In the usual explanation, the leaves fade, turning yellow in October. Eventually the leaves die, falling off of the trees. Hence, we often call the season "fall."
Biologically speaking, though, the season should be called "shed." The leaves, scientists will tell you, don't just fall off of the trees by happenstance. The tree intentionally drops the leaves to prepare for winter. And the changing of the colors of the leaves is an active process, not just a fading away.
As with most natural occurrences, the changes we see trees undergo in October are both more complex and interesting than they seem at first glance, says Robert Joly, associate professor of horticulture at Purdue University.
"There's something going on here that's inherently interesting. And it's really a dynamic process," Joly says. "Even my wife asks me to explain it to her every year."
Joly, a native of New Hampshire, says that each new school year the students in his introductory plant physiology class are fascinated to discover the biology of the process. "There's something about the trees changing colors in the fall that just captivates people," he says.
According to Joly, the essentials of how leaves change color have been known by scientists for more than 30 years. The first step to understanding the process is to know that tree leaves have three main pigments:
As temperatures begin to drop in the late summer, trees prepare for winter by stopping the production of chlorophyll. The chlorophyll already present in the leaves begins breaking down into simpler compounds, which are stored for the winter in the tree's twigs. As the chlorophyll is broken down, other colors -- the familiar yellow, orange, red, and purple colors of autumn -- are unmasked.
Trees with brilliant displays of yellow in the autumn include beech, honeylocust, poplars, birch and ginkgo trees.
For the best color show, clear days and cool nights are needed. The clear days allow photosynthesis to continue and allow the maximum production of anthocyanin. The cooler temperatures of autumn decrease the loss of nutrients, such as carbohydrates, through respiration and allow the pigments to accumulate. A mild or moderate drought will also increase the brilliance of the reds of autumn by stimulating anthocyanin production.
By the time trees sport the bright colors of October, they are completing their preparation for winter, a process that began during the hot, hazy dog days of mid-August. As the seasons slowly roll by almost imperceptibly day-by-day, the trees are able to perceive tiny changes in their environment.
"Trees are tuned in to a number of things, such as changes in the day length, light quality and temperature," Joly says. "The trees respond to their changing environment and transform these environmental changes into biological changes."
As autumn approaches, the tree begins breaking down chlorophyll in the leaves and moving the sugars and starches into storage cells in the twigs. The nitrogen in the chlorophyll also is moved into storage cells. "Nitrogen is valuable to plants, and each chlorophyll molecule contains four atoms of it," Joly says. "That's a heavy biological investment for that tree. It doesn't let that nitrogen just fall to the ground with the leaves."
Once the leaf is drained of its chlorophyll, the tree begins a process that removes the leaf. During the growing season, trees release two hormones -- ethylene, which causes the leaves to fall, and auxin, which counteracts ethylene. As winter approaches, ethylene levels rise as auxin levels drop. The ethylene acts upon a thin layer of cells near the base of the leaf called the abscission zone, triggering the production of enzymes that dissolve the polysaccharide "glue" that holds the cells together. This causes the cells in the abscission zone to separate and the leaf to drop to the ground.
"This is definitely not a passive process," Joly says. "The tree actively removes that leaf."
Trees shed their leaves each year for a variety of reasons. Fragile leaves often are damaged by insects, diseases or weather. The thin leaves also are easily damaged by freezing. Replacing the leaves each spring allows the tree to start fresh each year with a set of undamaged light collectors.
In the spring the tree will use the nitrogen it took up from the leaves to make amino acids, which are the building blocks of proteins used to make new tissue, such as new leaves. Some of the nitrogen also will be used to create new chlorophyll, so the tree can begin the process over again.
"It's pretty slick, isn't it?" Joly says.
"Some people view trees as objects that are practically lifeless," Joly says. "But they are living organisms that are exquisitely responsive to their environment."
Source: Robert Joly, (765) 494-6997; Internet, firstname.lastname@example.org
Writer: Steve Tally, (765) 494-9809; Internet, email@example.com
Purdue News Service: (765) 494-2096; e-mail, firstname.lastname@example.org
NOTE TO JOURNALISTS: A color photograph and a computer graphic showing the scientific processes that cause fall colors are available. Contact Steve Tally at (765) 494-2096.
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