Why Do Some Autumn Leaves Turn Red?

Why some leaves turn red or orange is not simple to explain. The red pigments in leaves are made by anthocyanins, which give color to such familiar fruits as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. Red maples, sugar maples, white ash, pin oak, sassafras, and other trees owe their brilliant hues of red to anthocyanin. 

Highland Scenic Highway

For years red pigments were thought to be leftover pigments, too, just as the yellow pigments. But current research has shown otherwise. As observed by seasoned fall color watchers, no two autumns are identical. Each fall, leaves differ in color and intensity. For instance, why does a maple go yellow one year and red the next? Red is the wild card color. It is not left over after the green is gone. Federal and state forest service researchers have discovered that the red pigments are produced under certain conditions by leaves during the approach and beginning of autumn. But, the how and the why of red pigment production are not fully known. 

Highland Scenic Highway

Temperature and light leading up to and during the fall season seem to be important. This leads to the third overall factor involved in leaf color change–the weather. Two weather conditions that apparently stimulate the production of anthocyanins, the red pigment, are sunshine and cool nights. Sunny days in September and October trigger the making of anthocyanins, and improve the chances for brilliant red leaves. Conversely, a majority of overcast days stifle red pigment production and lead to more yellows and browns. Orange leaves, by the way, occur when both yellow and red pigments are present. 

Summit Lake

Cool but not freezing temperatures at night also promote red pigment production. The thinking is that chilly nights prevent the leaves from sending all the sugars they made during the day into the twig and therefore they get trapped in the leaves. These trapped sugars are the backbone of anthocyanin synthesis and thus boost the red pigment production. At some time, which varies year to year, the first frost and first freeze will occur. Freezing conditions destroy the leaf's ability to manufacture the red pigments. So an early frost means an early end to colorful foliage.

Summit Lake

"Best Conditions for Brilliant Fall Colors" in a post to follow.

Dolly Sods