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Why do leaves change colors in the fall?

October 7, 2013
By Michael Green Fonte Staff Writer , The Inter-Mountain

Around this time of year, children sometimes ask, "Why do leaves change color?"

It's a question most adults take for granted. One would think the answer would be simple, but if one asks scientists specializing in trees, they'll admit that understanding that loss of green can be complicated.

The losing of leaves is an example of a biological term called abscission.

"Abscission is a physiological process that involves the programmed separation of entire organs, such as leaves, petals, flowers and fruit," said John C. Walker, professor and director of biological sciences at the University of Missouri. Walker said that on a cellular level, abscission works through a specialized barrier between the tree and the leaf stem. This barrier is known as the abscission zone, or AZ.

The AZ is similar to a lock placed on a river. The AZ manages water flow from the tree to the leaf. The water is used by the leaf's chloroplasts to perform photosynthesis. The chloroplasts used in this process are covered in the leaf's green pigments. To prepare for winter, the AZ starts limiting water flow to the leaf. Chloroplasts shut down, and green pigment diminishes to reveal other colors. The inevitable loss of the green pigment through the AZ's behavior is described by Walker as "programmed" because research demonstrates a signalling pathway from DNA to the AZ.

"The unmasking of the carotenoids accounts for the yellow fall leaf color of Ohio buckeye, yellow-poplar, sycamore, birches, hickories, ashes and many other tree species," Jeffrey Dawson, professor of tree physiology at the University of Illinois, Champaign-Urbana, explained. The darker leaves are caused by pigments called anthocyanidins. These anthocyanidins create the "pink, red, and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, and many other woody plants," Dawson said. Sap and other sugary substances play a role in creating the dark coloring, he added.

Regardless of leaf color, and whether any "leaf peepers" have caught a glimpse, once the AZ dries up and becomes hardened, the leaf will lose its grip and fall to the ground.

This year many have wondered if the high level of precipitation will affect foliage behavior, such as coloring onset, length or intensity. This is one thing scientists are still trying to figure out.

"Even though we had a wet summer, things seem on track for a good fall," said Howard Neufeld, professor of plant ecophysiology at Appalachian State University. "Because the weather has been cool at night and warm during the day, some trees could turn sooner, while others could last longer."

Neufeld said that in the science community, it's largely established that the weather during the leaf-changing season has an effect, not weather earlier in the year.

"There has been a study (spanning) over 20 years and it showed that earlier in the summer precipitation didn't play a role," he said.

The study Neufeld referenced looked at trees in New England. It could only say, "Much less is known, however, about how warming temperatures and precipitation and altered precipitation regimes affect autumn phenology (cycles), specifically as related to leaf coloration and senescence (leaf falling)."

Slightly more information was gleaned through studies conducted by Andrew Richardson, associate professor of organismic and evolutionary biology at Harvard University.

"Our results show that year-to-year variation in precipitation is correlated with variation in the timing and intensity of autumn colors," said Richardson. "But our modeling showed that adding more precipitation to the model didn't give us any more predictive power." In other words, Richardson says their work does show that more precipitation earlier in the year affects foliage behavior, but at this point they can't say in what way it affects the foliage change.

For information regarding leaf changing in West Virginia, visit the West Virginia Department of Commerce website for weekly fall foliage reports. Their first report was released on Oct. 3, and they will release the second report on Oct. 10. Dolly Sods, Blackwater Falls and Canaan Valley are currently areas with peak foliage.



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