How will the ongoing drought affect fall foliage?

Little River Canyon, DeKalb County (Alabama Department of Tourism)

Little River Canyon, DeKalb County (Alabama Department of Tourism)

HUNTSVILLE, Ala. – Although it has not started to feel a lot like fall, many people in the Tennessee Valley are already noticing some leaves beginning to change colors.

We have gotten lots of questions about how the ongoing drought might affect what kind of color display we can expect this year.

This post will answer your questions about what causes the leaves to change color in the first place, and will detail what impact the spring, summer and early-fall weather will likely have on the upcoming “leaf season.”

Why do leaves change colors?

Three factors influence autumn leaf color-leaf pigments, length of night, and weather, but not quite in the way we think.

The timing of color change and leaf fall are primarily regulated by the calendar, that is, the increasing length of night. None of the other environmental influences-temperature, rainfall, food supply, and so on-are as unvarying as the steadily increasing length of night during autumn.

As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature’s autumn palette.

How does weather affect fall color?

The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions-lots of sugar and lots of light-spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks.

A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.