Forestry & Wildlife
AUBURN UNIVERSITY, Ala. – The transition of cooler weather brings with it the stunning beauty of the vibrant fall colors. Often, it seems that the tree leaves make this transition overnight. What causes the striking transition to the beautiful fall shades of red, yellow, orange and even purple? Mallory Kelley, an Alabama Cooperative Extension System home grounds, gardens and home pests regional agent, shines some light on the magic of leaves changing colors.
Arguably, yellow leaves stand out the most in the fall as they glow in the sunlight. According to Kelley, to understand the transition from green to yellow, it is important to first understand where the leaf began.
“In the spring and summer, the green color seen in leaves is the chlorophyll,” Kelley said.
Photosynthesis, or the food-making process in the plant, takes place in the green chlorophyll-containing leaves.
“As days begin to shorten, so does the amount of sun available to the leaves,” she said. “Therefore, the living plant cells in leaves do not receive enough light to create food and begin to change color.”
Chlorophyll will begin to break down as temperatures and daylight hours continue to dwindle. This will allow the yellow color to show through the leaf. The beautiful yellow color has been in the leaf all along, it was simply covered by the chlorophyll. Eventually, the leaves will die and fall to the ground, providing nice piles to play in.
“The initial plant that tells me fall is here is the terribly invasive popcorn tree,” Kelley said. “You will start to notice them along fence rows and out in pastures along the roadsides.”
Although the yellow color change can seem like an overnight phenomenon, it is simply a chemical process causing the breakdown of chlorophyll.
Red, Orange and Purple Leaves
Since yellow leaves appear through the breakdown of chlorophyll, where do the striking red, orange or even purple shades come from? Surprisingly, every fall leaf starts as a yellow leaf and transition into other shades because of temperature changes. Kelley said warm sunny days with cool night temperatures, especially below 45 degrees F, will give leaves these extravagant shades.
“These warm day and cool night temperatures tend to raise the level of red coloration in leaves,” she said.
These temperatures trap the sugars produced during the warm sunny day inside the leaves, producing the array of fall colors. However, certain tree species will only show one type of colored leaves.
“Sugar maples take on a fiery orange color, hickories will only show yellow colors and oaks are primarily reddish brown to a brown shade,” Kelley said.
The colors can vary tree to tree based on factors like physical location or even genetics. Also, they can vary on the same tree.
“Leaves that are directly exposed to the afternoon sun may turn a bright red, while leaves on the shady side of the tree may only turn yellow,” she said.
From yellow to a fiery fall shade, no two fall leaf colors are the same. However, one thing remains constant; the transition of leaves in the fall is a phenomenon enjoyed by all. Alabama.Travel has a list of the best places in Alabama to see the fall leaf colors. For more information, visit the Alabama Extension website to learn more about fall leaves in Alabama.