Autumn is a season well known for its colorful foliage. The often-celebrated aesthetic displays, however, are actually part of a process that trees use to survive the winter.

As daylight hours decrease in the fall, there is less sunlight available to power photosynthesis – the chemical process that provides nutrients to trees by converting carbon dioxide and water into glucose. This, in combination with falling temperatures, signals the tree to stop producing food and prepare for a period of dormancy, which is similar to hibernation.

To do this, a tree turns off its food producers by slowly corking the connection between leaf-stems and its branches. This blocks the movement of sugars from the leaves to the tree as well as the flow of water from the roots to the leaves.  As a result, the leaves stop producing chlorophyll, the agent of photosynthesis and the reason for the green color of summer foliage. As the green fades, other chemicals that have been present in the leaves all along begin to show. These include xanthophyll and carotene, which produce yellow and orange leaves, respectively. Red to purplish colors are the result of anthocyanin, a chemical produced as a result of any remaining sugars trapped in a leaf.

While leaves change color every year, the timing and duration of the displays are largely dependent on temperature and rainfall. Dry, sunny days and cool nights are the ideal recipe for beautiful fall foliage. Warmer and wetter conditions, on the other hand, tend to dull and delay the color change. Extreme circumstances, such as frost or drought, can be a source of stress for trees and cause the leaves to fall off faster.

It should also be noted that different species of trees react to atmospheric conditions differently. Therefore, the more diverse a forest, the wider the range of colors in autumn.

Tree in Autumn. Credit: Melissa Fleming