The shift in the color of foliage is just one of several signs of the beginning of autumn.

Once again, the turning of the seasons has brought about a stirring display as the trees on and around campus have begun the changes in leaf color that are their first step toward surviving the coming winter.

The colors are expected to peak in central Iowa this week.

“Foliage color is mostly triggered by cooler temperatures and shorter day lengths,” said Rajeev Arora, plant stress physiologist and associate professor of horticulture. “These two environmental factors give trees a cue that winter is approaching.”

Arora said one of the reasons behind the color change trees experience in autumn is the decline in chlorophyll – the pigment that gives a plant its green color and enables it to make food for the plant by a process called photosynthesis. Light makes photosynthesis possible.

The shortening of the days that occurs as winter approaches prompts a plant to get rid of its chlorophyll, and the plant’s chloroplasts slowly begin to stop producing it.

“The chlorophyll starts to break down because it is no longer needed,” Arora said. “It cannot process the sunlight at these low temperatures anyway.”

The onset of the cooler temperatures and shorter days also cause trees to start producing other pigments that change the color of the leaves.

Some of these pigments are anthocyamins, which are responsible for the appearance of orange, red, and purple colors in leaves, and carotenoids, which are primarily responsible for yellows.

“Once the chlorophyll degrades, the manifestation of [other pigments] increases, since they have already been there for two or three weeks,” Arora said.

Many possible reasons have been put forth to explain this phenomenon.

Some scientists have theorized that the brightly colored pigments that plants produce during this time are an evolutionary mechanism designed to ward off insects like aphids by confusing their color.

Arora said this theory has not been put to test yet, however.

He said a more valid theory is that anthocyamins and carotenoids are produced at this time to protect the chloroplasts from too much light, since they have started the process of shutdown for winter and may be injured by too much sunlight.

“Ideal conditions for the peak production of [anthocyamins and carotenoids] are bright, cool, and clear days,” said Arora.