Every fall, Marilyn Krom tries to make a trip to Vermont to see its famously beautiful fall foliage.

This year, she noticed something different about the autumn leaves.

"They're duller, not as sparkly, if you know what I mean," Krom, 62, a registered nurse from Eastford, Conn., said during a recent visit. "They're less vivid."

Other "leaf peepers" are noticing, too, and some believe climate change could be the reason.

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Forested hillsides usually riotous with reds, oranges and yellows have shown their colors only grudgingly in recent years, with many trees going straight from the dull green of late summer to the rust-brown of late fall with barely a stop at a brighter hue.

"It's nothing like it used to be," said University of Vermont plant biologist Tom Vogelmann, a Vermont native.

He says autumn has become too warm to elicit New England's richest colors.

According to the National Weather Service, temperatures in Burlington have run above the 30-year averages in every September and October for the past four years, save for October 2004, when they were 0.2 degrees below average.

Warming climate affects trees in several ways.

Colors emerge on leaves in the fall, when the green chlorophyll that has dominated all spring and summer breaks down.

The process begins when shorter days signal leaves to form a layer at the base of their stems that cuts off the flow of water and nutrients. But in order to hasten the decline of chlorophyll, cold nights are needed.

In addition, warmer autumns and winters have been friendly to fungi that attack some trees, particularly the red and sugar maples that provide the most dazzling colors.

"The leaves fall off without ever becoming orange or yellow or red. They just go from green to brown," said Barry Rock, a forestry professor at the University of New Hampshire.

He says 2004 was "mediocre, 2005 was terrible, 2006 was pretty bad although it was spotty. This year, we're seeing that same spottiness."

"Leaf peeping" is big business in Vermont, with some 3.4 million visitors spending nearly $364 million in the fall of 2005, according to state estimates.

State tourism officials reject the notion that nature's palette is getting blander. Erica Housekeeper, spokeswoman for the state Department of Tourism and Marketing, said she had heard nothing but positive reports from foresters and visitors alike this year.

The problem is perception, Housekeeper says: Recollections of autumns past become tinged by nostalgia.

"Sometimes, we become our own worst critics," Housekeeper said.

People who rely on autumn tourism in New England are worried.

"I don't have a sense that the colors are off, but the timing is definitely off," said Scott Cowger, owner and innkeeper at the Maple Hill Farm Bed & Breakfast Inn at Hallowell, Maine.

"Some trees are just starting to change now," Cowger said Thursday. "It used to be, religiously, it was the second week of October when they were at their peak. I would tell my guests to come the second week if you want to see the peak colors. But it's definitely the third or fourth week at this point."

People in Northampton, Mass., are still waiting on fall color. If foliage-viewing is the goal, "I wouldn't send anybody down this way yet," Autumn Inn desk clerk Mary Pelis said this past week.

"The way things are going, the foliage season is the one sure thing for us," said Amie Emmons, innkeeper at the West Mountain Inn, in Arlington, Vt. "We book out two years in advance. It's very concerning if you think the business could start to be affected."