The Story Behind Fall Colors

It's that time of year again when trees fade from their various shades of green into their pops of orange, yellow, and red. Although the phenomenon of fall colors is well known, it is not common knowledge what causes the wide variety of shades we see every year.

So why do leaves change?

Leaves contain four main biochemicals responsible for fall colors: Chlorophyll, Carotenoids, Anthocyanin, and Tannins.

Chlorophyll is the energy-rich pigment that gives leaves their green color. It absorbs red and blue wavelengths of energy from the sun making it important for photosynthesis. As daylight begins to shorten, photosynthesis becomes less and less effective. As such, deciduous trees will pull back their photosynthesis resources to prepare for the winter dormant season. Since the nitrogen housed within chlorophyll is a critical resource, trees will break down the chlorophyll in their leaves in the fall.

Credit: Nikki Henger

Underneath the leaf’s strong green chlorophyll pigment lies carotenoids, anthocyanin, and tannins. These three chemicals are present year-round in deciduous trees and are responsible for the other colors we see. They are usually hidden by chlorophyll but their pigments become visible as chlorophyll is broken down.

Some species such as ashes have high levels of carotenoids and often appear yellow or orange in the fall. You may already be familiar with this compound because this is the same chemical that gives carrots their color! Other trees, such as maples, have high levels of anthocyanin. These trees tend to be bright red or purple in autumn. Oaks on the other hand are high in tannins so they usually turn brown. Tannins are also associated with bitterness and can be found in some of our favorite foods: black tea, chocolate, and wine.

Why are the colors different every year?

Sometimes the stunning beauty of fall colors comes as early as mid-September and other times the color stubbornly waits until October. Even if the timing is similar to years past, the colors are never the same. Some years they can be dull while others are breathtakingly vibrant. What’s going on?

Credit: Open Productions - Fall Colors 2017

The timing and vibrancy of fall colors are most influenced by day length, temperatures, and rainfall conditions before and during the fall season. Peak color tends to occur earlier when the region experiences an early light frost but can also end earlier if a hard frost occurs. Similarly, a severe drought can stress trees enough to cause an early, dull fall season but a mild summer drought can intensify colors. These goldilocks type of conditions apply to temperatures as well. A September with warm, sunny days and cool nights (around 32-45 °F) with few temperature swings is perfect for a vibrantly colored fall season. 

Given how many different factors and conditions go into creating the colors, it’s important to get out and experience them for the short window they appear!

Where can I find the best fall colors this year?

If you’re interested in where the fall colors are strongest, check out the Minnesota Department of Natural Resources’ Fall Colors Finder (MN) and the Travel Wisconsin Fall Color Report (WI) to see live maps and forecasts of where fall colors are peaking across both states.