Leaves' Fall Colors Have "Dirty" Secret, Study Finds

New Englanders are blaming this year's lackluster fall-color season on drought, but if you don't like the colors in your own backyard, you might blame the dirt, a new study says.

In an undergraduate research project, Emily Habinck, who has since graduated from the University of North Carolina at Charlotte, found that autumn leaf color is related to the richness of the soil.

She determined that on a North Carolina floodplain that was rich in nitrate—a nitrogen-containing nutrient—yellow-leafed trees dominated. But in the poorer soils of the hillside behind it, there were more reds.

Even among the trees that typically bear red leaves no matter the conditions, poorer soils made for redder hues.

Habinck based her study on her faculty advisor's observation that floodplain trees tended to be yellow and that soil nutrients might have something to do with it.

While Habinck was at work on the project, William Hoch, a plant physiologist at Montana State University, wrote a paper suggesting an additional link between the red-leaf pigment anthocyanin and autumn sunlight.

"It wasn't until I read his paper that it became a full story," Habinck said.

Leaf Protection

Leaves turn color in the fall as trees start shutting down their energy production and withdrawing nutrients into their roots.

"[The tree] pulls as many of these in as it can, then tries to drop just a skeleton of a leaf when it's done," Hoch said in a telephone interview.

But nutrient withdrawal takes time, and the process leaves the leaves vulnerable to damage from sunlight.

Anthocyanins protect leaves by "shading" them from excessive sunlight during the plant's relatively vulnerable autumn season, Hoch explained.

In a study of plants that had been genetically modified not to be produce anthocyanins, Hoch found that the modified plants were unable to send as many nutrients to their roots for winter storage.

"So the bottom line is that the plants that were able to produce red pigments were able to squeeze more of the nutrients out of their leaves than the ones that couldn't," he said.

Thus, Hoch says, plants living in nutrient-poor soils benefit more from anthocyanin than those living on better soils.

Scientists only recently made these connections, Habnick said, because when most other leaf-peepers are taking their fall-color tours, biologists are busy with academics.

"Most people's field season is in the summer," she said.