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Fall foliage 101

The science behind autumn’s great show of color

October 1, 2011
By CHRIS KNIGHT - Senior Staff Writer (cknight@adirondackdailyenterprise.com) , Adirondack Daily Enterprise

The days are getting shorter. The nights are getting cooler. The forests of the Adirondacks are being transformed into a brilliant sea of reds, golds and yellows.

Autumn has arrived.

While many people are in awe of the change of color that's now sweeping across the Park, the science behind that color change is just as interesting, and sometimes misunderstood. Why do the leaves change color? Why do the colors seem to be more intense or last longer some years? When will peak colors arrive?

Article Photos

Fall colors are reflected in the water at Lake Clear Outlet.
(Enterprise photo — Lou Reuter)

This year in particular, many people are wondering what impact Tropical Storm Irene and the spring floods will have on fall colors in the Adirondacks, which are a major part of the region's tourism-based economy.

To try and answer these and other basic questions about the science behind fall foliage, the Enterprise talked with a pair of experts: Randall Swanson, associate professor of forestry at Paul Smith's College, and Sean Reynolds, a state Department of Environmental Conservation forester based in Ray Brook.

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Fact Box

Color change in the Adirondacks

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Lake Placid: near 100 percent

Tupper Lake: 95 percent

Whiteface Mountain: 45 percent

Indian Lake: 60-80 percent

Old Forge: 95 percent

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(Source: foliage spotters from the I Love New York tourism program)

Why do fall colors appear?

Reynolds described the process of fall color as a defense mechanism. As summer ends and autumn arrives, deciduous trees and shrubs shed their leaves because it takes up too much energy to continue photosynthesis during the winter.

"What's happening now is there's a layer of cells that form at the base of each leaf," Reynolds said. "Once that layer completes its abscission, it totally shuts off flow to the leaf, and whatever sugars are in the leaf are trapped in there."

Once that connection between the leaf and the rest of the tree is blocked off, production of chlorophyll, a green food-making pigment, slows and stops, revealing other pigments that had been covered up by the chlorophyll.

"The chlorophyll can't be sustained when the tree goes dormant, and as it's not replenished, these other pigments - yellows an oranges called carotenoids - are showing up," Swanson said. "They were already there. They just weren't visible because of the masking effect of the chlorophyll."

While most of these pigments were already present in the leaf during the growing season, there is one pigment that's manufactured in the fall from the sugars trapped in the leaf. It's called anthocyanin.

"Anthocyanin is a pigment that is developed in the leaf that gives us the reds and purples," Swanson said. "Red maple produces anthocyanin in the fall. It seems to be that sunny days during that fall color period and cold nights will create more of this anthocyanin."

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The trigger

One of the most common misconceptions about autumn is that changes in leaf color are brought on by changes in temperature, namely that first clear, cold night that typically comes in mid- to late- September. Swanson said the main trigger is actually the change in the length of the day.

"It's not the intensity of light; it's the duration of the light," he said. "Trees are evolved in a particular geographic area to be in tune with day length, and so leaf color change is part of the shutting down for the growing season. Trees are triggered to say, 'OK, it's time to shut down,' due to the fact that the days are getting much shorter."

Since the amount of daylight in the Adirondacks starts to fade around the same time every year, the timing of our fall color change is usually consistent, Swanson said.

"People guess at it, but generally the last week in September and early October is going to be your peak because the photo period really doesn't change," he said.

While the loss of daylight is the primary trigger, there are other factors that can cause leaves to change color sooner, like stresses. For example, if a tree is changing in color in late August, that's a sign that there's something wrong, Swanson said.

"It's often a root issue: a compacted soil that's not getting water, a recently cut soil or an area where beavers have flooded and the soils have become saturated," he said. "You can see individual trees changing earlier than the normal time period because of significant stress."

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Intensity

The intensity of the fall colors each year depends on a number of variables, the first of which is simply the amount of leaves on the trees. Swanson said that's dictated by how good a growing year it was.

"Did we have a lot of moisture? Was it a good year for those trees? Did they put out a lot of leaves and did those leaves remain on the trees going into the fall season?" he asked. "That can play a large role."

Several years ago, the North Country was hit with a huge outbreak of forest tent caterpillar, which fed on the leaves of hardwood trees, particularly maples. That affected the intensity of the fall colors that year.

"Hillsides in St. Lawrence County got hit extremely hard, although it did impact us here in Franklin County," Swanson said. "These were hillsides where normally you'd have a great fall display, and people said, 'We didn't have great colors this year.' Well, the trees just didn't have the leaves because of this huge insect outbreak."

Drought can also impact the intensity of the fall colors, reducing the size and number of leaves a tree produces.

"If you have a drought, the tree simply says, 'I'm not going to keep putting out leaves, and I'm going to start slowing down sooner,' and in that year you wouldn't have as many leaves," Swanson said.

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Duration

How long the fall colors stick around depends on two factors: wind and rain. The lack of wind and rain prolongs the display, while strong wind and heavy rain can cause leaves to be lost before they develop to full color.

"Once that barrier in the petiole of the leaves is formed, if you get strong winds and hard rain you're going to have a short color season because the leaves get knocked off," Swanson said.

"As we progress into October and November, we get a lot of seasonal winds," Reynolds said. "If we have a good 30-mile-per-hour wind, we'll see a lot of leaves coming down. Heavy rains can also be a factor, and snow can't be ruled out this time of year."

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This year

Reynolds said he believes Tropical Storm Irene and the spring floods had an impact on the timing of this year's show of fall colors. He thinks the additional moisture in the soil led to a longer growing season for many trees, leading to a lag in the start of the color change.

"I feel like, based on my personal experience, we're a week to a week-and-a-half behind," Reynolds said. "The flooding and the rains were a factor in that. Usually by now we're seeing more leaf drop than we have been."

Swanson, however, said he thinks the timing of this year's colors is about the same as in prior years.

While some of the moisture brought by heavy rains can be a benefit, Swanson said localized flooding in some areas may have kept soils saturated to the point that it caused trees to show early leaf color or even die. He also noted that the high winds brought by Irene weren't much of a factor because when the storm hit in late August, the leaves were still firmly attached.

"The leaves were not going into that dormancy stage at that point," he said. "They weren't ready to fall off when the high winds came through. Individual trees came down, but we didn't see leaves ripped off trees in gigantic quantities, and we have a lot of foliage on the trees that are turning color."

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Eye of the beholder

Swanson said people's impressions of the display of color they see in the fall may be more psychological than anything.

"If you're out there at the right time on a sunny day, you're going to be that much more impressed by a color display," he said. "I really love those late afternoon sunny days when some trees just seem to glow. There's nowhere I'd rather be this time of the year."

"It's absolutely, personally and professionally, my favorite time of the year," Reynolds added. "It's cooler. The bugs are gone. And, of course, the colors are beautiful. It's just a nice time of the year to be out in the woods."

 
 

 

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