll, as with the Chlorophyll A molecule here.

We are used to seeing green leaves throughout the summer months. If you were to shrink down and take a walk inside that green leaf, you’d see that it is swamped with molecules that make it look green —a.k.a. chlorophyll moleculesChlorophyll has an important job inside a leaf. It takes light from the sun and converts it into energy that the tree uses to grow through a process called photosynthesis. 

In the fallin the north, we have less sun during the day and trees slow and then stop producing chlorophyll molecules. When this happens, other molecules that were already inside the leaf — just not as prominent as chlorophyll — begin to appear. Basically, leaves are chemically revealing other molecules already there, but we couldn't see them because the leaf was so swamped with chlorophyll.  

Why do leaves turn different colors?

Carotenoids are a family of molecules with related structures, all of them based on Beta carotene. Carotenoids can provide hues from yellow through orange to red; Beta carotene is orange, while lutein is yellow.

Other molecules besides chlorophyll can give color to a leaf. In fact, there are so many other color molecules that they are named in classes. A common class of molecules that give leaves a yellow-orange color are called carotenoids. Another class of molecules that make a leaf look red or purple are called anthocyanins. These color molecules also show up in our foods. You may ask: Why are carrots orange? Carotenoids. Why are apples red? Anthocyanins. 

What determines leaf color? 

Anthocyanins are a class of molecules that give a wide array of colors, including reds, purples and blues. Variations are possible in the core of the molecule. These are indicated with R groups in the structure above.

The other color molecules  anthocyanins and carotenoids — are a more diverse group than chlorophyll. Therefore, we see a spectrum of different colors. A given tree, say a black maple, might produce more of one type of color molecule, making it look a deep orange. A different black maple might produce less of that molecule and more of another, making it look yellow. It is the concentrations of these more diverse molecules that can make a tree a certain color or even different colors throughout the same tree or the same leaf.  

What makes fall leaves more colorful? 

A moist spring and dry and sunny late summer/early fall promote the most intense red and purple leaf colors.
  • Moist spring 
  • Summer that's not too hot or dry
  • Warm and sunny late summer and early fall 
  • Fall nights that are cool but not freezing  

 When is peak leaf color change in Wisconsin? Fall foliage map 2020. 

The color change in Wisconsin typically starts in mid-September and ends in October. The northern part of the state will begin to see change before the south. Check out Fall Color report will give you the estimated week of the peak in your area.