Posted 8:52 a.m. Thursday, Sept. 30, 2021

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

UWL scientists explain the basic science behind the beauty 

The trees of the Driftless Region are just beginning to change color with spots of bright oranges, deep reds and yellows. 

UW-La Crosse Biology Professor Tim Gerber predicts that this year’s fall leaf display in the Driftless Region will be more brilliant than a typical year. This is because of the wetter than average weather throughout August and into September. More moisture generally leads to more vibrant colors because it helps trees stay healthy and keeps sugar production up. Drought, like much of Minnesota has experienced this summer, is harder on trees and tends to dull the display.    

Increased precipitation later in the growing season also tends to produce earlier leaf color change. In a typical year, the La Crosse area will see a peak colors in mid-October, whereas this year the peak is expected to come the week of Oct. 1. Fall leaves will peak near the end of September and throughout the month of October in Wisconsin, according to Travel Wisconsin Fall Color Report

Why do leaves change color?  

Leaf color change is determined by several factors, which is why trees change at slightly different times during the fall season. Gerber and UWL Chemistry Professor Heather Schenck point to several factors that influence leaf color: 

  • Pigments inside the leaf - Production of Chlorophyll molecules that dominate the leaf during summer months stops, revealing other color molecules present in the leaf.
  • The shortening length of fall days - Trees know when days are getting shorter and nights are getting longer and they respond by slowing and then stopping the producing chlorophyll molecules.
  • Weather conditions - The best conditions in the fall for leaves are warm and sunny days, followed by cool and crisp but not freezing nights, according to the U.S. Forest Service. 
  • Soil conditions - Dry, sandy soil will lead to a less brilliantly colored leaf. 
  • Tree health -  Trees that are older or unhealthy tend to change sooner. 

Some of these conditions — namely soil, weather and tree health — vary from year to year, causing slight changes to the length of the fall foliage display and brilliance of the leaves each year.  

Below Schenck answers other 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 look inside green leaves, you’d see that they are 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 2021. 

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. The La Crosse area is estimated to peak the first week in October. Check out Fall Color report will give you the estimated week of the peak in your area.  

Listen in to Wisconsin Public Radio's The Morning Show featuring UWL Biology Professor Tim Gerber and UWL Chemistry Professor Heather Schenck sharing the science behind why fall leaves change color.