The science behind foliage - or lack thereof

"Where are all the colors?"

If you live in the Berkshires, chances are you've heard a leaf peeper, or two, mutter that while scanning the mountains this fall. We've been wondering ourselves as we rake up mounds of wet, brownish leaves with the occasional yellow spot of color.

Is this a bad year for foliage? And if so, why?

"I would agree with your readers that the colors have been muted this year, as compared to other years, but this is not unexpected or necessarily 'bad' from the perspective of an ecologist," said Timothy Flanagan, professor of environmental and life sciences at Berkshire Community College. "Each year really is different."

Flanagan — who said he answers this question every year in his ecology class — was happy to discuss foliage and explain the colorful science behind it.

"The fall foliage display is like nature's report card on what has happened during the entire growing season," he said. "We can't always predict the outcome but we can certainly enjoy the surprise."

What makes the leaves change color?

The short answer: chlorophyll, carotenoids and anthocyanins.

Like all things, even nature's color palette needs pigments.

"In summer time, the cells in leaves produce a lot of colorful chemical pigments that help them collect and capture energy from the sun," Flanagan said. "The most visible pigment to note is chlorophyll, as it absorbs blue and red wavelengths but glints like an emerald while strongly reflecting greenish light to our eyes."

Basically, it gives leaves their basic green color.

Meanwhile, carotenoid pigments help to absorb violet to green wavelengths and commonly reflect in orange and yellow as seen in carrots and bananas, explained Flanagan. "These pigments also help to protect chlorophyll from damage when exposed to bright summer sunshine."

As the trees prepare for dormancy, less chlorophyll is produced. Cue the fall colors.

"The loss of green will expose the formerly hidden yellow and orange carotenoids," he said. "The appearance of these colors is therefore quite reliable from year to year."

What makes a "good" year for foliage?

The brilliance of the colors in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture play a major role.

"So a late spring or bad summer drought can stress the trees, or a good growing season can boost the level of pigments and sugars and set the stage for a spectacular display," Flanagan said.

So, what's the perfect combination?

According to Flanagan, it's warm, sunny days alternating with very cool (but not too frosty) nights in the middle of autumn.

"When this happens, a group of pigments called anthocyanins respond to plant sugars and longer nights by adding intense crimson reds and purples to the mix of fall colors," he said.

What makes leaves fall?

In response to the shortening days and less sunlight, leaves begin the processes leading up to their fall and your hours of raking.

The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf.

"These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall," according to the USDA Forest Service. Once the leaves fall, they decompose and restock the soil with nutrients.


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