MADISON, Wis. – Every fall, thousands of Americans head for the woods to see summer extinguished in a blaze of color.
In Wisconsin, they celebrate Colorama. In New England, the visitors are called “leaf peepers.” They travel hundreds of miles north for the yellows, the oranges and especially the reds.
Leaf color theory. University of Wisconsin-Madison scientists have a new theory about why autumn leaves turn scarlet and why the hues are more vibrant some years than others.
They say that the red pigments – called anthocyanins – in plants such as maples, oaks, dogwoods and viburnums act like sunscreen.
“The pigments shade sensitive photosynthetic tissue in fall while trees reabsorb nutrients from their leaves,” said horticulturist Bill Hoch.
“Trees need to store as many of those nutrients as they can before the leaves drop.”
Co-authors Hoch, Eric Zeldin and Brent McCown lay out their ideas in an article featured on the cover of the journal “Tree Physiology.”
Several explanations. “The scientific literature contains many different explanations for why trees make anthocyanins in fall,” Hoch said. “Some theories account for the color change in one tree, but not in other species. Other ideas are clearly wrong.
“For example, the red does not come about because sugars are trapped in leaves and converted to anthocyanins.”
“Light that is too bright can inhibit photosynthesis any time of the year,” Hoch said. But in fall when trees are breaking down and reabsorbing important nutrients from their leaves, their photosynthetic tissues are especially unstable and vulnerable to too much light and other stresses.
How it works. Yet trees need the energy from photosynthesis to drive the processes that allow them to recapture as many of those nutrients as possible. Just as this process begins, leaves start producing large amounts of anthocyanins near the leaf surface.
The Wisconsin scientists argue that the pigments protect the leaves’ dwindling ability to generate energy during this period.
In addition to high light levels, other plant stressors such as near-freezing temperatures, drought and low nutrient levels trigger increased levels of the pigments.
At their best. The researchers’ theory agrees with the observation that autumn colors are best when the fall features dry weather with bright, sunny days, and cold nights.
It also makes sense of observations that the outer leaves of maple trees, for example, are more colorful than shaded leaves inside the canopy and leaves on the north side.
Hoch said their ideas also explain why most of our native maples and oaks in the Midwest and New England turn red, while European species such as the Norway maple do not.
“None of the European counterparts of these North American trees produce high levels of anthocyanins,” Hoch said.
“We think it’s because the weather in that part of the world is cloudier and warmer during fall. European species don’t need the protection of these pigments.”
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