Every autumn we look in melancholy at the falling leaves. But before falling, the leaves get yellow and orange with shades of red. But why this diversity in the color of the fallen leaves?

The undergraduate research project of Emily Habinck at the University of North Carolina at Charlotte, revealed that the color of the autumn leaf was connected to the amount of nutrients in the soil. She found that on a North Carolina floodplain with a soil abundant in nitrate (a nutrient that delivers nitrogen to the plants), yellow was the dominant color in the autumn "carpet", while the hillside, with poorer soils, were dominated by red during the fall. Even trees with typical red autumn leaves had even redder hues on poorer soils.

A 2006 research led by William Hoch, a plant physiologist at Montana State University, connected the synthesis of anthocyanin red pigments in leaves to fall sunlight. "It wasn't until I read his paper that it became a full story," Habinck said.

Leaves change color in the fall when trees stop the photosynthesis while withdrawing their nutrients from the leaves to be stored 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 told National Geographic.

But the food withdrawal is not a rapid process, and in the meantime, leaves are left vulnerable to harmful light waves. "Anthocyanins protect leaves by "shading" them from excessive sunlight during the plant's relatively vulnerable autumn season," Hoch explained.

Plants genetically engineered not to synthesize anthocyanins could not withdraw as many nutrients from their leaves. "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. Thus, plants living in nutrient-poor soils benefit more from anthocyanin than those living on better soils.", said Hoch.

"Scientists only recently made these connections, 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," said Habinck.