Study: Fall Colors are Sunscreen for Trees

The fiery red hues that seem to set forest leaves ablaze in autumn are produced in part as a result of the soil that trees grow in and help protect the trees in the winter, a new study finds.

The reasons why leaves change from their summer greens to brilliant yellows, oranges and reds has been something of a mystery to scientists because the process required energy, but didn't seem to benefit the trees.

Orange and yellow pigments, which exist in the leaf all year but only show up once the green chlorophyll leaves the leaf, were better understood than the ruddy pigments on sweetgum and red maple trees. But recent research has shed light on why trees bother producing these pigments, called anthocyanins, which also color raspberries, purple pansies, and red apples.

The anthocyanins were known to act as sort of an arboreal sunscreen, protecting leaves from harmful radiation and also keep leaves from freezing.

In 2003, plant physiologist William Hoch of Montana State University found that if anthocyanins were genetically blocked from leaves, they were very vulnerable to sunlight and so sent fewer nutrients to the plant's roots for winter storage.

The new study, which will be presented Oct. 29 at the Geological Society of America's annual meeting, went a step further and showed that producing anthocyanins is beneficial to trees that grow in nitrogen-poor soils because the pigments protect the leaves for longer so they can draw in as many nutrients as possible for storage in roots before winter sets in.

"It makes sense that anthocyanin production would have a function, because it requires energy expenditure," said study leader Emily Habinck, a former University of North Carolina graduate student.

So the scarlet hues that take over the leaves in the fall are the sign of a stressed-out tree just trying to survive.

"The rainbow of color we see in the fall is not just for our personal human enjoyment," said soil scientist Martha Eppes, also of UNC. "Rather it is the trees going on about their lives and trying to survive."