7 Oct

Map of the Week: 2021 Fall Foliage Map

by Jessie Woldstad

For many, it’s considered to be the most beautiful time of the year–the world’s trees erupt into a vibrant display of oranges, yellows, and reds. As the days fall shorter and summer nears its end, trees prepare themselves for the darker, colder months ahead. The Fall Foliage Prediction Map, created by the tourism site smokymountains.com, utilizes NOAA weather data (temperature, precipitation, and daylight) to predict when peak autumn foliage is expected across the United States.

Not only does the onset of autumn occur at different times across the United States–as the map suggests–but fall foliage also varies drastically in intensity by region. For instance, New England is known for its display of the most brilliant fall foliage in the country, while more southern or tropic regions typically have a lackluster show, if any fall colors at all. But why exactly is this? Turns out, it has to do with the reason for leaves changing color in the first place.

Autumn foliage is part of a tree’s food-making cycle; in particular, fall colors indicate that a tree is preparing itself for winter. Chlorophyll, which gives leaves their green color, collects energy from sunlight and converts it into carbohydrates in a fascinating process called photosynthesis. Because photosynthesis relies heavily on sunlight, the majority of a tree’s food supply is created in the spring and summer months, where days are long and there is plenty of sunshine. Once autumn begins and the days become shorter and darker, this process begins to wane. Eventually, chlorophyll in the leaves breaks down and food production comes to a halt. This breakdown of green chlorophyll results in beautiful reds, yellows, and oranges that comprise the autumn palette. In fact, these bright colors are present in the leaves all year, but are simply hidden by green chlorophyll.

So, if lower temperatures and shorter days indicate to trees that winter is coming, then in regions where days remain long and warm (like near the equator, for instance), plants have little reason to shed their leaves, staying green all year round. However, tropical plants aren’t the only ones who stay green. For evergreens, dark and cold winters are not an issue, as their needle-like and scale-like foliage is able to withstand freezing temperatures due to their waxy coating.

If you have ever noticed that some falls are more vibrant than others, this is due to several factors impacting color intensity of foliage from year to year. According to the U.S. Forest Service, a warm, wet spring will prime the trees for a colorful autumn; however, if spring is late, then that may delay the onset of fall foliage. A severely dry or hot summer will also delay fall colors. In general, fall foliage favors a mild summer with warm days and cool–but not freezing–nights, which would bring out the most vivid colors in the leaves.

(Image source: NASA) Fall colors begin to show in the Great Lakes region of the United States