Friday, October 9, 2009

Autumn colors emerge slowly—or not at all

Official: Fall’s delayed onset may be due in part to wet spring

Express Staff Writer

The powerful storm that dumped heavy snow clear to the Wood River Valley floor earlier this week sent the unmistakable message that autumn is here. Too bad the region's deciduous trees haven't received that message.

Up and down the valley and across much of central Idaho, forests that would have normally begun to display their brilliant fall hues of yellows, oranges and reds are instead mostly green or just muted browns. Local foresters say the season's leaf crop is also clinging to branches longer than normal, which contributed to havoc in the valley's towns this week as the heavy snowfall on leaves led to drooping branches, many of which broke under the strain.

In Hailey and Bellevue, and to a lesser extent in Ketchum, road crews were kept busy Monday morning cutting and clearing debris from damaged trees off downtown streets.

Those responsible for managing forests on federal lands in more rural parts of the valley are also wondering when the splash of bright fall colors will finally replace this fall's lackluster hues, if ever.

"I'm just not seeing the bright oranges and yellows," said Kurt Nelson, Ketchum District ranger for the Sawtooth National Forest. "It seems more that the leaves are turning brown."

Though day length is understood to be a significant contributor to autumn's color blast in the northern Rockies, alternating weather patterns—both current and from the previous summer and spring—can also impact the changing forest colors.

According to information provided by the U.S. Forest Service, the amount of moisture in the soil can have an effect on any given year's autumn hues. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns are exactly alike, Forest Service information states.

In early fall, in response to the shortening days and declining intensity of sunlight, leaves begin the processes that ultimately causes them to drop to the ground. 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 anthocyanin pigments, which are responsible for autumn's red, purple and blue colors. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to drop.

A late and wet spring—such as occurred this year—or a severe summer drought, can delay the onset of fall color. A warm and dry period during fall may also lower the intensity of autumn colors. Both those factors occurred this year, said Kim Pierson, forest botanist for the Sawtooth National Forest.

Pierson speculated that the amount of moisture the area received this spring—nearly four times the normal rainfall in June—led to the production of more chlorophyll in leaves, which gives them their green color. It's the loss of chlorophyll production once day length shortens that eventually causes leaves to begin changing color. With more chlorophyll in leaves this summer, the onset of fall color was likely delayed.

That, as well as the warm and dry September the region saw this year, may have tricked the leaves into staying green longer. Pierson said the only constant from year to year—steadily dwindling hours of daylight—just may not have been enough to initiate the shift to autumn colors.

"There wasn't any indication to the leaves to start changing," she said.

Also delayed by up to two weeks was the start of leaves dropping to the ground. Pierson fears that some of the trees damaged by Monday's snowfall could ultimately be susceptible to pathogens and other diseases, and die next year.

Pierson said that in the near term, even if leaves do begin to change color, the autumn display stands a good chance of being more muted than normal.

Jason Kauffman:

Why trees change color in the fall

A color palette needs pigments, and Mother Nature provides three that contribute to autumn's splendor: chlorophyll, carotenoids and anthocyanins. According to a U.S. Forest Service Web site, here's what those substances do and how they react to weather and climate conditions to make a colorful fall:

· Chlorophyll gives leaves their green color, and chemical reactions allow leaves to turn sunlight into sugar for food.

· Carotenoids are the yellows, oranges and browns of the color palette exemplified in bananas, corn and carrots.

· Anthocyanins produce the reddish-bluish hues of concord grapes, blueberries, plums and cherries. Responding to bright light and excess plant sugar, the majority of anthocyanins are produced in the fall.

· Chlorophyll and cartenoids are found in the chloroplasts of leaf cells during the growing season. A chloroplast is a specialized part of a leaf cell that converts light into plant food.

· During the growing season, chlorophyll is continually being produced and broken down, which gives leaves their greenish color. But as the nights get longer in autumn, the chlorophyll production slows down and stops, and all the chlorophyll is destroyed. Then, the carotenoids and anthocyanins that are present in the leaf are unmasked, setting the stage for fall's brilliant explosion of color.

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