This month changes in leaf color, heralding the approach of fall, have appeared across the north country. Experts said this week that while a wet summer may have stressed individual trees and caused them to color early, the majority of foliage is right on time.

"It's happening all over New York. Every place that I've been there's a lot of color on the trees," said Donald J. Leopold, distinguished teaching professor at the SUNY College of Environmental Science and Forestry, Syracuse.

Of the approximately 300 tree species in New York, and some 30 to 40 species in Watertown, Mr. Leopold said, there will always be certain trees, like red maples, that turn early.

Other individual trees can become stressed from environmental extremes, weather that is too hot and dry or too cool and wet, he said.

"Trees can't just get up and move and root systems can't adjust that rapidly," he said. "They're not tolerant of those extremes."

Susan J. Gwise, an educator with Cornell Cooperative Extension of Jefferson County, said individual trees would have had a particularly hard time this summer if they were in heavy clay soil saturated by rain.

"If there's not enough oxygen getting into the soil because the soil is saturated by water, yes, it puts a stress on the tree," she said.

Fungus or insects could also stress trees, causing them to turn early, she said.

"Obviously, some species turn before others," Ms. Gwise said, adding that she hasn't noticed any particularly unusual changes this year.

Stephen F. McLaughlin, a meteorologist with the National Weather Service in Buffalo, said the weather has been a bit cool for August, but nothing out of the ordinary.

Mr. McLaughlin compared this summer to last year's, when unusually warm and dry conditions stressed trees and caused them to turn early.

"The rainfall was quite heavy through a lot of the summer, but it's been a little lighter lately," he said. "It's been slightly wet but, with temperatures near normal, I wouldn't know if that would have any affect on the trees per se."

Any sense that fall is coming early may be simply a matter of marketing, said Cynthia A. Beckley, office manager for the town of Webb Visitors' Center in Old Forge.

She said the state tourism Web site, I Love New York, does not begin asking the town for foliage reports until September, even though local color may appear up to a month before.

"Any time after the first of August we start seeing the color change," she said. "I think we're right on schedule."