Colorful Colorado - The science behind our fall colors

by Amy Lentz, Weld County Horticulture Agent

Each year, after the summer season ends and ski season begins, there’s a two to three-week window of color that shifts its way across the state of Colorado. During this short period of time, vibrant colors such as red, orange, yellow and purple will brighten up the sometimes-dreary weather, giving us one last show before winter sets in. I was lucky enough this year to experience the fall colors in early September near Walden, in late September near Frisco, and currently along the lower elevations of the northern Front Range.

This year's predictions for aspen fall color across the state, from 9news.com

Various hues of yellow, orange and red from Aspen trees near Frisco in September.

As you observe these striking changes in the trees, shrubs and grasses in your surrounding landscape, you might wonder “What is the science behind the beauty?”

The genetics of the plant are a key factor in what color they will express during the fall season. Certain trees such as poplar, cottonwood, honeylocust and some ash trees will be various shades of yellow, while others such as maple, sumac and some oaks will lean more toward the orange and red tones.

Yellow fall color of a honeylocust tree.

 

Environmental conditions are another major factor. As the seasons change and we have shorter day lengths and cooler temperatures, a response is triggered in plants to shift their focus toward going dormant. Not only do these environmental conditions switch on the process of leaves changing color, they can also determine the intensity of fall color from year to year. Along with good soil moisture during the growing season, the warm days and cool nights of autumn help to trap sugars in the leaves, making some fall color displays more vivid.

Brilliant fall color in Jackson County near the Wyoming border in early September.

On a microscopic level, it’s the underlying plant pigments that lead us deeper into the science behind these amazing fall colors. So just how does the leaf make these interesting colors?

During the growing season, leaves have been producing the tree's food using chlorophyll, the dominant leaf pigment that is expressed as a green color. The chemical nature of the chlorophyll molecule allows it to absorb sunlight and use the energy to convert carbon dioxide and water to carbohydrates for the plant. Because the plant is actively growing and needs a large amount of food to be produced, chlorophyll is in high abundance and most of the leaves you see are green. Although other plant pigments are present in the leaves, the large amounts of chlorophyll present during the growing season will mask other, less abundant, pigments.  As the season progresses, the leaves will lose chlorophyll as they senesce, revealing our next pigment…

Carotenoids are also pigments made in the chloroplasts of plant cells, however some carotenoids remain in the cells and are unmasked after the chlorophyll degrades. Because these carotenoids absorb blue wavelengths of light, they tend to show up as leaves with a yellow hue.

Carotenoids expressed as yellow tones on a Kentucky coffee tree. 

The other variations of color that we see in fall foliage comes from yet another plant pigment called anthocyanin. This pigment is actually produced in the leaf of some plants prior to leaf senescence (leaf drop). It’s the combination of the anthocyanin and the carotenoids present that give us the other brilliant fall colors of red, orange, brown and purple. Anthocyanins and chlorophyll can produce brownish colors, while anthocyanins and carotenoids produce orange hues. Those leaves that show a red or purple color have a high proportion of anthocyanins.

Red fall color from anthocyanin production on an Autumn Blaze maple tree.

Red fall color of a burning bush, a common landscape shrub.

So, there you have it...some science behind the beautiful colors of the autumn season that we see not only on trees, but also on woody shrubs and ornamental grasses. These amazing fall colors are another reason we call our state 'Colorful Colorado'!