Extension experts explain fascinating fall foliage process
Colors lie dormant within leaves, then pop for fall
While going to and fro during daily routines this fall, people may scarcely notice the spectrum of colors subtly emerging around them, let alone stop to admire them before they are gone.
In only a short window of time, the fall season's artistic color show settles from late September through late December, wielding its brilliant canvas of colors before it is gone again.
The science behind the magnificent color changes, and the necessity of them, are changes that must occur for the survival of the tree. To understand this process requires backtracking to previous seasons.
"The entire purpose is for deciduous trees, [trees that drop their leaves], to shed their leaves to prepare for winter," said Shon Bishop, small farms specialist with the Barry County Extension.
During spring and summer, leaves, like factories, make food for the trees. Chlorophyll, the green pigment in the cells of the leaves, takes in sunlight, transforming carbon dioxide and water into carbohydrates such as sugars and starches through the process of photosynthesis.
As the end of summer approaches, a dormancy process is triggered when daylight decreases and temperatures drop, and the leaves stop making food. A layer of cells form where the leaf stem attaches to a branch or twig, blocking metabolites from entering or leaving the leaf. Sugars, still being manufactured by the leaf, are blocked from leaving, and begin turning into colorful pigments such as red or purple.
The chlorophyll then breaks down, allowing the beautiful hues of yellow, oranges and reds to come forth.
"There's a chemical message that goes throughout the trees' vascular system," Bishop said. "As the photo period begins to decrease and the sun begins to dip to the southern horizon, hormones are secreted, chlorophyll breaks down and the leaves fall to protect the tree. Old timers' reasoning for the changes are so the tree can get the maximum amount of light from the south as possible [to prepare for winter]."
While the show may only be witnessed for a brief period, the colors are actually contained within the leaf throughout the growing season.
"What's interesting is that the colors are always there," Bishop said. "The colors we do see are comprised mainly of lutein and anthocyanin, which are primarily those yellows and reds. Orange is a pigment of yellow and red."
After completing their grand production production of eye-popping colors, the leaves must go for the tree to survive. The leaves begin to drop when the special layer of cells that form where the stems attach to the tree begin to sever the tissues that support the leaf. The tree seals the cut, and when the leaf eventually falls off or is blown away by the wind, a leaf scar is left. Not all trees shed their leaves, but most of the broad-leaved trees in the north do.
"On the underside of the leaf, you have a constant source of moisture, if it is not humid," Bishop said. "The tree is having to feed those leaves. For trees still in full bloom, if it gets into the teens, it can cause permanent cellular damage, and the leaves won't come back."
Colors usually peak around the third weekend in October, but some years are better than others.
"It's always questionable until the last minute what will happen," said Extension horticulturist David Trinklein, adding that about once a decade, colors really pop.
And, each tree species is on a different schedule, as leaves, shrubs and vines turn colors at different time. But, overall in Missouri, color changes begin in the north and move south. Different trees stand out in different part of the state. For instance, sugar maples make autumn colors practically glow with bright golds, yellows and reds, whereas oaks have reddish-brown hues.
"Sugar maples are absolutely beautiful," Bishop said. "But there's a myriad of species here that beautify the landscape. A host of trees can be extremely pretty depending on external conditions."
Those same conditions determine actual color intensity, which depends on factors like temperature, moisture, light and water, even throughout the year. The best fall color displays are a result of a good supply of mild, sunny days and cool nights, along with moisture for colors to intensify.
"Contrary to popular belief, frost is not necessary for trees to begin their color show," Trinklein said. "Early frosts may even tarnish leaf color."
"Based on the moisture that's there, you have the variations when you see those peak colors," Bishop said.
"Whatever the outcome, fall leaf colors are a treat we are privileged to witness only once each year," Trinklein said. "Therefore, take time to enjoy them."
For more information, visit the Extension at http://extension.missouri.edu/p/G5010 for the guide, Autumn Colors.