Rain, temperature, sunlight produce our fall color

By Jim Thornton

Ohio certified volunteer naturalist

As fall approaches, many people say it is their favorite time of the year. For some, it’s the cool evenings or pumpkin-spiced everything. For others, it’s baseball playoffs and football. For many of us who live in an area with the right climate and diverse foliage, our fall treat is seeing much of this foliage change from lush greens to beautiful yellows, oranges, browns and reds. The biggest providers of this fall color are leaf-shedding (deciduous) trees and shrubs.

Many factors, including rainfall and temperature, play into how spectacular a fall we will have, but the key to the change in leaf color is the amount of sunlight the plant receives during the fall season.

Photosynthesis (the chemical process that allows a plant to use sunlight to produce food) relies on chlorophyll, the chemical that gives plants and leaves their green color. As the night lengthens and the temperatures turn cooler, the plants slow down chlorophyll production. This process also cuts off the connecting layer of cells between the plant and the leaf, leaving various chemicals in the leaf.

As the green chlorophyll breaks down and fades, other pigments begin to show. The leaves then display the orange and yellow colors (the carotenoids). They were always there, just masked by green during the growing season.

The reds and purples come from anthocyanins which are made from the sunlight reacting with sugars left in the leaves. Thus, you may see leaves on some trees turning a red to purple color on the outer leaves toward the sun. The tans and browns we see are the tannins which remain when all the other colors are gone.

The amount of and length of color we get to see varies, depending on conditions. A poor weather year (especially drought) may cause the plant to start cutting off the supply of chemicals to the leaves earlier in the year, causing the leaves to fall early with little color display. Early frosts can sharply curtail the production of anthocyanins, dulling colors. Good sun allows the anthocyanins to work their magic. I have seen Staghorn Sumac (Rhus typhina) on the full sun side of a lake with deep red leaves, and on the other, shady side of the lake, the same tree species was shedding pale yellow leaves.

Keep in mind we have few deciduous conifers in our area. My favorite for fall is the Tamarack or American Larch (Larix laricina), which turns to a beautiful yellow before shedding all its needles.

What’s this year’s prediction? Well, with lots of disease factors from a wet spring and summer, coupled with a very dry August and early September, we’re not exactly sure. The expectation would be a shorter window of colors that are not as intense. But the sunny days and early onset of cool nights should help the leaves present more intense red and purple tones.

For details and Ohio’s fall color report, visit http://go.osu.edu/fallcolorreport.

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